Think of foliated rocks as something that is foiled. Adding foil creates a layer, so foliated rocks are layered rocks.
There are two main types of metamorphism:
- Regional metamorphism: This is caused by geological processes that affect a wide area. This process affects many sedimentary rocks because they’re already in layers. For example, limestone in mountains or other formations turns into marble as pressure from tectonic plates push the layers together.
- Contact metamorphism: This is caused by the heat of magma covering the rock(s). Most of the rocks formed by this process are called hornfels. For example, sandstone becomes quartzite and shale becomes a hornfels containing a variety of minerals.
An example of contact metamorphism,
where magma changes the type of rock over time
There are two types of textures on metamorphic rocks:
- Foliated: These rocks typically have layers as they’re usually formed by pressure exerted from one direction. They give off a “platy” look.
- Non-foliated: These rocks don’t have layers, and they are described as “massive” due to their non-uniform look.