Metamorphic Rock Help

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


Igneous rock is formed as a result of the Earth’s internal “engine,” while sedimentary rock formation depends on external climate and conditions. Metamorphic rock, however, takes place after these rock types have already formed. It is created by transforming igneous or sedimentary rock into something new.

Of the three major rock types, igneous, metamorphic, and sedimentary, metamorphic rock is the chameleon rock. It transforms into different types of rocks depending on the factors that it is exposed to within the Earth. This rock type is both a wonder and a headache to geologists. Since metamorphic rock begins originally as something else, it can be confusing as to whether it is the original rock or a transformed version. To solve this problem, geologists gather clues from the surrounding area or an outcrop from which the sample rock is found.

Besides being intruded upon by magma regularly, the Earth’s crust is subjected to stresses within the crust and mantle that cause it to break and bend forming fault folds . These forces often center along thin, winding belts when folding. They also combine with magma intrusion and extrusion while pushing up mountain ranges. The rocks within a mountain range are not only under extreme pressure, but heated by magma intrusion as well. These stresses deform and recrystallize rock to different degrees. Pressure and temperature can also change previously metamorphosed rocks into new types.

Rock-forming and destroying processes have been active since the Earth was first formed. When sedimentary and igneous rocks are exposed to extreme pressure or medium heat, they are changed. They become metamorphic rocks , which form while deeply buried within the Earth’s crust. It is important to remember that metamorphism does not just melt existing igneous or sedimentary rock, but transforms it into a denser, compacted rock.

Metamorphic rocks are formed from rocks that were originally another type and were changed into a different form.

Metamorphic Rocks

The name metamorphic comes from the Greek words, meta and morph , which mean “to change form.” Geologists have found that nearly any rock can become a metamorphic rock. When existing rock is shoved and pressurized, its minerals become unstable and out of equilibrium with the new conditions, causing them to change.

Remember the chameleon? When a chameleon moves from a gray rock to a bright green leaf, he changes his skin color to the same as his environment. By adjusting to his new conditions, the chameleon protects himself and comes into equilibrium with his surroundings. The process of metamorphism is similar. When a rock is slowly moved through tectonic processes to a new temperature or pressure environment, its original chemical and physical conditions are changed. In order to regain stability in the new conditions, chemical and physical changes take place. With metamorphism, mineral changes always move toward reestablishing equilibrium. Common metamorphic rocks include slate , schist , gneiss , and marble with many grades in between.

Most of the time, metamorphic rock is buried many kilometers below the crust which allows increasing temperatures and pressures to affect it. However, metamorphism can also happen at the surface. When geologists study soils under hot lava flows, they find metamorphic changes. The three main forces responsible for the transformation of different rock types to metamorphic rock are:

  • internal heat from the Earth,
  • weight of overlying rock, and
  • horizontal pressures from rock that changed earlier.

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

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