A metamorphic rock used to be some other type of rock, but it was changed inside the Earth to become a new type of rock. The word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph).

During metamorphism the mineral content and texture of the protolith are changed due to changes in the physical and chemical environment of the rock. Metamorphism can be caused by burial, tectonic stress, heating by magma, or alteration by fluids.

A rock undergoing metamorphism remains a solid rock during the process. Rocks do not melt during most conditions of metamorphism.

Even though rocks remain solid during metamorphism, fluid is generally present in the microscopic spaces between the minerals. This fluid phase may play a major role in the chemical reactions that are an important part of how metamorphism occurs.

Metamorphic rocks provide a record of the processes that occurred inside Earth as the rock was subjected to changing physical and chemical conditions. This gives the geologist literally “inside information” on what occurs within the Earth during such processes as the formation of new mountain ranges, the collision of continents, the subduction of oceanic plates, and the circulation of sea water into hot oceanic crust.

Figure 1. The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism.

Metamorphism is the addition of heat and/or pressure to existing rocks, which causes them to change physically and/or chemically so that they become a new rock. Metamorphic rocks may change so much that they may not resemble the original rock.

All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Figure 2.

Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals.

Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Hornfels is shown in table 1.

Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated.

The two main types of metamorphism are both related to heat within Earth: The reason rocks undergo metamorphism is that the minerals in a rock are only stable under a limited range of pressure, temperature, and chemical conditions.

The type of rock undergoing metamorphism is a major factor in determining what type of metamorphic rock it becomes. In short the identify of the protolith plays a big role in the identity of the metamorphic rock.

the atoms will likely be rearranged into new mineral forms within the rock. Therefore, not only does the protolith determine the initial chemistry of the metamorphic rock, most metamorphic rocks do not change their bulk (overall) chemical compositions very much during metamorphism.

Temperature is another major factor of metamorphism. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes.

This is because temperature inside the Earth increases along what is called the geothermal gradient, or geotherm for short. Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism.

Tectonic processes are another way rocks can be moved deeper along the geotherm. Faulting and folding the rocks of the crust, can move rocks to much greater depth than simple burial can.

Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. Pressure is a measure of the stress, the physical force, being applied to the surface of a material.

Lithostatic pressure is the pressure exerted on a rock by all the surrounding rock. The source of the pressure is the weight of all the rocks above.

If pressure does not apply equally in all directions, differential stress occurs. There are two types of differential stress.

At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum stress. Shear stress pushes one side of the rock in a direction parallel to the side, while at the same time, the other side of the rock is being pushed in the opposite direction.

Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below.

This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. If any of these flat minerals are growing under normal stress, they will grow with their sheets oriented perpendicular to the direction of maximum compression.

Such a rock is said to be foliated, or to have foliation.

Most commonly, if there is a fluid phase in a rock during metamorphism, it will be a hydrous fluid, consisting of water and things dissolved in the water. Less commonly, it may be a carbon dioxide fluid or some other fluid.

The fluid phase can also influence the rate at which mineral crystals deform or change shape. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase.

However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. Most metamorphism of rocks takes place slowly inside the Earth.

Metamorphism usually involves slow changes to rocks in the solid state, as atoms or ions diffuse out of unstable minerals that are breaking down in the given pressure and temperature conditions and migrate into new minerals that are stable in those conditions. This type of chemical reaction takes a long time.

As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism.

This is not far beyond the conditions in which sediments get lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look somewhat like its protolith. Low grade metamorphic rocks t.

This activity will help you assess your knowledge of the definition and examples of foliated rocks. For this activity, print or copy this page on a blank piece of paper.

Neatly write the LETTER of your answer on the appropriate blank space provided before the number and your EXPLANATION below the sentence.

__________ 1.) a.) Metamorphic rocks have distinct b.) layers, textures, and c.) patterns that can easily be identified.

__________ 2.) a.) Metamorphic rocks are formed as a result of applying extreme b. heat and c.) direction to existing rocks.

__________ 3.) The layers in a.) foliated rocks are caused by natural b.) shearing which is the change of direction the c.) heat is applied.

__________ 4.) The a.) temperature, the b.) length at which the rock is buried, and the c.) amount of time of the process are some of the factors involved in the formation of foliated rocks.

__________ 5.) a.) Phyllite is a b.) foliated rock formed from schist as its c.) source rock.

1.) a. Foliated rock is a type of metamorphic rock having identifiable layers, textures, and patterns.

2.) c. Extreme heat and pressure are applied to existing rocks to form metamorphic rocks.

3.) c. The change in direction of applied pressure causes layers in foliated rocks.

4.) b. When something is below the surface, the measurement is referred to as the depth at which something is buried.

5.) a. Gneiss is formed from schist as a source rock.

Glacier ice, like limestone (for example), is a type of rock. Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite).

Most glacier ice forms through the metamorphism of tens of thousands of individual snowflakes into crystals of glacier ice. Each snow flake is a..

Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite). The mineral ice is the crystalline form of water (H2O).

Each snow flake is a.. Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies.

Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.Intrusive Igneous Rocks:Intrusive, or plutonic, igneous rock forms.. Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies.

Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.Intrusive Igneous Rocks:Intrusive, or plutonic, igneous rock forms.. Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms.

Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock.

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface.

Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks: Common sedimentary rocks include sandstone..

Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite. A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral matter.

A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite.

Common rocks include granite, basalt.. At the head of the valley in Yosemite National Park – as if on a pedestal – stands Half Dome.

Half Dome, which stands nearly 8,800 feet (2,682 meters) above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized thousands of feet beneath the Earth’s surface. The..

It is smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8,800 feet (2,682 meters) above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized thousands of feet beneath the Earth’s surface.

Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations.Geologic maps at many scales and from many sources are listed in the National Geologic Map Database.Some geologic maps can be purchased in hard copy through the USGS Store.Download digital geologic maps for entire states from the USGS Mineral Resources Online Geospatial Data..

Our National Parks are the showcases of our nation’s geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history.

The website has listings for regions of the country. selected parks are listed within each region..

The National Park Service has websites for most individual parks that include information about their geology and natural history. A source of information from the USGS is our Geology and Ecology of National Parks website.

selected parks are listed within each region.. Ever wondered what the difference between a rock and a mineral was.

Originally, any rock now classed as metamorphic, started out as a different kind of rock. They’ve just changed from an earlier form of rock, whether they were sedimentary, igneous, or even some other kind of metamorphic rock.

Slate is a type of metamorphic rock. ©.

In most cases, some combination of these factors works together to change the rocks. Metamorphism is not a process that melts rocks.

Minerals metamorphosize into new minerals by rearranging the components within. Or some kind of reaction occurs to the fluids that enter the rocks and therefore change them.

Rocky mountains are made up of metamorphic rocks. ©Steve Boice/Shutterstock.com.

This occurs at the subduction zones when continental collisions occur as two plates converge. This type of metamorphism forms most of the foliated metamorphic rocks, including slate, phyllite, schist, and gneiss.

The heat of nearby magma combined with solid rock and igneous intrusion creates contact metamorphism. This basically means that it’s not pressure but rather contact with hot magma and extreme temperatures that create metamorphic changes.

Sometimes, this form of metamorphism is referred to as high-temperature, low-pressure metamorphism. Hornfels is commonly the result of this kind of metamorphosis.

This type of metamorphism occurs when rocks interact extensively with super-hot fluids for a period. Often, these heated fluids are the result of nearby hot magma.

This form of metamorphism should be reasonably clear by the name. It occurs when rocks are buried beneath sediments deep enough to create the stress of pressure at the lithostatic level.

This kind of metamorphism occurs deep in the earth ad subduction levels (tectonic plates level). Subduction takes rocks deep into the earth relatively quickly, putting immense pressure on the rocks and metamorphosizing them.

Metamorphic rock containing staurolite and almandine garnet. ©.

Fluids are often present during metamorphism, filling in microscopic spaces in the rocks that are changing. Metamorphic rocks provide geologists with an insight into the happenings deep within the earth, as they bear the evidence or record of the changes they undergo.

Think of slate, for example. It is made of fine-grained interlocking crystals that have created flat rocks.

The crystals line up in the rock, making it appear a bit wrinkly and shiny at once (flecks of mica). This is a foliated rock.

This appearance of bands is the semi-interlocking of crystals forming those flat “squashed” foliation. Basically, it could be said that metamorphic rocks may have 5 characteristics, depending on how they are formed.

Not all metamorphic rock will have shimmer and shine (non-foliated rocks don’t), but if you hold up the rock to light and see shininess on the rock, you have a foliated metamorphic rock on hand. The small flecks are created by crystals in the rocks.

Tiny, reflective dots and spots in a rock usually indicate you’ve got a metamorphic rock. These are those spiny specs of minerals like mica that sparkle and glimmer in light.

When stripes or bands, even narrow ones, appear, looking like ribbons or veins in the rock, you more likely than not have a metamorphic rock. These are different than sedimentary rock, though, which have layers that look a bit like stacked pieces and have distinctive textures.

The grainy appearance is quite clear in this Mylonite. ©.

These don’t usually reflect the way crystalline rocks do, but the grains are distinctive, with obvious bands and stripes. These patterns and grains are a strong indicator that you’re probably dealing with metamorphic rock.

We do have some articles that dig deeper into types, formation, and others, but these FAQs may help answer some basic questions you have. Interestingly enough, any type of rock can become a metamorphic rock.

Existing metamorphic rocks can also be metamorphosized again, as well. It all depends on the various conditions the rocks encounter which determine if they remain as is, melt into igneous rock, or morph into a new kind of rock altogether.

©iStock.com/Rawf8. There are many, many common metamorphic rocks that we interact with and have become familiar with because of their use in architecture, home improvement, and other structures, as well as commonly seen in nature.

There are two primary types of metamorphic rocks: foliated and non-foliated. Foliated metamorphic rocks are strongly banded types of metamorphic rocks.

They have a platy or sheet-like structure to them. Non-foliated metamorphic rocks don’t have that sheet-like structure, but rather are formed through other conditions, like heat and fluids, which change the rock without flattening them.

The photo featured at the top of this post is ©. Enter your email in the box below to get the most mind-blowing animal stories and videos delivered directly to your inbox every day.

Where Does Metamorphism Occur. Where Does Metamorphism Occur.

With this background, let’s now examine the geologic settings on Earth where metamorphism takes place, as viewed from the perspective of plate tectonics theory. Because of the wide range of possible metamorphic environments, metamorphism occurs at a wide range of conditions in the Earth.

That’s because the geothermal gradient (the relation between temperature and depth), the extent to which rocks endure compression and shear during metamorphism, and the extent to which rocks interact with hydrothermal fluids all depend on the geologic environment. Thermal or Contact Metamorphism Geologic settings of metamorphism.

Heat flows from the magma into the wall rock, for heat always flows from hotter to colder materials. As a consequence, the magma cools and solidifies while the wall rock heats up.

As a consequence of the heat and hydrothermal fluids, the wall rock undergoes metamorphism, with the highest-grade rocks forming immediately adjacent to the pluton, where the temperatures were highest, and progressively lower-grade rocks forming farther away. The distinct belt of metamorphic rock that forms around an igneous intrusion is called a metamorphic aureole or contact aureole (figure above a).

The local metamorphism caused by igneous intrusion can be called either thermal metamorphism (see Pottery Making—An Analog for Thermal Metamorphism), to emphasize that it develops in response to heat without a change in pressure and without differential stress, or contact metamorphism, to emphasize that it develops adjacent to the contact of an intrusion with its wall rock.

Contact metamorphism occurs anywhere that the intrusion of plutons occurs. In the context of plate tectonics theory, plutons intrude into the crust at convergent plate boundaries, in rifts, and during the mountain building that takes place where continents collide.

