Metamorphic Rock

Metamorphic Rocks are rocks that are exposed to a lot of heat and a lot of pressure and this causes them to change.  Most of the changes happen many, many years later.  Many of the metamorphic rocks are found on the surface of the Earth where there is a lot of pressure and heat that comes from the magma that is under the surface.

A metamorphic rock deformed during the Variscan orogeny

Even though rocks can be made from the same type of minerals, they become classified by how they are made and not by what kinds of minerals they have.

Kinds of Metamorphic Rocks

There are many different kinds of metamorphic rocks such as:

  • Anthracite
  • Schist
  • Gneiss
  • Marble
  • Slate
  • Quartzite
  • Granulite

Anthracite

Anthracite is a metamorphic rock that looks very shiny.  This is a type of coal and it is high in carbon.  When a rock looks shiny, it is also called a rock’s luster.

Quartzite, a type of metamorphic rock

Marble

Marble is an interesting rock because it is a metamorphic rock that was once a sedimentary rock.  This rock followed the rock cycle and because of the sedimentary rock was under so much pressure and heat, it turned the rock into a metamorphic rock.

Limestone is the sedimentary rock that marble is formed from.  Marble is very durable but it can be dissolved if someone puts different kinds of acid or lemon juice on it.

Slate

Slate is another metamorphic rock that was formed from a sedimentary rock.  Slate was made from mudstone that was under a lot of heat and pressure and it formed slate.

Folded foliation in a metamorphic rock

When slate is formed, clay is replaced by mica.

Quartzite

Quartzite is once again a metamorphic rock that changed from the sedimentary rock, sandstone.

Granulite

Granulite is a metamorphic rock that changed from an igneous rock, basalt to a metamorphic rock due to heat and pressure.

Schist

Schist is a rock that comes from slate.

What Does Metamorphic Mean?

The word “metamorphic” comes from the world “morph” which means “to be changed or transformed.”  The reason that metamorphic means changed, is because metamorphic rocks are made when other rocks change because of heat or pressure.

There has to be a lot of heat and a lot of pressure for this to happen.

Metamorphosis

When rocks start to change, this is called metamorphosis.  When metamorphosis happens, the rocks that are close to the surface of the Earth are changed by magma.

Why Are Metamorphic Rocks Different?

Metamorphic rocks are different because not only can they be formed by igneous and sedimentary rocks; they can also be formed by other metamorphic rocks.

When a metamorphic rock is formed, it has to have a lot of pressure and a lot of heat for a very, very long time.

Contact or Thermal Metamorphism

When rocks are changing, this is called contact metamorphism.  This happens when bits of rocks are broken down or when they are changed by the heat from the magma.

When magma goes up into the Earth’s crust, it is changing all of the rocks around it.  When this happens, there is so much heat that new rocks are being formed.

Regional Metamorphism

When rocks are buried beneath the surface of the Earth, deep into the Earth, the rocks begin to change.  These changes happen from the chambers of magma that reach these rocks.

Even though the rocks are really hot and there is a lot of pressure, the rock will not melt because of the pressure.  If the rock did melt instead of staying solid, it would form into one of the other two types of rocks either a sedimentary rock or an igneous rock.

When there is more pressure, the rocks will change more.  If the rock is really deep and it is around more heat, it can change more too.

A great example of this is that mud can turn into shale if it is under just a little bit of pressure.  But when the shale is under even more pressure, it turns into slate.

When a rock is close to the surface of the Earth, and it is exposed only to a little bit of pressure, it causes the rock to sometimes have different layers and to split easy.  This is called foliation.

Foliated Rocks

When a rock has different layers of grain in them, these rocks are called foliated rocks.

Non-foliated Rocks

Rocks that have no grain in them are called non-foliated rocks.

How Do Metamorphic Rocks Get to the Surface?

There are different ways that metamorphic rocks get to the surface of the Earth such as volcanic activity and erosion.  When erosion happens, it can lift rocks out of the ground and bring them to the surface.

More Facts About Metamorphic Rocks:

  • Most of the crust of the Earth is made up of metamorphic rocks.
  • The Taj Mahal is a type of marble that is a metamorphic rock.
  • The lower crust of the Earth is made up of metamorphic rock except for parts of it that has igneous rocks through it.
  • One type of metamorphic rock is called serpentine.  This rock was first an igneous rock called peridot that was exposed to extreme heat and pressure.
  • Slate first started as mud at the bottom of the ocean or sea.
  • In Brazil, the coastline is made of metamorphic rocks.

What Did You Learn?

  • What is a metamorphic rock?
    A metamorphic rock is a rock that has changed because of a lot of heat and a lot of pressure.
  • What does the word metamorphic mean?
    The word metamorphic means to change and rocks that are metamorphic rocks have changed over time.
  • What are metamorphic rocks called that have different layers of grain in them?
    Metamorphic rocks that have different layers of grain are called foliated rocks.
  • What are the two types of metamorphism that metamorphic rocks go through?
    The two types of metamorphism are contact metamorphism and regional metamorphism.
  • What makes metamorphic rocks different?
    Metamorphic rocks are different than other rocks because they are made up of rocks that change and become metamorphic rocks or made up of other metamorphic rocks.