Metamorphic Rocks Chapter 3.4

Metamorphic Rocks The word metamorphic means to “change form”. Metamorphic rocks are formed when one type of rock changes or transforms into another by the extreme pressures and temperatures that can be produced on the earth. Under ordinary circumstances, the only place you can find these extreme temperatures and pressures is deep within the earth’s crust.

Two Types of Metamorphism There are two main types of metamorphism. Contact Metamorphism occurs when magma comes into close contact with a rock and doesn’t melt it, but cooks it. This causes a recrystallization of the rock to occur. Regional Metamorphism occurs when large regions such as mountain ranges and tectonic plates move, causing extreme pressures and temperatures to recrystallize the rocks.

Contact vs. Regional Metamorphism Contact Metamorphism – Cooking, but not melting Regional Metamorphism – Cooking, pushing, bending, and twisting all at the same time.

Classification of Metamorphic Rocks The two different agents of metamorphism both cause two very different textures for metamorphic rocks. Foliated Metamorphic Rocks are formed by Regional Metamorphism (Pressure). Foliation gives a layered or banded appearance. Nonfoliated Metamorphic Rocks are formed by Contact Metamorphism (Heat). Nonfoliated Metamorphic Rocks often have a coarse crystalline texture.

Foliated Metamorphic Rocks created by Regional Metamorphism Schist has many varieties but one of the more common types seen here has many layers of muscovite mica present Slate is a Foliated Metamorphic Rock with a dull appearance Phyllite is similar to Slate but notice the wavy or glossy sheen

More Foliated Metamorphic Rocks Gneiss (pronounced nice) has a banded or tiger-striped appearance Another type of Schist, called Garnet Schist has small Garnet Gemstones in it.

Progression of Metamorphic Rocks Metamorphic Rocks can transform into different metamorphic rocks depending on the conditions. Slate for example can change into Phyllite, Phyllite into Schist, and Schist in Gneiss.

Nonfoliated Metamorphic Rocks created by Contact Metamorphism Quartzite, created by Quartz Sandstone that got “cooked” Marble, created by Limestone that got “cooked” Anthracite Coal is similar to Bituminous Coal, but notice how shiny it is. This is what happened to Bituminous Coal that gets “cooked”

Metamorphic Rock Classification A Parent Rock is the name of a rock that metamorphic rock originated from. A good example is limestone, which is the Parent Rock of Marble.