At depths greater than about 8 to 15 km, depending on the geothermal gradient, temperatures may be great enough for metamorphic reactions to begin, and low-grade metamorphic rocks form. Metamorphism due only to the consequences of very deep burial is called burial metamorphism.

Near the Earth’s surface (in the upper 10 to 15 km) this movement can fracture rock, breaking it into angular fragments or even crushing it to a powder. But at greater depths, rock is so warm that it behaves like soft plastic as shear along the fault takes place.

We call this process dynamic metamorphism, because it occurs as a consequence of shearing alone under metamorphic conditions, without requiring a change in temperature or pressure. The resulting rock, a mylonite, has a foliation that roughly parallels the fault (figure above b).

Dynamic metamorphism takes place anywhere that faulting occurs at depth in the crust. Thus, mylonites can be found at all plate boundaries, in rifts, and in collision zones.

As a consequence, rock that was once near the Earth’s surface along the margin of a continent ends up at great depth beneath the mountain range (figure above c). In this environment, three changes happen to the protolith: (1) it heats up because of the geothermal gradient and because of igneous activity.

and (3) it undergoes compression and shearing. As a result of these changes, the protolith transforms into foliated metamorphic rock.

Since the metamorphism we’ve just described involves not only heat but also compression and shearing, we can call it dynamothermal metamorphism. Typically, such metamorphism affects a large region, so geologists also call it regional metamorphism.

Such belts may be hundreds of kilometres wide and thousands of kilometres long. Hydrothermal Metamorphism at Mid-Ocean Ridges Hot magma rises beneath the axis of mid-ocean ridges, so when cold seawater sinks through cracks down into the oceanic crust along ridges, it heats up and transforms into hydrothermal fluid.

Eventually, the fluid escapes through vents back into the sea. these vents are called black smokers.

This pliable and slimy muck is a mixture of very fine clay minerals and quartz grains formed during the chemical weathering of rock and water. Fine potter’s clay for making white china contains a particular clay mineral called kaolinite, named after the locality in China (called Kauling, meaning high ridge) where it was originally discovered.

Such bricks can be used for construction only in arid climates, because if it rains heavily, the bricks will rehydrate and turn back into sticky muck drying clay in the sun does not change the structure of the clay minerals. To make a more durable material, brick makers place clay blocks in a kiln and bake (“fire”) them at high temperatures.

Potters use the same process to make jugs. In fact, fired clay jugs that were used for storing wine and olive oil have been found intact in sunken Greek and Phoenician ships that have rested on the floor of the Mediterranean Sea for thousands of years.

In other words, firing causes a thermal metamorphic change in the mineral assemblage that composes pottery. The extent of the transformation depends on the kiln temperature, just as the grade of metamorphic rock depends on temperature.

To produce porcelain fine china the clay must partially melt at even higher temperatures up to 1400C. Just as it begins to melt, the potter cools it relatively quickly.

Metamorphism in Subduction Zones Blueschist is a relatively rare rock that contains an unusual blue-coloured amphibole. Laboratory experiments indicate that formation of this mineral requires very high pressure but relatively low temperature.

So to figure out where blueschist forms, we must determine where high pressure can develop at relatively low temperature. Plate tectonics theory provides the answer to this puzzle.

They realized that because prisms grow to be over 20 km thick, rock at the base of the prism feels high pressure (due to the weight of overburden). But because the subducted oceanic lithosphere beneath the prism is cool, temperatures at the base of the prism remain relatively low.

The heat may be sufficient to melt or even vaporize rock at the impact site, and the extreme compression of the shock wave causes quartz in rocks below the impact site to undergo a phase change and become a more compact mineral called coesite. The changes in rock due to the passage of a shock wave are called shock metamorphism.

Figure 10.8 is an outcrop of the Gore Mountain amphibolite at the Barton Mine, near North Creek in the Adirondack Mountains of New York. This mine once produced garnet for use as an abrasive and to make sandpaper.

The biggest garnets in this photo are about 10 cm across, but some garnets from the mine are up to a meter across. Petrologists call large crystals like these megacrysts.

Recent thermodynamic modeling by Shinevar et al. (2021), however, suggests that a small amount of melt was possibly present.

Some garnets have a layer of plagioclase between the garnet and the hornblende. Textures of this sort – where one mineral armors another – suggest that the garnet-forming reaction may have stopped before going to completion.

(2021) concluded that the Gore Mountain amphibolite equilibrated at 9-10 kbar and at a temperature of about 950 oC, an exceptionally high metamorphic temperature if no melting was involved. The abnormally large garnets formed because of a prolonged period of metamorphism and an influx of metamorphic fluids.

21 November 2018. Posted by Callan Bentley.

I’d like to return to the South Mountains today for a more comprehensive look at the rocks exposed there. The South Mountains offer a geometrically-relatively-simple example of a metamorphic core complex.

There you see a granite, and a deformed granite. Granite is an igneous rock: one of those rocks that forms from the slow crystallization of magma, deep underground.

But as you work your way down through the photo, the rock takes on more foliation, and the size of the big chunky feldspar crystals is reduced dramatically. Quartz crystals are “ribboned” out into long wraithlike things.

The exact boundary between the granite and the smeared-out version of the granite is impossible to put your finger on here – the deformation is progressively more and more pronounced from top to bottom in this photo. By the time we’re at the bottom, though, we’ve got a mylonite.

Mylonite is a structural rock term. it’s independent of the composition of the rock that got deformed.

If they can smear out, they can hope to someday be lucky enough to be transformed into mylonite.

Mylonites were first described at Loch Eriboll in Scotland’s Northwest Highlands, where Charles Lapworth gave them their name and interpreted their fine grain size and foliation to be the result of thrust faulting. Rather than faults being crisp breaks in the crust, sometimes they could be broader zones of smeared-out rocks that accomodated the relative movement between two big blocks of rock in a diffuse zone of deformation.

In the South Mountains, the massive phaneritic granite you find at lower levels transitions at Dobbins Lookout into something very fine indeed:.

This little outcrop (of granitic-protolith mylonite) has a couple of tiny kinks running through it. These kink bands are like very crisp folds, and they only occur in rocks that have a strong mechanical layering, as here with the mylonitic foliation.

(There is also at least one proper microfault in that image. See if you can find it.

It’s worth mentioning that mylonites are often lineated as well as foliated. While I’ve been showing you photos of the trace of the foliation surface so far, I can also aim the camera lens right down onto that foliation surface, and look at the lineations lying within it, like pencils aligned on the surface of a desk.

In the South Mountains, that’s an east-northeast/west-southwest orientation, and it’s the direction the hanging wall block slid off the top of the metamorphic core complex. In other words, it’s the direction of crustal extension.

There’s some serious grain size reduction between the first lineation photo and the second. That’s mylonitization for you.

Okay, now let’s zoom out and look at the overall geology of the mountain range, before diving back in to outcrop photos. Here is a geologic map of the South Mountains by Steve Reynolds and Julia Johnson of Arizona State University, shared with geologists attending the January 2018 Structural Geology and Tectonics Forum at ASU:

Steve Reynolds interprets the structural geology of the range as representing an episode of crustal extension, with deep, warm rocks experiencing mylonitization at first, then rising toward the surface and progressively colder conditions.

So now, with that set-up, it’s finally time for some photographs of the brittle deformation that overprints the ductile… The next few photos show brittle faulting offsetting well-developed mylonitic foliation:.

I’ve switched the orientation of the trace of foliation in the next three photos. If you look closely, you’ll see that both the light-colored rock (deformed granite) and the dark colored rock (deformed some-darker-colored-rock) are foliated:

…Zooming in closer….

Here’s another outcrop, quite striking, especially with that foliation-perpendicular contact between foliated-dark and foliated-light so crisp and sharp:.

There are lots of little microfaults in there too, disrupting the contacting and giving it a stair step sort of morphology. I think this was the most profound example I saw: Everything you’re looking at in this next photo is mylonite, just in two different colors, and the orientation of the trace of foliation is ~horizontal in this photo in both rock types:

I was really struck in the field by how profoundly crisp the vertical contacts were, a clear indication there was no more subsequent mylonitization of these rocks after faulting shuffled them into the arrangement we see today.

There might be some foliation-parallel slip, too. We saw that in the orientation of the pseudotachylytes I mentioned the other day.

the modern outcrop breaks cleanly across them. They aren’t crumbly.

There’s plenty of that up (structurally) above in the (map-scale) chloritic breccia. The little train of three boudin-like fault blocks at upper right was nice, too:

All told, these are an intriguing set of rocks and structures, and I’m glad I got the chance to explore them in more detail than when I first visited this site in January. Posted in: arizona, faults, foliation, granite, lineation, structure No Comments/Trackbacks ».

Contributing Author: Dr. Peter Davis, Pacific Lutheran University.

Metamorphic rocks, meta- meaning change and –morphos meaning form, is one of the three rock categories in the rock cycle (see chapter 1). Metamorphic rock material has been changed by temperature, pressure, and/or fluids.

And metamorphic rocks themselves can be re-metamorphosed. Because metamorphism is caused by plate tectonic motion, metamorphic rock provides geologists with a history book of how past tectonic processes shaped our planet.

Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock. Most metamorphic processes take place deep underground, inside the earth’s crust.

Rock texture is changed by heat, confining pressure, and a type of pressure called directed stress. Temperature measures a substance’s energy—an increase in temperature represents an increase in energy.

At high temperatures atoms may vibrate so vigorously they jump from one position to another within the crystal lattice, which remains intact. In other words, this atom swapping can happen while the rock is still solid.

Heat-driven metamorphism begins at temperatures as cold as 200˚C, and can continue to occur at temperatures as high as 700°C-1,100°C. Higher temperatures would create magma, and thus, would no longer be a metamorphic process.

Pressure is the force exerted over a unit area on a material. Like heat, pressure can affect the chemical equilibrium of minerals in a rock.

Stress is a scientific term indicating a force. Strain is the result of this stress, including metamorphic changes within minerals.

When pressure is exerted from rocks above, it is balanced from below and sides, and is called confining or lithostatic pressure. Confining pressure has equal pressure on all sides (see figure 6.2) and is responsible for causing chemical reactions to occur just like heat.

Confining pressure is measured in bars and ranges from 1 bar at sea level to around 10,000 bars at the base of the crust. For metamorphic rocks, pressures range from a relatively low-pressure of 3,000 bars around 50,000 bars, which occurs around 15-35 kilometers below the surface.

Directed stresses are generated by the movement of lithospheric plates. Stress indicates a type of force acting on rock.

In contrast to confining pressure, directed stress occurs at much lower pressures and does not generate chemical reactions that change mineral composition and atomic structure. Instead, directed stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals.

Directed stresses produce rock textures in many ways. Crystals are rotated, changing their orientation in space.

Conversely, they may grow larger as atoms migrate. Crystal shapes also become deformed.

For example, recrystallization increases grain size much like adjacent soap bubbles coalesce to form larger ones. Recrystallization rearranges mineral crystals without fracturing the rock structure, deforming the rock like silly putty.

A third metamorphic agent is chemically reactive fluids that are expelled by crystallizing magma and created by metamorphic reactions. These reactive fluids are made of mostly water (H2O) and carbon dioxide (CO2), and smaller amounts of potassium (K), sodium (Na), iron (Fe), magnesium (Mg), calcium (Ca), and aluminum (Al).

In addition to using elements found in the protolith, the chemical reaction may incorporate substances contributed by the fluids to create new minerals. In general, this style of metamorphism, in which fluids play an important role, is called hydrothermal metamorphism or hydrothermal alteration.

Fluids-activated metamorphism is frequently involved in creating economically important mineral deposits that are located next to igneous intrusions or magma bodies. For example, the mining districts in the Cottonwood Canyons and Mineral Basin of northern Utah produce valuable ores such as argentite (silver sulfide), galena (lead sulfide), and chalcopyrite (copper iron sulfide), as well as the native element gold.

