They are not made from molten rock — rocks that do melt form igneous rocks instead. Earth movements can cause rocks to be deeply buried or squeezed. As a result, the rocks are heated and put under great pressure.
They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks. One mineral which does this is olivine, which changes from olivine isolated silica tetrahedra to spinel a much more tightly-bonded structure to perovskite a still more highly compressed structure.
This kind of pressure is usually due to tectonic forces. It changes rocks by changing the structure of minerals and by changing the orientation of mineral grains, particularly platy minerals like mica or clay. Fluids which metamorphose rock are not pore fluids remaining from when sedimentary rocks were deposited. Instead, they come from two main sources: hydrothermal fluids from magmatic intrusions and dehydration of minerals, like clay, which contain water in their structures hydrous minerals.
Whatever the source, fluids contain ions dissolved from other rock or from their original source. As fluids percolate through rocks, they can exchange ions with the existing minerals and thus change the chemical makeup of those minerals. The other way fluids change minerals is by hydrating minerals which previously did not contain water.
Either way, fluids change the chemical makeup of minerals, turning them into new minerals, which changes the rocks which were made of the previous minerals. This process of change by fluids is called metasomatism. Types of Rock Metamorphism. Some kinds of metamorphism: Burial Bury rocks deeply enough and they will warm up and change.
This form of metamorphism is found anywhere where sediments and rocks are buried deeply, and should strike you as being pretty similar to diagenesis, which we discussed last time. The line between diagenesis and burial metamorphism is fuzzy. Regional Caused by widespread moderate-to-high temperatures and pressures, as opposed to localized changes along faults or near magmatic intrusions.
You find this type of metamorphism in mountain building regions and near subduction zone volcanism. Contact Caused by high temperatures near magmatic intrusions. Found in volcanic regions subduction zones, hot spots and mountain building zones. Cataclastic Caused by grinding along fault zones. Found along major faults like the San Andreas Fault in California , in mountain building zones, and in deformation regions associated with subduction zones.
Hydrothermal Caused by hot fluids percolating through rocks. Found anywhere where hot fluids can percolate through rocks, notably along mid-ocean ridges. Metamorphic Rocks and Rock Textures. Three major texture and rock types for metamorphic rocks that you need to know: Foliated Rocks Characterized by parallel planes formed through directed pressure and preferred growth orientations of certain platy minerals.
Two common kinds are schist and gneiss, which have been used in a great many really bad geological puns. Non-foliated Rocks Don't have those planes, usually because they are made of mineral grains which are cubic or spherical, and therefore have no preferred orientation. Two common examples are marble and amphibolites. Deformational Caused by cataclastic metamorphism. The most common rock of this kind is called a mylonite; there is a big mylonite belt in the mountains south of Palm Springs, CA.
Metamorphic Grade. Geologists who study metamorphic rocks have come up with the concept of metamorphic grade to describe how 'metamorphosed' a rock is. More specific distinctions can be made through lab experiments in which various kinds of rocks are squeezed and heated up and the changes observed. Through this kind of work, geologists have found a set of index minerals , which are common minerals which form under particular combinations of pressure and temperature.
Armed with knowledge from these experiments, field geologists can go out and make maps of mineral location to determine how metamorphism is distributed over large regions of rocks. Metamorphic Facies. Keep that in mind: you get different minerals from different rocks under the same conditions and you get different minerals from the same rocks under different conditions.
Geologists have formalized these statements into a system of classifications for rocks by pressure and temperature conditions, so that a given combination of pressure and temperature will give a specific class of rocks. These classifications are called metamorphic facies. This is vital information for figuring out past tectonic conditions in the region, since certain facies form in certain plate tectonic environments. For example, blueschists form under low temperatures and moderate-to-high pressures, which indicates that the material which metamorphosed was shoved down into the Earth so quickly it didn't have much time to warm up.
What kind of plate tectonic environment displays these features? Subduction zones! Once material is weathered from rocks, it is transported away and later deposited somewhere else, and eventually is turned into new rocks. Such rocks are called sedimentary rocks , and they're the subject of this lecture. What are sediments? What is a sedimentary rock?
Sediments are loose particles of former rocks. They can either be bits of rock ranging from mineral grains all the way to boulders or material which was dissolved and then later precipitated from water to form solid crystals.
After these sediments are deposited, they may be buried and undergo a set of physical and chemical changes which turn them into solid rock. Rocks formed from sediments are called sedimentary rocks. Types of Sedimentary Rocks. There are three main types of sedimentary rocks I want you to know about: Clastic Made from solid particles which were weathered from previous rocks. All sedimentary rocks go through a cycle similar to this one: Weathering from previous rocks Erosion of weathering products Transport of particles from weathering region to someplace else Deposition of sediments Burial of sediments Diagenesis We will illustrate these processes with clastic rocks, then discuss chemical and biochemical rocks.
Clastic Rocks: Transport. For example, a river can transport clastic particles of all sizes when it is flowing rapidly, high in the mountains or during a flash flood. As the current slows, the river drops big particles first, then the next size, then the next, and so on. When the current is very gentle, only the smallest particles are carried.
This phenomenon is called sorting. Another phenomenon resulting from transport of clastic particles is called rounding. Basically, as particles are moved downstream, they bounce around and chips are taken off, especially around the corners. In this way, the sharp edges are worn away and a smoother, rounder particle is left. The farther a particle travels, the rounder and smoother it will be.
Clastic Rocks: Deposition. Rivers and all other modes transport clastic material downstream and then drop it someplace; this process is called deposition or sedimentation. Places where this occurs are called sedimentary environments. As sediments are deposited, they build layers of material. These layers are buried by later sediments, which are buried by later sediments, and so on and so on.
As more and more sediments are deposited, the sediments near the bottom get heated up and squeezed more and more. Explanation: Weathering is the process of breakdown of rocks through the action of wind, air, water and by the action of organisms. Related questions What are some examples of metamorphic rocks? How does contact metamorphism differ from regional metamorphism? What is regional metamorphism?
Why is contact metamorphism associated with igneous intrusions? How is foliation related to the dominant regional stresses? What is a metamorhpic rock and how are they formed?
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