Unit: Earth’s Composition
Concept: Metamorphic Rocks: Engage Tab_ Discovery Education
How Are Metamorphic Rocks Formed?
Metamorphic rocks are formed under high heat and pressure in Earth's crust. Over a long period
of time, the heat and pressure can cause minerals in the rocks to change. The minerals can
recrystallize; the rocks can also change their shape, texture, or composition. Other rock types,
such as igneous or sedimentary rocks, can become metamorphic rocks. For example,
sedimentary rocks are formed when small particles of other rock types slowly cement together. At
some point, a sedimentary rock could be exposed to heat or pressure, such as when the rock is
in a subduction zone. Subduction zones are areas where one piece of Earth's crust is forced
underneath another piece of Earth's crust. The heat and pressure present in a subduction zone
can cause the sedimentary rock to become metamorphic rock.
Misconception: Only a rock's texture changes during metamorphism.
Although it is true that the texture of rocks can change when rocks undergo metamorphism,
texture isn't the only thing that can change. The mineral composition of the rock can change
when the minerals in the rock recrystallize. The heat and pressure can also cause new crystals to
form. Metamorphism can also change the shape and appearance of rocks.
What Are the Two Main Types of Metamorphic Rocks?
There are different kinds of metamorphic rocks. You read that when rocks undergo
metamorphism, they can change texture, shape, or composition. Sometimes when rocks change,
the mineral grains within the rocks change their appearance. They become stretched into long
layers or separated into different bands according to their composition. These metamorphic
rocksare called foliated rocks. Rocks like schist or slate are examples of foliated metamorphic
rocks. Not all metamorphic rocks are foliated. Another type of metamorphic rock, called
nonfoliated, doesn't have distinct layers or bands of minerals. Rocks like marble or hornfels are
examples of nonfoliated metamorphic rocks.
What Is the Difference Between Regional and Contact Metamorphism?
There are different ways that rocks can change and become metamorphic rocks. Two ways are
regional metamorphism and contact metamorphism. In regional metamorphism, rocks over a
large geographic area are exposed to heat and pressure. For example, tectonic forces like
subduction can cause large areas of rock deep within Earth's crust to melt. Regional
metamorphism is the change of rock types over a large area. Contact metamorphism is a change
of rock types on a much smaller scale. In this type of metamorphism, rocks are exposed
to heat from magma or lava. The heat from the magma or lava can cause rocks to melt and
transform into metamorphic rocks.