Hot, circulating fluids expelled by the crystallizing granite reacted with and dissolved the surrounding limestone and dolostone, precipitating out new minerals created by the chemical reaction. Hydrothermal alternation of mafic mantle rock, such as olivine and basalt, creates the metamorphic rock serpentinite, a member of the serpentine subgroup of minerals.

Some hydrothermal alterations remove elements from the parent rock rather than deposit them. This happens when seawater circulates down through fractures in the fresh, still-hot basalt, reacting with and removing mineral ions from it.

The mineral-laden water emerges from the sea floor via hydrothermal vents called black smokers, named after the dark-colored precipitates produced when the hot vent water meets cold seawater (see chapter 4). Ancient black smokers were an important source of copper ore for the inhabitants of Cyprus (Cypriots) as early as 4,000 BCE, and later by the Romans.

If you are using an offline version of this text, access the quiz for section 6.1 via the QR code.

Metamorphic rock textures are foliated, non-foliated, or lineated are described below. Table 6.1: Metamorphic rock identification table.

Certain minerals, most notably the mica group, are mostly thin and planar by default. Foliated rocks typically appear as if the minerals are stacked like pages of a book, thus the use of the term ‘folia’, like a leaf.

These linear objects can also be aligned within a rock. This is referred to as a lineation.

If they lie on a plane with mica, but with no common or preferred direction, this is foliation. If the minerals line up and point in a common direction, but with no planar fabric, this is lineation.

this is both foliation and lineation. Foliated metamorphic rocks are named based on the style of their foliations.

Slate is a fine-grained metamorphic rock that exhibits a foliation called slaty cleavage that is the flat orientation of the small platy crystals of mica and chlorite forming perpendicular to the direction of stress. The minerals in slate are too small to see with the unaided eye.

In fact, original sedimentary layering may be partially or completely obscured by the foliation. Thin slabs of slate are often used as a building material for roofs and tiles.

This chapter is a photo gallery of igneous and metamorphic rocks. Different parts of the chapter have brief introductions to put the photos in context, but mostly this chapter is meant only to provide many visual examples of many different rocks.

For more detailed navigation, use the Table of Contents on the right side of this web page. Most rocks in this collection have a hand sample photo, a plane-polars (PP) thin-section view, and a crossed-polars (XP) thin-section view.

Many rocks (and we are adding more) have a link to a 3D rotatable model of the sample. The rotatable images open in a different tab on your browser.

Many of the photos in this chapter were taken specifically for this book. Others came from existing University of North Dakota websites, and others came from a number of different internet sources.

Granitoids are a diverse group of plutonic rocks that contain some combination of essential quartz, plagioclase, and alkali feldspar. Two (or three) of these minerals are always present in granitoids.

All contain essential quartz with feldspar. The feldspar can be plagioclase or alkali feldspar, or both, depending on the kind of rock (Figure 16.1).

Micas (biotite and sometimes muscovite), and amphiboles or pyroxenes of various sorts, are commonly present. Many other minerals may be present in small amounts.

8 cm across space holder. 16.3 Thin-section view (PP) of the same aplite.

16.4 Thin-section view (XP) of the same aplite. Aplites are rocks equivalent to fine-grained granites.

The photos above show the Silver Plume aplite from near Boulder, Colorado. The sugary-textured rock contains predominately quartz, microcline, plagioclase, and lesser amounts of biotite.

In the plane polars view, the feldspars have rough surface due to incipient alteration. some show traces of twinning.

Dark colored grains are biotite. In crossed polars, the quartz and feldspar have typical 1st-order interference colors.

16.5 Biotite-hornblende granite. 8.5 cm across.

2.5 mm across. 16.7 Thin section (XP) view of the same granite blank space.

The coarse-grained granite seen above comes from St. Cloud, Minnesota.

Cloud are well known as sources of building stone, generically called the St. Cloud granite, whether the stone is truly granite or not.

Cloud granite is porphyritic, containing conspicuous large pinkish microcline crystals up to 1-2 cm across. Other light-colored mineral grains are gray plagioclase and glassy quartz.

The PP thin-section view shows light brown flakes of biotite with green hornblende. Clear quartz and feldspar surround them.

In crossed polars, the interference colors of the mafic minerals do not show – being masked by the minerals’ strong colors. The large grain along the bottom edge of the view is K-feldspar, and just above it is a large quartz crystal displaying undulatory extinction.

Figure 16.8 is a photo of one of the many abandoned quarry pits in the St. Cloud area.

10 cm across place holder. 16.10 PP view of the St.

2.5 mm across. 16.11 The same thin-section view, but with XP light.

Cloud, Minnesota. It is mostly pink K-feldspar, gray euhedral to subhedral plagioclase, and black biotite.

Lesser amounts of hornblende and quartz are also present but difficult to discern. In plain-polarized light, the thin section shows brown biotite and pale-green hornblende.

Two very high-relief dark-brown titanite crystals stand out. In some parts of this thin section, the hornblende contains remnants of augite, although it is not obvious in this view.

Biotite displays 2nd-order colors. Plagioclase is zoned and a bit altered to sericite, and titanite displays its normal very high birefringence.

9 cm acrossspace holder space. 16.13 The same tonalite seen in thin section (PP view).

16.14 XP view of the same thin sectionthis is a space holder. Tonalites are plutonic rocks characterized by essential quartz and plagioclase.

Mafic accessory minerals include biotite, hornblende, and clinopyroxene. The photos above are the Bonsall Tonalite from San Diego County, California.

In the PP thin-section view, clear grains are quartz and plagioclase. Plagioclase has slightly higher relief than quartz and shows specks and lines due to alteration.

In the XP view, the diagonal lath of hornblende displays twinning and up to high 2nd-order interference colors. Biotite has its typical pebbly texture with up to 2nd-order colors.

Quartz-poor granitoids are not as common as their quartz-rich cousins. These rocks contain 0-20% quartz with K-feldspar, plagioclase, or both.

As with other granitoids, there are many variations because of different minor and accessory minerals that may be present. Micas, amphiboles, and pyroxenes of various sorts, and other minerals, are common accessories.

5 cm across space holder. 16.17 A different syenite from the same area.

2.5 mm across. 16.18 The same thin-section view with XP light place holder.

The coarse-grained syenite from Cuttingsville, Vermont (Figure 16.16) contains mostly light-colored perthitic orthoclase and Na-rich plagioclase. The mafic minerals are biotite with lesser amounts of hornblende.

Most of the dark minerals seen in the hand sample are biotite. The thin-section views are of a different syenite.

The biotite contains inclusions of ilmenite. Below and to the left of the biotite is a large grain of greenish-brown hornblende that shows 60o-120o cleavage angles.

Under crossed polars, both mafic minerals appear brown due to the strong coloration of the mineral grains. Feldspars and quartz have their characteristic black-gray-white interference colors.

16.19 Monzonite. 17 cm across space holder.

7 mm across. 16.21 XP view of the same thin section.

The hand sample of monzonite above is from an unknown location in France. Besides the two feldspars, it contains a small amount of mafic minerals.

The thin sections views above are of a different monzonite, one from the Monzoni Massif in Italy, the type locality for this kind of rock. The colorless grains in the PP view are orthoclase and plagioclase.

In crossed polars, the pyroxene has strong 2nd-order or 3rd-order interference colors. It is hard to see hornblende’s interference colors because the strong color of the mineral masks them.

Plagioclase shows polysynthetic twinning. Some K-feldspar grains show simple Carlsbad twinning.

8.5 cm acrossholder. 16.23 Thin section (PP) view of the same quartz monzonite.

16.24 Thin section (XP) view of the same quartz monzonite. Quartz monzonites contain quartz and subequal amounts of plagioclase and K-feldspar.

This quartz monzonite from Garfield, Colorado, contains phenocrysts of very light-pinkish K-feldspar and white plagioclase in about equal proportions. The two feldspars have about the same color in the hand sample.

The mafic minerals are biotite and hornblende. The rock contains 10-25% quartz.

When we think of the word metamorphosis, we usually think of caterpillars turning into butterflies, or perhaps an actor taking on a role and becoming that character so convincingly that we can’t tell where the actor ends and the character begins. However, the term can apply to rocks, as well.

When an existing rock does this, it is referred to as a metamorphic rock. There are three types of rocks on the earth: metamorphic, sedimentary, and igneous.

Now, though, through these unique processes, they have changed forms. Very specific conditions are required to turn a rock into another type of rock, as you’d probably assume based on what most of us know or perceive about these solid materials.

Mountains are formed in belts in what is known as regional metamorphism. ©.

There are many types of rocks within igneous, metamorphic, and sedimentary rocks. ©natkom/Shutterstock.com.

This process usually happens deep within the Earth, whether beneath solid land or in the ocean floor. It may occur upon collision of the tectonic plates, as well.

There must be a geological uplift for these rocks to surface where we humans may find them and use them. As soil erodes over time, this uplift occurs, revealing these deeply bedded rocks.

There are multiple types of metamorphism, though, which form these rocks. Eclogite, one of the many types of metamorphic rocks.

Metamorphism is a natural process that occurs within the Earth’s crust or deep within at the subduction zone (tectonic plates). Super-hot mineral-rich fluids, extreme pressure, extreme heat, or some combination thereof initiates the process.

The rocks are not melted but rather flattened and reshaped. Chemical changes occur in the minerals contained within the existing rocks.

These new rocks, formed by these processes, are metamorphic rocks. Andesite is an extrusive igneous rock named after the Andes Mountains where it is found in abundance.

A lot of folks confuse igneous rocks and metamorphic rocks, which makes sense. Both rocks are “changed” in a sense, often by exposure to heat.

Chemical processes or the compaction of different types of rocks change metamorphic rocks. Igneous rock is rock or magma that melts and cools to form this new type of rock.

Extreme heat produces both metamorphic rock and igneous rock. Magma or lava forms igneous rocks.

Rocky mountains around the world are typically formed in metamorphic response to shifts in the Earth’s crust or tectonic plates. ©Nyker/Shutterstock.com.

This form of metamorphism usually occurs in long belts, which is why the name “regional” has been attached to this form. This is often how mountain ranges are formed.

This type of metamorphic rock tends to occur in long belts. Different types of metamorphic rock are created depending on the gradients of heat and pressure applied.

©visdia/Shutterstock.com. As the name indicates, extreme heat induces thermal metamorphism.

These are areas deep within the Earth’s crust where magma forms and expels, upwelling to create environments of extreme heat. It is within these environments that thermal metamorphism occurs.

Another metamorphic process will use extreme pressure beyond 100 megapascals of force (1 megapascal is equivalent to 145 pounds per square inch). This occurs within the Earth’s crust where the tectonic plates shift and create this immense force.

As the plates rub together, shearing and crushing of the plates occur, which is the primary cause of this type of metamorphism. The result is smaller, broken particles making up the rocks.

Even tinier particles ground down by this massive pressure create mylonitic rocks. In some circumstances, both heat and pressure work together to form metamorphic rocks.

This form of metamorphism occurs along mountain belts, ranges, and tectonic plates. These rocks are some of the most commonly formed metamorphic rocks.

Metamorphic rocks are formed in different ways, as noted above, but the results are either foliated (banded, striped, or layered) rocks or non-foliated (non-striped or banded) rocks. These are the two classifications of metamorphic rocks.

While less obviously banded, if you look at slate you can see patterns very distinctively giving it the unique texture it has. ©.

This is the mass pressure at work, often in conjunction with immense heat, elongating the minerals within and creating foliated (striped, banded, or striped layers). Foliated metamorphic rocks are easy to identify by these visible bands or stripes within the rocks.

