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Types of Metamorphic Rocks

Metamorphic rocks are formed when rocks are changed under heat and intense pressure.

Of the rocks that form the continental land mass, most are metamorphic rocks.There are three different ways rocks can be changed by heat and pressure. The process of changing is called metamorphism.

Metamorphic rock


The first kind of metamorphism happens when rocks simply get buried by new rocks formed on top of them. The weight on top of rocks forces them deeper into the earth. The deeper the rocks are pushed into the ground the more they become exposed to heat. The heat and pressure cause the rock to change into a new rock. This type of metamorphism is called regional metamorphism and is the most common form of metamorphism.However,if the heat melts the rock and then the rock cools down it is an igneous rock, not a metamorphic rock.

Another way metamorphic rocks form is called contact metamorphism.Sometimes magma pushes its way up through the crust along cracks in a rock formation. As the magma cools it forms igneous rock.This kind of geologic feature is called an igneous intrusion. Although the rock in the intrusion is igneous rock, the rock the magma contacts can be changed by the heat of the magma and the pressure of the surrounding rock.

Devil’s Tower in Wyoming was formed by an igneous intrusion.


Scientists call the third way metamorphic rocks form a tectonic process.Sometimes I think scientists like to use long words to make it harder for non-scientists to figure out what they are talking about. Here’s what they are talking about:The large plates that make up the crust of the earth are always moving.Sometimes they slam into each other.Sometimes they grind past each other. Sometimes the plates are pulling apart forming large cracks called rifts.When plates pull apart it is called a divergent boundary.On some boundaries one plate dives under another plate. Geologists call this type of boundary a convergent boundary.When the plates bump and grind past each other, the geologists call this boundary a transform fault boundary.A famous example of this type of fault in the United States is called the San Andres Fault.At that plate boundary, the North American plate is moving mostly south and the Pacific plate is moving mostly north, which means that in 15 million years Los Angeles and San Francisco will be neighbors.I wonder if that will solve the debate about which is the better town. In any case, metamorphic rocks are formed from the pressure and heat caused by the plates crashing into each other.

The San Andreas Fault can be seen from the sky

The San Andreas Fault in California.


So what rocks are metamorphic rocks? Examples of these rocks are marble, schist,slate, gneiss (pronounced “nice”). All of these rock types are formed by heat and pressure. So what makes them different? The answer is simple.Metamorphic rocks are formed from different rocks.Marble is made from sedimentary rock called limestone and sometimes dolomite.Gneiss is formed from ingenious rock, like granite.Schist is formed from sedimentary rocks like mudstone or siltstone. Slate is formed from shale.As a matter of fact, slate and shale look so much like each other, the best way to tell them apart is to lightly tap them with a metal object like a coin. Slate and shale make different sounds when tapped. When tapped, slate has a slightly more metallic sound than shale. Shale makes a kind of hollow thumping sound.

Marble is commonly used for sculptures

Because of its beauty, marble is used for many purposes.

Metamorphic rocks are one of the three main types of rocks and are the most common of rock on the continental plates.Fossils may be found in metamorphic rocks,but only if the metamorphic rock was formed from a sedimentary rock that already had the fossil in it. However, the fossil is most likely going to be crushed, warped, or somehow changed because the process that changes sedimentary rock into metamorphic rock will change the fossil, too. Marble is a metamorphic rock used by artists to create sculptures and to decorate buildings and other things. Slate was used to make chalkboards and is still used to make pool tables. Keep your eyes open! Metamorphic rocks are everywhere.


Written for the KidsKnowIt Network by: Brandon Guymon