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Types of Rock Metamorphism Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 1, 2011

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 been worn 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.

Practice problems of this concept can be found at: Metamorphic Rock Practice Test

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