Metamorphic rock containing staurolite and almandine garnet – a distinctively metamorphic rock without the banding. ©.

Non-foliated rocks occur when minerals are irregular, not elongated or uniform. Under the massive pressure, these minerals compress but don’t align into sheets or platy layers.

Marble is one of the most well-known metamorphic rocks out there, as it has been used for millennia for carving into statuary and creating ornaments in homes, palaces, and more. ©.

We find them as they surface through erosion of the soil by many circumstances and then we use them for many purposes. Some of the most commonly used include:

The photo featured at the top of this post is ©. Enter your email in the box below to get the most mind-blowing animal stories and videos delivered directly to your inbox every day.

KEY CONCEPTS. Metamorphic rocks, and the processes that create them, are key parts of the rock cycle that relates igneous, sedimentary, and metamorphic rocks.

The preexisting, or parent rocks, are called protoliths. Protoliths can be igneous, sedimentary, or metamorphic rock of all sorts.

These changes record geologic processes and events of the past. Consequently, metamorphic petrologists often study metamorphic rocks to interpret rock histories.

This kind of metamorphism, called regional metamorphism, creates large metamorphic terranes, regions characterized by distinctive metamorphic rocks and intensity of metamorphism that may vary laterally. Regional metamorphism occurs because both pressure and temperature increase with depth in Earth (Figure 8.3).

The photos below show two outcrops of regional metamorphic rocks. The schist outcrop on the left (Figure 8.4) is in Vermont’s Green Mountains.

The gneiss seen in outcrop on the right (Figure 8.5) is much older. it is from Precambrian terrane 750 km northwest of the Green Mountains, near Sudbury, Ontario.

8.5 Outcrop of gneiss near Sudbury, Ontario. Mountain building brings rocks from deep in Earth to the surface.

Other examples are found in Precambrian shields, relatively flat-lying areas that may be thousands of kilometers across, that are the exposed roots of ancient mountains. The gneiss seen in Figure 8.5 is from the Canadian Shield in central Ontario.

This occurs when magma that intrudes the crust rises close to, or all the way to, the surface. In such cases, heat from the magma can cause contact metamorphism that affects shallow or surface rocks.

As seen in Figure 8.6, contact metamorphism leads to the development of metamorphic zones called contact aureoles, or skarns, that wrap around an intrusion. Aureoles may be anywhere from a few centimeters to many kilometers thick.

The width of an aureole mainly depends on the size of the intrusion and how much fluid (mostly H2O and CO2) it gives off. Aureoles often develop concentric zones or layers, each containing distinct metamorphic minerals and mineral assemblages that reflect the maximum metamorphic temperature attained and the amount of metasomatism.

Occasionally metamorphism occurs without significant tectonism or magmatism. For example, metamorphism called hydrothermal metamorphism may occur because of hot water flowing through rock in areas next to hot springs or other geothermal areas.

These changes, mostly chemical in nature, can occur without significant increases in temperature and pressure. Typically, however, hydrothermal metamorphism is associated with regional or contact metamorphism.

This uncommon form of metamorphism, occurs because of shearing and deformation associated with faults and fault zones where rocks move past each other. The metamorphism produces fractured and granulated rocks that contain elongated mineral grains.

Figure 8.7 shows an outcrop where dynamic metamorphism has occurred along a meter-wide fault zone. Impact metamorphism, also called shock metamorphism, is related to dynamic metamorphism because it also involves physical changes involving crushing and deformation.

The products may include high-pressure metamorphic minerals such as coesite or stishovite, both polymorphs of quartz. Highly granulated, deformed, and shattered rocks are common, and sometimes intriguing structures called shatter cones develop.

Shatter cones are akin to the damage that a pebble does when it strikes the windshield of your car. Regional and contact metamorphism account for most metamorphic rocks.

Some geologists have also described another kind of metamorphism, called burial metamorphism, but it is really just high-temperature diagenesis. Metamorphic grade is a general term we use to describe the temperature at which metamorphism occurs.

Rocks metamorphosed at low temperature may change only very slowly, and some changes may not go to completion. Rocks that form at high temperatures generally do not have the same problems.

Low-grade metamorphic rocks form at low temperatures, generally between 150 and 450 °C. They mostly form at low pressures, too.

Low-grade metamorphic rocks are often fine grained. Because they are hard to study and frequently do not represent chemical equilibrium, many metamorphic petrologists prefer to study higher-grade rocks.

It contains serpentine and chlorite, both hydrous minerals, that formed during metamorphism of a mafic protolith. Medium-grade metamorphism, forming at temperatures between 400 and about 600 °C, often produces rocks containing conspicuous metamorphic minerals we can easily see and study.

High-grade metamorphic rocks, which form at temperatures greater than about 600 °C, are usually quite coarse-grained and contain minerals easily identified in hand specimen. Most form at high pressures.

Depending on its composition, a high-grade metamorphic rock may undergo partial melting, also called anatexis, so both metamorphic and igneous processes contribute to its evolution. When this happens, the rock, strictly speaking, is no longer a metamorphic rock.

Other kinds of rocks, especially those that contain little H2O, may remain completely solid to temperatures as great as 1100 °C. Prograde metamorphism occurs when low-grade or unmetamorphosed rocks change mineralogy or texture in response to a temperature increase.

During progressive metamorphism, a series of reactions occur as the degree of metamorphism increases. Rock mineralogy changes multiple times before equilibrating at the highest temperature conditions.

For example, many metamorphic rocks are deep in Earth where pressure and temperature are great. They were never unmetamorphosed rocks at low pressure and temperature.

Still other rocks may only partially equilibrate during metamorphism. Some metamorphic rocks form by retrograde reactions (metamorphism causing high-temperature rocks to change into low-temperature rocks).

Upon uplift and cooling, retrograde metamorphism may replace original high-temperature mineral assemblages with low-grade minerals. Figure 8.11 compares paths of prograde and retrograde metamorphism.

The Laws of Thermodynamics say that rocks will change mineralogy in response to increasing temperature (prograde metamorphism), so why don’t they undergo opposite (retrograde metamorphism) changes when temperature decreases as the rock reaches Earth’s surface. If rocks always went to equilibrium, we should have no samples of high-grade rocks or minerals to study.

For example, we have samples of diamond-bearing kimberlite, like the specimen seen in this photo (Figure 8.12), that are unstable and should break down at Earth’s surface. Several considerations help answer these questions:

Metamorphic rock formations hold a mysterious allure, captivating the imagination with their stunning beauty and intriguing origins. These formations, created through the process of metamorphism, offer a glimpse into the forces that have shaped our planet over millions of years.

Metamorphism is the geological process by which rocks undergo profound changes in their mineralogy, texture, and structure due to high temperatures, intense pressure, or the presence of chemically active fluids. It is a slow and transformative process that occurs deep within the Earth’s crust, where rocks are subjected to immense heat and pressure over long periods of time.

Metamorphic rocks are formed through a combination of heat, pressure, and geological activity. The process begins when rocks are buried deep within the Earth’s crust, where they experience increasing temperatures and pressure due to the weight of overlying rocks.

As the rocks are subjected to further compression, they become denser and more compact, resulting in the formation of metamorphic rocks.

The Grand Canyon is renowned for its awe-inspiring beauty and is home to several metamorphic rock formations. The Vishnu Schist, a prominent formation in the canyon, is a metamorphic rock that originated from ancient sediments and has been transformed by intense heat and pressure.

Nestled in the General Carrera Lake, the Marble Caves are a breathtaking example of metamorphic rock formations. Carved by the erosive action of the lake’s waters, these caves showcase the beauty of marble, a metamorphic rock formed from limestone.

The Meteora is an extraordinary complex of towering rock formations, perched high above the plain of Thessaly. These rocks are primarily composed of metamorphic rocks such as schist and gneiss, which have been shaped by the forces of weathering and erosion over millions of years.

Each famous metamorphic rock formation possesses its own unique beauty, shaped by the specific geological processes that occurred over time. The Grand Canyon’s Vishnu Schist, for example, displays a stunning array of colors ranging from deep reds to vibrant yellows, showcasing the transformative power of metamorphism.

Meanwhile, the Meteora’s towering rock formations, with their sheer cliffs and jagged peaks, evoke a sense of grandeur and majesty.

Famous metamorphic rock formations hold immense geological significance, providing valuable insights into the Earth’s history and the processes that have shaped its surface. By studying these formations, geologists can unravel the complex interplay between tectonic forces, heat, and pressure that drive metamorphism.

Visiting and exploring famous metamorphic rock formations is an enriching experience that allows us to witness the wonders of nature up close. Many of these formations have designated visitor centers and guided tours, providing educational resources and insights into their geological significance.

By respecting the environment and practicing responsible tourism, we can continue to appreciate the magnificence of these rock formations.

Famous metamorphic rock formations often have a profound impact on local communities, both economically and culturally. These formations attract tourists from around the world, boosting the local economy through increased tourism revenue.

Many communities have embraced their metamorphic rock formations as symbols of their heritage, incorporating them into folklore, art, and local traditions.

Preserving famous metamorphic rock formations is crucial for maintaining their beauty and ecological integrity. Several conservation efforts are underway to protect these natural wonders from human activities and environmental degradation.

Additionally, sustainable tourism practices and responsible visitor management are essential to minimize the impact of human activities on these delicate ecosystems.

Famous metamorphic rock formations offer a glimpse into the immense power and beauty of the Earth’s geological processes. From the awe-inspiring Grand Canyon to the ethereal Marble Caves and the majestic Meteora, these formations captivate our imagination and remind us of the vastness and diversity of our planet.

On the eve of the New Year-2021, World of Stones USA is presenting a unique gift to its readers, fans, and customers. Those are sightseeing places where nature lovers like to roam during the Holiday Season-2020-21.

Of course, we have infused some avid knowledge regarding natural stones, types, and formation patterns in concise ways. Let’s dive into the world of stones.

[The Wave, Coyote Buttes, northern Arizona, Utah, USA – By Francesco Conti.]. According to Wikipedia, “The Coyote Buttes area is an exposure of cross-bedded aeolian Jurassic Navajo Sandstone.

Coyote Buttes is a section of the Paria Canyon-Vermilion Cliffs Wilderness. It is managed by the Bureau of Land Management (BLM).

It is located at the south of US 89 halfway between Kanab, Utah and Page, Arizona.

Wikipedia described Petra as the historic and archaeological city around Jabal-Al-Madbah in a basin surrounded by mountains.

[Wadi Rum is a protected area covering 720 square kilometers of dramatic desert wilderness in the south of Jordan By Supapisit Charoenngam]. Wadi Rum is known as the valley of the moon.

[Amazing Rock Formations on the Cote Granit Rose in Brittany, France By Karl Allen Lugmayer.]. Here, pink granite stones are found abundantly.

Thus, it becomes an attraction for tourists across the globe.

The area is made up of two salt lakes surrounded by mountains and dunes. Erosion by winds has created huge rock formations.

[Warm tone and soft light, soft edge, sunset at Delicate arch, Archesh National Park, Utah, the USA By Wisanu Boonrawd.]. Arches National Park, Utah consists of more than 2,000 arches.

Delicate Arch is one of the pieces of evidence.

By Victor Maschek.]. The formation of black basalt stone evident on the seashore where black sand also adding flairs in the scene.

[Spheric Moeraki Boulders on the Koekohe Beach, Eastern coast of New Zealand. Long time exposure – see is blurred By Filip Fuxa.].

These are Mudstone boulders and formed with the erosion of rocks by sea waves. They were found to consist of mud, clay, and fine silts clasts cemented by calcite.

[Rock Formation in Joshua Tree National park – Jumbo Rocks By awphoto.]. Joshua tree national park is located in southeastern California, east of Los Angeles and San Bernardino, near Palm Springs.

