(49) Earth Science

Metamorphism of rocks




Hydrothermal Metamorphism

 This type of metamorphism is common with mid-ocean ridges where the crust is spreading and growing as a result of the outpouring of hot lava. The ocean water that bubbles through the hot, fractured basalts of the ridge margins becomes heated, causing chemical reactions between the surrounding ridge rock and seawater. These chemical changes produce metamorphosed basalt.

Hydrothermal metamorphism can also take place on land, when fluids from igneous rock intrusions percolate through surrounding country rock, causing a regional metamorphism.


Higher temperature and pressure metamorphic boundaries mark the lower limits of magma production. With a good amount of water, magma formation starts at a lower temperature. When there is little water, magma doesn’t form until higher temperatures are reached. This allows different types of metamorphic rock (schists, gneisses, and amphibolites) to form in different areas depending on the amount of fluid present.

Different types of layering are also possible depending on fluid intrusion, as well as temperature and pressure factors. When there is a variety of metamorphic rock types in an area, geologists find that a combination (mixed) rock has formed. Alternating layers of granite and schist form a mixed rock called migmatite.

A combination metamorphic rock type that contains both igneous and metamorphic rock is known as migmatite.

Burial Metamorphism

When layers of sedimentary rock become heavier and heavier, they get pushed further down into the crust, where they heat up and take on the temperature of the surrounding rock. We learned that when this happens, digenesis causes the transformation of sedimentary rock minerals and their textures. It happens at temperatures below 2008C.

As a result of increasing temperature and pressure in sedimentary rock layers, by ever heavier upper layers, diagenesis slowly continues and changes sedimentary rock layers over time through the process of low-grade burialmetamorphism.

This type of metamorphism often causes partial mineral changes in sedimentary rock with some bedding layers left unaffected. Burial metamorphism usually causes wide folding of sedimentary rock layers within the greater changes of regional metamorphism.

Cataclastic Metamorphism

Cataclastic metamorphism takes place in the same areas as igneous activity along plate margins, oceanic, and continental hot spots, and deformed mountain ranges.

Tectonic plate movement causes high-pressure metamorphism by crushing and shearing rock away as a result of plate movement. When metamorphism happens along a fault, the transforming heat comes from intense friction and pressure going on between massive plates as they grind past each other.

Broken and metamorphic rock fragments found along a metamorphic rock fault are called fault breccia. This rock type has minerals that crystallize at either extreme temperature or the high pressure and low temperature associated with extreme frictional stress. This type of metamorphism is often part of regional metamorphism.

Regional Metamorphism

Regional metamorphism is the most widespread kind of metamorphism. This takes place over a much greater crustal area where both temperatures and pressures are high. Geologists use the term regional metamorphism when talking about large-scale metamorphism rather than that found locally near specific igneous rock intrusions or faults. Most regional metamorphism takes place in the deeper levels of the crust, along the margins of clashing and subducting tectonic plates, where rock is deformed and forced into a new direction. Regional metamorphism is fueled by the Earth’s internal heat.

Regional metamorphism happens when a chunk of strata originally at the surface becomes deeply buried and subjected to squeezing horizontal stresses. When this happens, the sedimentary rock cracks, buckles, and is folded gently or severely depending on the amount of ongoing pressure. As the folds are shoved further down, heating increases and crystals begin to form as the sedimentary rock is changed into metamorphic rock. The speed and length of sedimentary burial affects the temperature and pressure it sees. For example, if the sediment is pushed down quickly in a subduction zone, it doesn’t have time to heat up because of the high-pressure environment. However, if the downward movement is slow, the temperatures usually keep pace with the surrounding rock and mineral formation is slower, more complete, and gradual.

Regional metamorphism affects large structures across a broad stroke of the landscape. It involves the uplifting and down warping of stressed and deformed landmasses in the middle of mountain building. When both pressure and temperature increases are involved in regional metamorphism, it is called dynamothermal metamorphism.

Since regional metamorphism covers a large geographical area, the minerals and textures throughout the area are found in zones. Some areas may be near magma intrusion sources and contain zones of metamorphic and igneous rock. Some fairly undisturbed areas will look very different than those found nearer active tectonic areas. The main thing to remember is that in a broad region of metamorphism, the areas of changed rock can be found in horizontal and vertical positions.

Regional metamorphism produces rocks such as gneiss and schist.Regional metamorphism is caused by large geologic processes such as mountain building. These rocks, when exposed to the surface, show the unbelievable pressure that causes rocks to be bent and broken during the mountain uplifting process.