Groundwater has eroded the edge, joints of monzonite, and cut the stones into symmetrical shapes.

It is on an unhabituated island. It is made of Basal hexagonal rocks jointed in columns.

The rock formation started into a blocky tetragonal pattern and resulted in hexagonal columns of Basalts. The fracture of rocks is taken place in a perpendicular orientation to cooling surfaces.

It is a closeup of orange granite rock formations at Bunker Bay, Western Australia. It is a rich scenic spot with beautiful pure white sand, limestone cliff, and low granite headland.

[Beautiful rocky formations of Matopos National Park, Zimbabwe By Vladislav T. Jirousek.].

Weathering has formed smooth whale-back shapes that look like a balancing rock. Granite is made up of Matopos Batholith.

[Scenic view of the Romanian mountains with various rock formations By Dragosh Co.]. Indeed, great scenic view of the Carpathian mountain in Romania.

The entire mountain belt is rich with iron, gold, and silver. It is the reason Trajan brought 165 tons Gold 330 tons of silver by winning in the conquest of Dacia.

[Hvitserkur, giant rock with the shape of a dinosaur at Hunafjoraur, taken at the blue hour with a long exposure time By Menno Schaefer.]. Hvitserkur, a giant rock is a monolith dyke made from erosion by the sea.

It is 15-meters-high but only 2 meters thick. So, local have fixed it with cement-concrete base in 1955.

[The hexagonal rocks of the Giant’s Causeway By SAPhotog.]. The hexagonal rocks of the Giant’s Causeway are a story of volcanic rock formation before 50 to 60 million years.

Once the lava settles on the earth’s surface, it gets cooling rapidly due to cool and moist air contact. Cooling brings molecular changes in the mass of rock and shrinking brings cracks in it.

[The valley is near St Just has the Brison rocks not far offshore. By Andy Fox Photography.].

It is a fine example of a glacial valley. Its rock formations are smooth and rounded.

[limestone pinnacles formation at Gunung Mulu national park Borneo Malaysia. By Juhku.].

It is a mountainous area that consists of metamorphosed sedimentary rocks, chiefly limestone, sandstone, and shells.

Weathering has formed pinnacles in the mountain.

[Ariel View of rock formations in Albany Western Australia By Bruce Retriever.]. It has a notorious coastline due to death waves.

Gneiss also present there. They are some 1300-1600 million years old.

[Colorful rock in Quebrada de Humauaca, Argentina by thoron.]. Quebrada de Humauaca, Argentina has spectacular rock formations.

The spectrum of colors also reflects in the sand that is made from the rock erosions. The Mudrock layer seems sandwiched between the silt layers.

[Bungle Bungles rock formation in Purnululu National Park, Australia by iacomino FRiMAGES.]. According to UNESCO World Heritage, it is the superlative example of beehive-shaped karst sandstone rising 250 meters above the surrounding semi-arid savannah grasslands.

Unique depositional processes and weathering have given these towers their spectacular black and orange banded appearance, formed by biological processes of cyanobacteria (single-cell photosynthetic organisms) which serve to stabilize and protect the ancient sandstone formations.

[Akakus (Acacus) Mountains, Sahara, Libya – Bizarre sandstone rock formations By Patrick Poendl]. It is a mountain range in the Sahara Desert famous for its pictogram art on the stones.

[Spitzkoppe, Damaraland, Namib Desert, Namibia By CherylRamalho.]. The site depicts the dramatic granite rock formation.

Moreover, golden sunlight at dawn creating shadows and natural illumination in the scene. The age of rocks is believed to be 120 million years.

[Great Ocean Road national park in Victoria, Australia – By Taras Vyshnya.]. The Great Ocean Road traverses through cliffs compose of the strata of sandstone and limestone rock formation.

[Unique geological formations in Cappadocia, Turkey. By DenisProduction.com.].

[Stunning rock formation in Tsingy de Bemaraha, Madagascar]. The scene is of the forest of needles.

The erosion has created a dramatic steep superimpose of limestone. It has an awesome yet dangerous bio-ecosystem.

[The layers of blue sedimentary chert, Rainbow Rock, Oregon By Weldon Schloneger]. The blue rainbow rocks are chert, which are hard sedimentary rocks made of sand and calcium compounds.

Thereby, it contains fossils of the sea bed.

[Aerial view of Göreme National Park, Tarihi Milli Parki, Turkey by Naeblys.]. You will find the typical rock formations of Cappadocia with fairy chimneys and desert landscapes.

Metamorphism can create new rocks form from pre-existing rocks based on the pressure, temperature, and the fluid activity in the rock. Metamorphic rocks are formed from some original rock type (igneous, metamorphic, or sedimentary) and are referred to as the parent rock (protolith).

Depending on the degree or intensity of metamorphism (metamorphic grade), various elements will be rearranged and new minerals will form. The composition of the original rock and the intensity of the metamorphism determine how much the rock will change.

Increasing temperature will progressively recrystallize the minerals in rocks, and makes them harder and denser than the original rock, but all changes during metamorphism occur while the rock is solid.

Figure 7-2. Paleoproterozoic metamorphic rocks of the Grand Canyon.

Gneissic rocks along Bright Angel Creek (right). There are different types of metamorphic environments depending on the METAMORPHIC GRADE.

Diagenesis. > Very low temperature and pressure.

burial metamorphism).

> Low to high pressures and temperatures. > Directed pressure causes the platy minerals to RE-ALIGN in a more parallel orientation, forming a layered or foliated metamorphic rock texture.

Contact metamorphism. > High temperatures and low pressures.

> High-temperature mineral RECRYSTALLIZATION occurs. > Occurs locally near igneous intrusions.

The size of the aureole depend on the volume of the heat-generating intrusion, but the effects are typically more localized than regional metamorphism. > Magma-driven hydrothermal fluid activity is commonly associated with this type of metamorphism (contact metamorphism is typically gradational with hydrothermal metamorphism).

Blueschist metamorphism. > High pressures and low temperatures.

> Occurs during rapid tectonic burial at subduction zones.

> Localized high pressures and temperatures related to fault zones and shear zones. > Brittle deformation (crushing, grinding) includes cataclastic metamorphism that produces fault gouge and fault breccia.

Impact metamorphism. > Related to the localized, but very high pressures associated with meteorite/comet impacts.

At the introductory level, metamorphic rocks are probably the easiest to recognize and identify. This is because there are only two major textural types and the rocks are fairly distinctive.

We’ll keep it simple for now. There are two basic types of metamorphic rock texture, foliated and non-foliated (see Figure 7-3).

Figure 7-3. Foliated and nonfoliated metamorphic rocks and textures.

Non-foliated Mazatzal Quartzite in Barnhardt Canyon in the Mazatzal Mountains (right).

Foliated metamorphic textures. > In layered or foliated metamorphic rocks, the platy minerals are aligned in a parallel orientation.

> The specific type of foliation (slaty cleavage, schistosity, gneissic banding) can also be related to the intensity of metamorphism. > Foliated rocks are primarily distinguished by their specific type of foliation.

IMPORTANT NOTE – Some rocks lacking abundant amounts of platy minerals will not form a well-developed foliation, even if they are subjected to intense pressure.

> In massive or non-foliated metamorphic rocks, there is no preferred orientation of grains or crystals. > Clast boundaries are recrystallized, making the rock more resistant.

> Non-foliated rocks are primarily identified by their composition (mineral content). For example, test to see if the rock reacts with HCl (it will fizz if made of calcite), or scratch the rock to check its hardness (quartz is hard, H=7.

> Because they lack layering, nonfoliated textures are typically associated with contact metamorphism.

Foliated Rocks.

> A foliated, very fine-grained metamorphic rock. > The platy minerals (micas) are too small to be seen without the aid of a microscope.

> Color is highly variable (black, greenish gray, purple, red, etc.). > Metamorphic grade – Low / low temperatures and high directed pressures (regional metamorphism).

> Arizona examples – Breadpan Formation (central Arizona). Texas Gulch Formation (central Arizona).

Phyllite. > A foliated, fine-grained metamorphic rock.

> Metamorphic grade – Low-moderate / moderate-high temperatures and directed pressures (regional metamorphism). > Protoliths – Derived primarily from mudstone.

Schist. > A foliated, coarsely crystalline metamorphic rock.

> Platy minerals (like muscovite and biotite) commonly predominate, which may give schist an almost “fish scale” appearance. > Breaks along irregular, sub-planar surfaces.

> Protoliths – Derived from a several possible protoliths (like mudstone or assorted volcanics). >> If derived from mudstone, the schist may contain typical metamorphic minerals such as garnet, staurolite, or kyanite.

>> If derived from basalt, green or black amphibole is the common mineral and may be termed a greenschist or amphibolite. > Arizona examples – Pinal Schist (central and southern Arizona), Vishnu and Brahma Schists (Grand Canyon).

Gneiss. > A foliated, coarsely crystalline, distinctly banded metamorphic rock.

> The light-colored minerals typically include quartz and feldspar, while the dark minerals are generally biotite and/or amphibole. > Metamorphic grade – High / high temperatures and directed pressures (regional metamorphism).

Typically termed “granitic gneiss” if derived from granitic rocks. > Arizona examples – Catalina Gneiss (in Sabino Canyon near Tucson), Estrella Gneiss (western part of South Mountain in Phoenix), Brahma Gneiss (Grand Canyon).

Nonfoliated Rocks.

> A non-foliated, crystalline metamorphic rock. > Composed mostly of recrystallized calcite or dolomite (carbonate minerals).

> Metamorphic grade – Low to high / moderate to high temperatures and low to high directed pressures (contact / regional metamorphism). > Protoliths – Derived from carbonate rock (limestone or dolostone).

Quartzite. > A non-foliated, crystalline metamorphic rock.

> In many quartzites, the sedimentary features (bedding, cross beds or ripple marks) from the sandstone are still preserved. > Recrystallized silica grains create a very hard rock that does not react with HCl acid.

> Protoliths – Derived primarily from sandstone under conditions of high temperature and low-to-high directed pressure (contact / regional metamorphism). > Arizona examples – Mazatzal Quartzite (central Arizona), Dripping Springs Quartzite (central Arizona), Bolsa Quartzite (southeastern Arizona).

Other Metamorphic Rock Types. There are many other varieties of metamorphic rocks.

Metabasalt/ Greenstone / Amphibolite. > Va.

Schist (/ˈʃɪst/ SHIST) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates.

These are often interleaved with more granular minerals, such as feldspar or quartz.

Schist can form from many different kinds of rocks, including sedimentary rocks such as mudstones and igneous rocks such as tuffs. Schist metamorphosed from mudstone is particularly common and is often very rich in mica (a mica schist).

Otherwise, the names of the constituent minerals will be included in the rock name, such as quartz-felspar-biotite schist.

The word schist is derived ultimately from the Greek word σχίζειν (schízein), meaning “to split”, which refers to the ease with which schists can be split along the plane in which the platy minerals lie.

Geologists define schist as medium-grained metamorphic rock that shows well-developed schistosity. Schistosity is a thin layering of the rock produced by metamorphism (a foliation) that permits the rock to easily be split into flakes or slabs less than 5 to 10 millimeters (0.2 to 0.4 in) thick.

Typically, over half the mineral grains in a schist show a preferred orientation. Schists make up one of the three divisions of metamorphic rock by texture, with the other two divisions being gneiss, which has poorly developed schistosity and thicker layering, and granofels, which has no discernible schistosity.

Schists are defined by their texture without reference to their composition, and while most are a result of medium-grade metamorphism, they can vary greatly in mineral makeup. However, schistosity normally develops only when the rock contains abundant platy minerals, such as mica or chlorite.