Schist rocks are metamorphic in origin. In other words, they started out as something else and were changed by external factors. Schists can be formed from basalt, an igneous rock; shale, a sedimentary rock; or slate, a metamorphic rock. Through tremendous heat and pressure, these rocks were transformed into this new kind of rock.

Schist is a medium-grade metamorphic rock. Medium-grade rock has been subjected to more heat and pressure than another rock such as slate. Slate, a low-grade metamorphic rock, needs lower temperatures for metamorphic changes to take place.

Schist is a coarse-grained rock with easily seen individual mineral grains. Since it has been compressed tighter than slate, schist is often found folded and crumpled. A lot of its original minerals have been transformed into larger flakes. Schists are usually named with reference to their original minerals. Biotite mica schist, hornblende schist, garnet mica schist, and talc schist are all different types of schist that come from different original minerals.

Gneiss rocks are also metamorphic in origin. Some gneiss rocks started out as granite, an igneous rock, but are changed by heat and pressure. Many gneiss rock samples have flattened mineral grains that have been smoothed flat by extreme heat and pressure and are aligned in alternating horizontal patterns.

Gneiss is a high-grade metamorphic rock. It has been the focus of much more heat and pressure than schist. Gneiss, a coarser rock form than schist, has distinct and easily seen banding. This banding is made up of alternating layers of different minerals. Gneiss can be formed from sedimentary rock such as sandstone or shale, or it can be created from the metamorphism of the igneous rock, granite. Since gneiss can come from granite, the same minerals found in gneiss are also found in granite. Along with mica and quartz, feldspar is the most important mineral found in gneiss. Gneiss is often used as a paving and building stone due to its attractive banding.

Dynamic Metamorphism

Dynamic metamorphism also results from mountain building. Huge extremes of heat and pressure cause rocks to be bent, crinkled, smashed, compacted, and sheared. Metamorphic rocks are generally harder than sedimentary rocks because of their tough formation environment and are hard or harder than igneous rocks. They form the bases of many mountain chains and are exposed as outcrops only after short-lived outer rock layers have beenworn away. Metamorphic rocks discovered in mountainous regions today provide geologists with clues as to the location of ancient mountains on modern-day plains.

Geologists use these clues to figure out the temperatures that change different rock types into metamorphic rock. The crystal arrangement of different rock samples gives them a good idea as to the temperatures that the specific sample has been exposed to during its lifetime.

Retrograde Metamorphic Rock

Sometimes a rock type is changed into a high-grade rock at one point, then later exposed to low temperatures and changed to another type of rock. When this happens, it is known as retrograde metamorphism. Retro means to go backwards in development.

An easy way to think of it is to picture butter. When butter is heated, it melts and turns into a liquid. When the temperature cools, the butter, which has separated into slightly different forms, goes back into a solid state. Later, if the butter is left out and melts at room temperature, it will eventually sour and return to its basic components.

Sometimes, geologists find rock that has been through more than one change. This is usually seen during microscopic crystal examination or through chemical analyses.


There are three main factors that cause pressure increases and the formation of metamorphic rocks. These are:

* The huge weight of overlying sedimentary layers,

* Stresses caused by plates clashing during mountain building, and

* Stresses caused by plates sliding past each other, like the shearing forces along the San Andreas Fault (western United States).

Pressure or stress from tectonic processes or the weight of overlying rock causes changes in mineral texture. The two types of pressure that are applied to existing rock are confining pressure and directed pressure. Confining pressure is an all around pressure. Like atmospheric pressure at the surface of the Earth, confining pressure is present within the mantle’sdepths. Extreme confining pressure changes a mineral’s structure by squeezing its atoms tighter and tighter until new minerals with denser crystalline structures are formed.

Directed pressure happens in a specific direction. When extreme squeezing pressure is applied in one direction, it’s like toothpaste in a tube; it is forced in one direction. When clashing plates are compressed, the force is applied in one direction. Since heat decreases a rock’s strength, when pressure is applied in one direction, a lot of folding and deforming goes on when temperatures are high.

Depending on the type of stress applied to a rock, the minerals in metamorphic rock are squeezed, stretched, and rotated to line up in a specific direction. This is how directed pressure affects the size and shape of metamorphic rock minerals undergoing change by heat and stress. For example, during recrystallization of micas, crystals grow within the planes of their sheet-silicate structures and align perpendicular to the directed pressure. Geologists use this type of metamorphic mineral to figure out the pressures that specific samples have been exposed to during their history.

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