The ease with which the rock splits along the aligned grains accounts for the schistosity. Though not a defining characteristic, schists very often contain porphyroblasts (individual crystals of unusual size) of distinctive minerals, such as garnet, staurolite, kyanite, sillimanite, or cordierite.

Because schists are a very large class of metamorphic rock, geologists will formally describe a rock as a schist only when the original type of the rock prior to metamorphism (the protolith) is unknown and its mineral content is not yet determined.

Mineral qualifiers are important when naming a schist. For example, a quartz-feldspar-biotite schist is a schist of uncertain protolith that contains biotite mica, feldspar, and quartz in order of apparent decreasing abundance.

Lineated schist has a strong linear fabric in a rock which otherwise has well-developed schistosity.

Nonhydrostatic stress is characteristic of regional metamorphism where mountain building is taking place (an orogenic belt). The schistosity develops perpendicular to the direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers.

At the microscopic level, schistosity is divided into internal schistosity, in which inclusions within porphyroblasts take a preferred orientation, and external schistosity, which is the orientation of grains in the surrounding medium-grained rock.

For example, the clay minerals in mudstone are metamorphosed to mica, producing a mica schist. Early stages of metamorphism convert mudstone to a very fine-grained metamorphic rock called slate, which with further metamorphism becomes fine-grained phyllite.

If the metamorphism proceeds further, the mica schist experiences dehydration reactions that convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into a gneiss.

Chlorite schist is typically formed by metamorphism of ultramafic igneous rocks, as is talc schist. Talc schist also forms from metamorphosis of talc-bearing carbonate rocks formed by hydrothermal alteration.

This may be of algal origin.

In geotechnical engineering a schistosity plane often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, tunnel, foundation, or slope construction. A hazard may exist even in undisturbed terrain.

This caused a massive landslide that killed 26 people camping in the area.

The outcome of metamorphism depends on pressure, temperature, and the abundance of fluid involved, and there are many settings with unique combinations of these factors. Some types of metamorphism are characteristic of specific plate tectonic settings, but others are not.

As metamorphic processes go, burial metamorphism takes place at relatively low temperatures (up to ~300 °C) and pressures (100s of m depth). To the unaided eye, metamorphic changes may not be apparent at all.

The metaconglomerate formed through burial metamorphism does not display any of the foliation that has developed in the metaconglomerate in Figure 10.10. Figure 10.25: Metaconglomerate formed through burial metamorphism.

John (2014) CC BY 2.0 view source. Names given to rocks that are sold as building materials, especially for countertops, may not reflect the actual rock type.

While these terms might not provide accurate information about the rock type, they generally do distinguish natural rock from synthetic materials. An example of a synthetic material is the one referred to as quartz, which includes ground-up quartz crystals as well as resin.

Regional metamorphism refers to large-scale metamorphism, such as what happens to continental crust along convergent tectonic margins (where plates collide). The collisions result in the formation of long mountain ranges, like those along the western coast of North America.

The deeper rocks are within the stack, the higher the pressures and temperatures, and the higher the grade of metamorphism that occurs. Rocks that form from regional metamorphism are likely to be foliated because of the strong directional pressure of converging plates.

Sedimentary rocks have been both thrust up to great heights—nearly 9 km above sea level—and also buried to great depths. Considering that the normal geothermal gradient (the rate of increase in temperature with depth) is around 30°C per kilometre in the crust, rock buried to 9 km below sea level in this situation could be close to 18 km below the surface of the ground, and it is reasonable to expect temperatures up to 500°C.

These rocks are all foliated because of the strong compressing force of the converging plates. Figure 10.26: Regional metamorphism beneath a mountain range resulting from continent-continent collision.

Dashed lines represent temperatures that would exist given a geothermal gradient of 30 ºC/km. A sequence of foliated metamorphic rocks of increasing metamorphic grade forms at increasing depths within the mountains.

At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 10.27). Water within the crust is forced to rise in the area close to the source of volcanic heat, drawing in more water from further away.

Figure 10.27: Seafloor (hydrothermal) metamorphism of ocean crustal rock on either side of a spreading ridge. Source: Karla Panchuk (2018) CC BY 4.0, modified after Steven Earle (2015) CC BY 4.0 view source.

Chlorite and serpentine are both hydrated minerals, containing water in the form of OH in their crystal structures. When metamorphosed ocean crust is later subducted, the chlorite and serpentine are converted into new non-hydrous minerals (e.g., garnet and pyroxene) and the water that is released migrates into the overlying mantle, where it contributes to melting.

Figure 10.28: Greenstone from the metamorphism of seafloor basalt that took place 2.7 billion years ago. The sample is from the Upper Peninsula of Michigan, USA.

John (2012) CC BY 2.0 view source. At subduction zones, where ocean lithosphere is forced down into the hot mantle, there is a unique combination of relatively low temperatures and very high pressures.

The lower temperatures exist because even though the mantle is very hot, ocean lithosphere is relatively cool, and a poor conductor of heat. That means it will take a long time to heat up, can be several hundreds of degrees cooler than the surrounding mantle.

Figure 10.29: Regional metamorphism of oceanic crust at a subduction zone occurs at high pressure but relatively low temperatures. Source: Steven Earle (2015) CC BY 4.0 view source.

Glaucophane is blue, and the major component of a rock known as blueschist. If you have never seen or even heard of blueschist, that not surprising.

Most of the blueschist that forms in subduction zones continues to be subducted. It turns into eclogite at about 35 km depth, and then eventually sinks deep into the mantle, never to be seen again.

One such place is the area around San Francisco. The blueschist at this location is part of a set of rocks known as the Franciscan Complex (Figure 10.30).

The blue colour of the rock is due to the presence of the amphibole mineral glaucophane. Source: Steven Earle (2015) CC BY 4.0 view source.

Heat is important in contact metamorphism, but pressure is not a key factor, so contact metamorphism produces non-foliated metamorphic rocks such as hornfels, marble, and quartzite. Any type of magma body can lead to contact metamorphism, from a thin dyke to a large stock.

A large intrusion will contain more thermal energy and will cool much more slowly than a small one, and therefore will provide a longer time and more heat for metamorphism. This will allow the heat to extend farther into the country rock, creating a larger aureole.

Thus, aureoles that form around “wet” intrusions tend to be larger than those forming around their dry counterparts. Figure 10.31: Schematic cross-section of the middle and upper crust showing two magma bodies.

The lower body is surrounded by rock that is already hot (and probably already metamorphosed), and so it does not have a significant metamorphic aureole. Source: Steven Earle (2015) CC BY 4.0 view source.

Contact metamorphism can take place over a wide range of temperatures—from around 300 °C to over 800 °C. Different minerals will form depending on the exact temperature and the nature of the country rock.

When rocks are heated up or put under a lot of pressure, they can change drastically. This is because the minerals that make up the rocks form only at certain temperatures and pressures.

Graphite and diamond are two minerals that are both made entirely out of carbon. If we put graphite under a huge amount of pressure, the carbon atoms will be squeezed together and will rearrange themselves into the more compact crystal structure of diamonds.

Everything remains solid while the metamorphism occurs. We know from experiments that certain minerals form only at very specific temperatures and pressures.

Something must have happened to bring the rock up to the surface where we could find it. If we find a rock that formed at high temperatures (such as marble), we know that the rock must have been heated up.

We often find metamorphic rocks in mountain ranges where high pressures squeezed the rocks together and they piled up to form ranges such as the Himalayas, Alps, and the Rocky Mountains. Metamorphic rocks are forming deep in the core of these mountain ranges.

A good example of where this has happened is the Appalachian Mountains in the Eastern United States. The Appalachian Mountains used to be a very large mountain range.

Gneiss is a foliated metamorphic rock that is characterized by its banded appearance. The bands are composed of different minerals, which are aligned in parallel layers.

Gneiss pronounced “nais,” (/naɪs/ NICE) is a high-grade metamorphic rock characterized by its gneissic banding and coarse-grained texture. Its genesis is attributed to intense regional metamorphism, where pre-existing igneous or sedimentary rocks are subjected to elevated pressures (>2 kbar) and high temperatures (>600°C).

The metamorphism causes the original rock to recrystallize, and the new minerals form in bands that are parallel to the direction of stress. Gneiss is typically formed at high temperatures and pressures, and it is often found in regions that have experienced tectonic activity.

Gneiss’s key features:.

The composition of gneiss can vary widely depending on the original rock it was formed from and the intensity of the metamorphic process it underwent.

Main minerals: Other minerals:

These might include zircon, apatite, magnetite, and more.

Gneiss is formed by the process of metamorphism. Metamorphism is the transformation of rocks under high temperatures and pressures.

The new minerals are typically larger and more ordered than the original minerals. 1.

Mineral Transformations: Gneiss formation involves the breakdown and recrystallization of minerals in the protolith.

Fluid-Rock Interaction: Metamorphic fluids like water and carbon dioxide play a crucial role in gneiss formation.

Deformation and Foliation: During metamorphism, the protolith often experiences deformation, such as shearing or folding.

There are many different types of gneiss, depending on the composition of the original rock and the conditions of metamorphism.

Granitic Gneiss: This type is rich in quartz and feldspar, giving it a light-colored appearance similar to granite. Micaceous Gneiss: Characterized by a high concentration of mica minerals like biotite or muscovite, resulting in a sparkly or flaky texture.

Orthogneiss: Formed from the metamorphism of igneous rocks, like granite or diorite. These gneisses often retain some of the original igneous textures, such as large crystals (phenocrysts).

They typically have finer-grained minerals and may show remnants of sedimentary layering, like bedding planes, disrupted by the metamorphic process. Banded Gneiss: Exhibits distinct, alternating light and dark bands.

Augen Gneiss: Contains oval-shaped feldspar crystals (augens) within the bands. Migmatite: This type of gneiss is a mixture of igneous and metamorphic rocks.

The melt then flows through the rock, and it cools and recrystallizes to form new minerals. The banding in gneiss is caused by the segregation of different minerals during metamorphism.

As the rock is heated, the denser minerals sink to the bottom of the rock, while the less dense minerals float to the top. This process of segregation leads to the formation of bands of different minerals.

KEY CONCEPTS. Metamorphic rocks form when heat, pressure, or chemically reactive fluids cause changes in preexisting rocks (Figure 9.1).

The changes that occur may involve changes in rock texture, in the minerals present, and in overall rock composition. So, metamorphic rocks, and the processes that create them, are key parts of the rock cycle that also includes igneous and sedimentary rocks and processes.

In Chapter 8 we introduced the Laws of Thermodynamics. In that chapter we were considering mostly melting reactions, but below the solidus, where melt is absent and metamorphism occurs, the principles of thermodynamics also apply.

When energy is minimized, the system is at chemical equilibrium and the phases present are the stable phases. Rocks are chemical systems defined by their compositions, and they follow the rules of thermodynamics like any other kind of system.

If the pressure or temperature change, a different assemblage and different compositions may become stable, and chemical reactions may occur to regain stability. Changes in the minerals present, or changes in their compositions are kinds of chemical processes called phase changes.

Such changes occur to minimize physical energy – just as phase changes occur to minimize chemical energy. Rocks may develop textures, called metamorphic fabrics.

For example, Figure 9.2 is a photo of a gneiss with a foliation (a striped or layered appearance) due to parallel layers of different compositions. And, within each layer, black biotite crystals are aligned parallel to the foliation.

Overall, the changes that occur during metamorphism vary depending on rock composition and metamorphic conditions. Sometimes chemical changes dominate, and sometimes physical changes dominate.

Although metamorphic changes can occur in response to either heat, pressure, or reactive fluids, petrologists often think of temperature as most important. Part of this perspective is because heat often promotes metamorphic reactions more than pressure or fluids do.

So, petrologists commonly talk about metamorphic grade, which is a relative scale of metamorphic intensity based on temperatures of metamorphism. Low-grade rocks -rocks metamorphosed at low temperature – may change only very slowly, and some changes may not go to equilibrium.

However, there are many kinds of metamorphic rocks, and some of them are more chemically reactive than others. Low-grade metamorphic rocks typically form at temperatures between 200 and 450 °C.

Most low-grade metamorphism involves minerals in a protolith reacting with water to produce hydrous minerals such as chlorite. The photo in Figure 9.3 is an example of a low-grade mafic metamorphic rock, a gold-bearing greenschist from the Homestake Mine in South Dakota.

Low-grade rocks can be difficult to study due to fine grain sizes, and the minerals in them frequently do not represent chemical equilibrium. For these reasons, many metamorphic petrologists prefer to study higher-grade rocks.

Many schists are medium-grade rocks. High-grade metamorphic rocks, which normally form at temperatures greater than about 600 °C, are usually quite coarse-grained and contain minerals easily identified in hand specimens.

Many high-grade rocks develop gneissic textures, like the texture seen in Figure 9.1. Others, like the high-grade garnet granulite in Figure 9.4, lack foliation.

Metamorphism occurs over a wide range of pressure and temperature. Most rocks will begin to show metamorphic changes when heated to 150-200 oC, and some can be heated to temperatures greater than 900 or 1,000 oC before they begin to melt and enter the realm of igneous rocks.

During regional metamorphism, high temperatures and thus high-grade rocks, are associated with high pressures. During contact, or other kinds of metamorphism, this may not be the case.

For the most part, these rocks equilibrated at pressures less than 10 kbar (Figure 9.5). Rocks called blueschist or eclogite form at higher pressures.

Some mantle xenoliths, carried to the surface from deep in Earth, equilibrated at pressures greater than 100 kbar – off scale in Figure 9.5. But mantle xenoliths contain rocks that formed at high pressure and temperature, not metamorphic rocks that started out at low pressure and temperature and subsequently became high-grade rocks during metamorphism.

But rare rocks, such as some from Antarctica’s Napier Complex, equilibrated at more than 1,000 oC. To reach such temperatures without melting requires that the rocks were exceptionally dry during metamorphism, meaning they contained almost no H2O or CO2.

Often, they weather to produce clays, zeolites, or other minerals that are more stable. And after a sedimentary rock forms, diagenetic reactions may take place that produce new minerals.

But weathering and diagenesis are considered low-temperature sedimentary processes. Petrologists reserve the term metamorphism for changes that occur between conditions of weathering/diagenesis and melting.

Adding more complications – many minerals created by alteration or diagenesis also form during low-temperature metamorphism. Sometimes we identify metamorphic rocks by their fabrics, but metamorphic fabrics develop slowly and incipiently at low temperatures and may be difficult to distinguish from sedimentary textures.

Depending on its composition, with more heating, a high-grade metamorphic rock may undergo partial melting, also called anatexis, so both metamorphic and igneous processes contribute to its evolution. When this happens, the rock becomes a kind of rock that, strictly speaking, is no longer a metamorphic rock.

So the upper temperature of metamorphism varies with rock composition. Some high-temperature rocks, called migmatites, develop textures that appear to be part way between metamorphism and melting.

The name migmatite comes a Greek word for mixed rock. Heat is thermal energy that can move (flow) from one place to another or from one substance – such as rock, magma, or water – to another.

Three processes can transfer heat: conduction, convection, and radiation. but within Earth, heat transfer by radiation is insignificant (Figure 9.8).

Thus, for example, heat is always flowing from Earth’s hot interior to the cooler surface by conduction. And if a (hot) magma intrudes the (cooler) crust, the magma will cool as heat is conducted grain-by-grain into the surrounding rock, causing the rock to warm.

Convective heat transfer is the transfer of heat due to the flow of material, such as the flow of water, of hot magma and, occasionally rock. It is more efficient than conductive heat transfer.

Rocks are formed on Earth as igneous, sedimentary, or metamorphic rocks. Igneous rocks form when rocks are heated to the melting point which forms magma.

Metamorphic rocks form from heat and pressure changing the original or parent rock into a completely new rock. The parent rock can be either sedimentary, igneous, or even another metamorphic rock.

The diagram above shows you how the rocks on Earth have been changed continually over time from one rock type to another. This changing of rock types is called the “Rock Cycle”.

Solid rock can be changed into a new rock by stresses that cause an increase in heat and pressure. There are 3 main agents that cause metamorphism.

Temperature increases can be caused by layers of sediments being buried deeper and deeper under the surface of the Earth. As we descend into the earth the temperature increases about 25 degrees Celsius for every kilometer that we descend.

The great weight of these layers also causes an increase in pressure, which in turn, causes an increase in temperature.

The descending of rock layers at subduction zones causes metamorphism in two ways. the shearing effect of the plates sliding past each other causes the rocks coming in contact with the descending rocks to change.

When rock melts it is then considered igneous not metamorphic, but the rock next to the melted rock can be changed by the heat and become a metamorphic rock. The diagram above shows you where metamorphic rock (YELLOW ZONE) can be produced at a subduction zone.

There are 3 factors that cause an increase in pressure which also causes the formation of metamorphic rocks. These factors are.

The huge weight of overlying layers of sediments.

Stresses caused by plates colliding in the process of mountain building.

Stresses caused by plates sliding past each other, such as the shearing stresses at the San Andreas fault zone in California.

Very hot fluids and vapors can, because of extreme pressures, fill the pores of existing rocks. These fluids and vapors can cause chemical reactions to take place, that over time, can change the chemical makeup of the parent rock.

Metamorphism can be instantaneous as in the shearing of rocks at plate boundaries or can take millions of years as in the slow cooling of magma buried deep under the surface of the Earth.

There are three ways that metamorphic rocks can form. The three types of metamorphism are Contact, Regional, and Dynamic metamorphism.

Contact Metamorphism occurs when magma comes in contact with an already existing body of rock. When this happens the existing rocks temperature rises and also becomes infiltrated with fluid from the magma.

Contact metamorphism produces non-foliated (rocks without any cleavage) rocks such as marble, quartzite, and hornfels.

In the diagram above magma has pushed its way into layers of limestone, quartz sandstone and shale. The heat generated by the magma chamber has changed these sedimentary rocks into the metamorphic rocks marble, quartzite, an hornfels.

Regional Metamorphism occurs over a much larger area. This metamorphism produces rocks such as gneiss and schist.

These rocks when exposed to the surface show the unbelievable pressure that cause the rocks to be bent and broken by the mountain building process. Regional metamorphism usually produces foliated rocks such as gneiss and schist.

Dynamic Metamorphism also occurs because of mountain-building. These huge forces of heat and pressure cause the rocks to be bent, folded, crushed, flattened, and sheared.

Metamorphic rocks are almost always harder than sedimentary rocks. They are generally as hard and sometimes harder than igneous rocks.

Many metamorphic rocks are found in mountainous regions today and are a good indicator that ancient mountains were present in areas that are now low hill or even flat plains. Metamorphic rocks are divided into two categories- Foliates and Non-foliates.

Foliates are composed of large amounts of micas and chlorites. These minerals have very distinct cleavage.

Slate, as an example, will split into thin sheets. Foliate comes from the Latin word that means sheets, as in the sheets of paper in a book.

Silt and clay can become deposited and compressed into the sedimentary rock shale. The layers of shale can become buried deeper and deeper by the process of deposition.

Because these layers are buried, temperatures and pressures become greater and greater until the shale is changed into slate. Slate is a fine-grained metamorphic rock with perfect cleavage that allows it to split into thin sheets.

Slate is produced by low grade metamorphism, which is caused by relatively low temperatures and pressures.

Slate has been used by man in a variety of ways over the years. One use for slate was in the making of headstones or grave markers.

The problem with slate though is its perfect cleavage. The slate headstones would crack and split along these cleavage planes as water would seep into the cracks and freeze which would lead to expansion.

Today headstones are made of a variety of rocks, with granite and marble being two of the most widely used rocks. Slate was also used for chalk boards.

Today it is not very advantageous to use this rock because of its weight and the splitting and cracking over time.

Schist is a medium grade metamorphic rock. This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock.

The individual grains of minerals can be seen by the naked eye. Many of the original minerals have been altered into flakes.

Schists are usually named by the main minerals that they are formed from. Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this.

Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to more heat and pressure than schist.

This banding has alternating layers that are composed of different minerals. The minerals that compose gneiss are the same as granite.

Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneouse rock grantite. Gneiss can be used by man as paving and building stone.

Non-Foliates are metamorphic rocks that have no cleavage at all. Quartzite and marble are two examples of non-foliates that we are going to study.

Quartzite is composed of sandstone that has been metamorphosed. Quartzite is much harder than the parent rock sandstone.

Quartzite looks similar to its parent rock. The best way to tell quartzite from sandstone is to break the rocks.

Marble is metamorphosed limestone or dolomite. Both limestone and dolomite have a large concentration of calcium carbonate (CaCO3).

Marble has many color variances due to the impurities present at formation. Some of the different colors of marble are white, red, black, mottled and banded, gray, pink, and green.

Marble is much harder than its parent rock. This allows it to take a polish which makes it a good material for use as a building material, making sink tops, bathtubs, and a carving stone for artists.

Marble is quarried in Vermont, Tennessee, Missouri, Georgia, and Alabama.

Write the answers to the following questions in complete sentences on a piece of paper.

Write a definition in your own words of what a metamorphic rock is.

What are the three agents of metamorphism.

What are the three types of metamorphism.

In your own words write a definition of the rock cycle.

The Himalayas, a vast mountain range including the highest peaks in the world, stretch approximately 1,500 miles across portions of India, Pakistan, Nepal, Bhutan, Afghanistan and China. Like all mountain ranges, the backbone of the Himalayas is comprised of rock layers.

In order to understand why certain rocks are found in the Himalayas, it helps to be familiar with the basics of the Himalaya’s geologic history. The Himalayas were produced by the motion of tectonic plates, which essentially brought India — which was once an island — crashing into Eurasia.

Geologists recognize six distinct rock zones in the Himalayas, separated by fault zones. Some zones are composed primarily of one rock classification, while others feature a more diverse mixture.

Igneous rocks form as a result of lava or magma cooling and solidifying. There are two main types of igneous rocks.

Two of the Himalayas’ major rock zones are comprised primarily of igneous plutonic rocks. Specific plutonic rock types in these zones include:.

As their name implies, sedimentary rocks form when loose sediments on the Earth’s surface become compressed and bonded together. Many of the rocks found in the Himalayas are sedimentary, and actually once laid on an ocean floor millions of years ago when India was an island.

Within some of the Himalayas’ sedimentary rocks, fossils of ancient plants and animals can be found. Metamorphic rocks are rocks whose composition has been changed by heat, pressure or chemical processes.

Additionally, metamorphosed forms of some sedimentary rocks occur in the region, such as quartzite, a metamorphosed type of sandstone. slate, a metamorphosed form of shale.

Some metamorphic rocks in the Himalayas have even been found to contain garnets.

Over the centuries, we have been seeing new and different phenomenon or creations on this planet. One such creation is the forming of rocks.

These formations are quite scenic and attract a lot of people for various purposes like hikes, treks, photography, picnics, and more.

There are also large landscapes that are home to just hundreds of such rock formations. The recent trend of exploring and travelling has increased the hype of these places.

Keeping in mind the different reasons for the rock formation, there are several types of rock formations in the world. If you’re new to this exploring, then keep this guide which will lead you to the most famous, natural or the closest rock formations which keen traveler must know about.

One of the top 10 rock formations in the world are Hoodoos. Hoodoos attract thousands of tourists owing to its unique shapes and sizes.

The rocks are made of sandstone and have been eroded over the years to form such shapes. In Hoodoos, you can spot rock formations as small as 5 feet and as large as 100 feet.

The area has also been declared as protected area due to the number of fossils which have been found there. Location: Ah-She-Sle-Pah Wilderness Area, North Mexico.

Image Source. If you are looking for unique natural rock formations in the world, Giant’s Causeway would top the list.

Nearly 40,000 columns are a part of this rock formation which is a result of volcanic eruptions. The columns are a result of collection and cooling of the lava.

Location: County Antrim, Northern Ireland. Image Source.

The desert area in Arizona is one of the several places where you can find one of the best natural rock formations in the world. Located very close to the Page town, Horseshoe Bend is a must visit.

The site is quite famous for rafting as it allows the tourists to have a complete view of the place. Depending on the time you visit Horseshoe Bend, you will see it in a different light.

Location: Page town, Arizona. Suggested Read: 10 Most Popular Resorts In Ireland: Epitomes Of Class, Elegance, And Stunning Views.

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Image Source. Another one of the wonders in Arizona is Wave Rock.

It is known widely to have a very magnificent yet confusing view. The place is open to just 20 tourists per day and is a great spot for hiking.

The rocks have various waves like patterns on them due to years of erosion and exposure to sun. You can get some great photographs clicked at this site.

Image Source. Devil’s Tower is a famous rock landmark in USA and is highly celebrated.

The rock is famous for its unique features and its high place in local traditions. The high tower of multiple scaling rocks is a big feature in USA.

The phenomenon behind the formation of this rock is unclear till date. Most scientists think that the rock has been formed by cooling of lava and exposure to the atmosphere.

Suggested Read: 24 Fabulous Places To Visit In Ireland: The Ultimate Irish Bucket List. Image Source.

Over the years, the Old Harry Rocks have lost a major part to erosion. The little stump visible only during low tides was known as “the wife”.

The rock formations are made of chalk and keep changing their shape each year. The continuous waves keep eroding the rocks.

It is the perfect place for scenic, tourist-y pictures.

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Image Source. Formed of Limestone and Shale, the reddish coloured rock is a major tourist attraction in Quebec.

Located in the middle of the sea, the 400 million years old rock formation looks splendid till date. The rock looks like a lobster claw.

Location: Perce, Quebec, Canada. Suggested Read: 13 Things To Do In Niagara Falls, Canada: An Experience Below A Majestic Waterfall.

Standing tall at the height of 8,850 meters, Mount Everest is the highest mountain peak in the whole world. The mountain evolved about 400 million ago and is one of the most famous sedimentary rock formations.

It is a sedimentary rock formation composed of limestone, pelite, shale, and marble. The mountain also has a deposit of several marine fossils.

If you are someone who loves adventure, you must plan a trip to Mount Everest. Location: Tibet-Nepal border.

Located in the crevasses of the Kjerag Mountain, this is the most thrilling rock formation in the world. For decades, a rock has been stuck between the crevasses and has become a famous tourist site.

It is easy access and quite daring.

Location: Norway. Image Source.

The round shaped rock has a sharp split in the middle. Although the split has been caused due to expansion and contraction in water, it looks quite scenic.

If you are in New Zealand, you must plan a trip to the rock during high tides. Location: Abel Tasman National Park, New Zealand.

These are some of the rock formations in the world that are worth a mention. If you are planning a trip anytime soon to one of these places, plan your holiday with Travel Triangle and travel tension free.

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Schist is a type of metamorphic rock characterized by its foliated texture, which means it possesses distinct layers or bands of minerals that have undergone significant physical and chemical changes due to heat, pressure, and other geological processes. The term “schist” is derived from the Greek word “schízein,” meaning “to split,” referencing the rock’s tendency to easily break along its foliation planes.

These conditions cause the minerals within the rock to re-crystallize and align themselves in parallel layers, giving schist its characteristic foliation. The minerals that make up schist can vary widely, but common minerals found in schist include mica (such as biotite and muscovite), quartz, feldspar, and various other minerals.

The layers of schist are often visible to the naked eye, making it relatively easy to distinguish from other types of rocks. One of the notable features of schist is its ability to cleave along the planes of foliation, resulting in flat, sheet-like pieces.

Schist is commonly found in regions with a history of intense tectonic activity and mountain-building processes. The formation of schist is often associated with regional metamorphism, where large areas of rock are subjected to pressure and heat over long periods due to the collision of tectonic plates or other geological forces.

Its unique texture and appearance have also made it a subject of interest for geologists, researchers, and enthusiasts alike. Type: Medium-grade metamorphic rock.

Grain size – Fine to medium grained. can often see crystals with the naked eye.

Colour – Usually alternating lighter and darker bands, often shiny. Mineralogy – Mica minerals ( biotite, chlorite, muscovite), quartz and plagioclase often present as monomineralic bands, garnet porphyroblasts common.

Name origin: The name is derived from the Greek word that means “to split.”. Contents.

However, there are several common minerals that are often found in schist, contributing to its characteristic appearance and properties. Here are some of the key minerals that can be present in schist:

Additionally, the degree of metamorphism can affect the mineralogy and texture of the rock, leading to further variations in composition. Classification based on Mineral Composition:

Here are some common types of schist categorized by their mineral composition: Classification based on Geological Setting:

Here are the main categories: Remember, these classifications provide a framework to understand the diversity of schist types.

Schist is a metamorphic rock characterized by its distinct foliation, layering, mineralogy, texture, parent rock relationships, and metamorphic grade. Here’s an overview of these characteristics:

Its characteristic foliation and mineral alignment make it an easily recognizable rock type. The various types of schist, such as mica schist, garnet schist, and amphibolite schist, are named based on their dominant minerals or significant features.

The formation of schist involves several key processes: The specific sequence of these processes and the resulting type of schist formed depend on factors such as the mineral composition of the original rock, the temperature and pressure conditions, and the presence of fluids that facilitate mineral reactions.

Overall, the formation of schist is a complex interplay of geological processes that transform existing rocks into the distinct metamorphic rock type we recognize today. Schist formations are found in various parts of the world and are associated with different tectonic settings and geological histories.

These are just a few notable regions with extensive schist formations. Schists can be found in many other parts of the world as well, each with its own geological history and tectonic context.

Schist has several economic significances due to its unique properties and mineral composition. Some of the key economic aspects associated with schist include:

The uses mentioned above highlight the versatility and value of schist in various industries and applications. Landforms and Landscapes: Influence on Terrain and Topography:

Here are some ways schist influences terrain and topography: Schistose Rocks in Erosion and Weathering:

In summary, schist’s unique characteristics, including its foliation, layering, and resistance to erosion, have a significant influence on the development of landforms and landscapes. The alternating bands of more and less resistant material contribute to ridge-and-valley topography, while the weathering and jointing of schistose rocks create distinct features such as talus slopes, domes, and cliffs.

Both are foliated metamorphic rocks in which individual minerals can be seen with the naked eye. The difference is that gneiss is generally more coarsely crystalline and has color banding and schist smells bad.

From 4 to 5 on the Moh’s scale, which is only indicative of its relative hardness against other rocks and minerals. What is schist made of.

When a volcano erupts the magma (lava) runs down into the holes and hardens making schist. AKA: schist is made of magma.

What is the parent rock of mica schist.

The original parent rock (or protolith) of mica schist is shale. Phyllite could also be considered the parent rock as mica schist is a more highly metamorphosed phyllite.

Metamorphic rocks are formed through the process of metamorphism, which involves the transformation of existing rocks due to high pressure, temperature, or chemical changes. Examples of metamorphic rocks include marble, slate, and gneiss.

The study of metamorphic rocks provides valuable insights into the Earth’s geological history and the processes that shape its surface. One of the three main types of rocks along with igneous and sedimentary rocks, have a unique relationship with igneous rocks.

In fact, metamorphic rocks often start their journey as igneous rocks and undergo metamorphism, a transformation due to intense heat and pressure deep within the Earth’s crust. This process, which can span millions of years, alters their shape, texture, and composition, effectively making them changed or evolved versions of the original igneous rocks.

These rocks, which can initially be igneous, sedimentary, or even metamorphic, experience a transformation known as metamorphism – signifying a ‘change in form’. This metamorphosis occurs deep within the Earth’s crust, under intense pressure and temperature.

Consequently, the pebbles or shells typically found in sedimentary rocks may morph into shiny crystals in the metamorphic rocks, further adding to their allure. As integral parts of the rock cycle, metamorphic rocks bear witness to the dynamic and enduring transformation of rock types over time.

This metamorphic process is far from instantaneous, often spanning millions of years. Thus, any metamorphic rock you encounter during a nature walk is not just a simple stone, but a testament to an epic geological journey.

This transformation is a result of a process known as metamorphism, where the structure and composition of rock layers are altered as they are thrust deeper into the Earth over time. Schist, often rich in minerals like mica and quartz, exhibits a sparkly aesthetic and can be found across the globe, including regions of the United States, Canada, and Europe.

Ultimately, schist exemplifies the transformative power of heat, pressure, and time, morphing simple rocks into distinctive and aesthetically pleasing entities. Gneiss, a type of metamorphic rock pronounced ‘nice’, is an engaging subject for children’s learning due to its intriguing formation process and its capacity to reveal Earth’s history.

The stripes are layers of minerals such as quartz, feldspar, and mica, which, over time, undergo a transformation due to the intense heat and pressure, becoming a tougher rock. One of the remarkable attributes of Gneiss is that it serves as a historical record of the Earth, with each layer reflecting different environmental conditions from millions of years ago.

Foliation, a unique characteristic found in certain metamorphic rocks like slate, highlights the intriguing power of earth’s natural processes. This characteristic, which refers to the specific alignment of mineral grains in flat, sheet-like layers within the rock, is a result of intense pressure during formation that causes the minerals to line up in a particular direction.

The fascinating world of metamorphic rocks reveals a deep connection with plate tectonics, as they are shaped in the Earth’s depths through intense heat and pressure from shifting tectonic plates. This process, aptly compared to a rock recycling program, transforms existing rocks into new types.

This metamorphism, a testament to Earth’s deep history, can span millions of years. Hence, each metamorphic rock encountered is a glimpse into the powerful forces of plate tectonics that have shaped our planet over eons.

This typically transpires during significant geological events like tectonic plate collisions, which give rise to mountain ranges. Gneiss, a rock showcasing a layered or banded look, is a prime example of the product of regional metamorphism.

Contact metamorphism is a geological process initiated when magma, the molten rock from Earth’s core, interacts with the solid rock surrounding it. This interaction alters the form, structure, and composition of the original rock, thus leading to the formation of metamorphic rocks.

Marble and quartzite are common examples of rocks created through contact metamorphism, which originate from limestone and sandstone respectively. It’s noteworthy to mention that these rocks, unlike other metamorphic rocks, lack layering or banding as they are formed through direct exposure to magma heat, as opposed to gradual pressure over time.

Its journey begins as a fine-grained sedimentary rock, typically shale, composed of clay or volcanic ash. The transformation process, which involves exposure to extreme heat and pressure within the earth’s crust, causes this shale to metamorphose into slate.

The impressive strength and durability of slate make it a popular choice for various practical applications, including roofing, flooring, and blackboards. The variety of colors it exhibits – ranging from grey, green, and purple, to black – can be attributed to the different minerals present during its formation process.Earth Science Fun Facts for Kids All About Metamorphic Rock – a Diagram Showing Magma Heat Causing Rocks to Change.

Question: Can igneous rocks become metamorphic rocks.

These rocks are typically formed deep in the Earth’s crust where one plate is pushed beneath another plate – a subduction zone. __________.

Answer: As long as the original rock or protolith is subjected to heat (temperatures greater than 150 to 200 °C) and pressure, it may undergo a physical or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.