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Types of Metamorphic Rocks

So you’ve learned the basics about metamorphic rocks, now what? Let’s learn about the different types of metamorphic rocks, foliation, and why this happens.An image of a canyon formed of marble, one of the types of metamorphic rocks.
 
As a reminder, metamorphic rocks are rocks changed by very high heat and pressure. The heat comes from magma in the mantle. Earth’s upper layers put pressure on rocks below the surface, which is where these rocks form.
 
Additionally, the name ‘metamorphic rock’ comes from two words: meta (meaning to change) and morph (meaning form). Metamorphic rocks can start out as many different types of rocks.
 

What is Foliation, and Why Does it Happen?

Foliated Metamorphic rocks form deep below the Earth’s crust. It happens when there is a great but unequal pressure. This pushes minerals in the rock to re-align themselves, causing a striped look

An image of a river surrounded by slate rocks. Because the minerals are lined up, it flakes into flat platforms.

Slate. Slate is a foliated metamorphic rock.

There are a few different types of Foliated Metamorphic Rocks:

  1. Slate – Slate forms when there is low heat and low pressure. It’s very smooth and usually aligns parallel to itself. It is also very useful, we use it today for pool tables and roofs!
 
  1. Phyllite – Phyllite forms when there is low to medium heat and pressure. It has a crystallized appearance, making it look shiny. The layers in Phyllite rocks are much wavier than Slate.
 
  1. Schist – Schist forms when there is medium to high heat and pressure. These have larger crystals that align with the other layers in the rock.
 
  1. Gneiss – Gneiss (pronounced nice!) is created when there is extremely high heat. It looks very grainy and has alternating light and dark minerals in it. 

    An image of gneiss rock, due to weathering the lines look like swirls.

    Layered gneiss. How gneiss!

What are Non-Foliated Rocks? What’s the difference?

Non-Foliated Metamorphic rocks form when there is a high temperature, but low pressure. Because they’re created under these conditions, they do not have the layered appearance that Foliated rocks do and they are much more smooth. Two examples of Non-Foliated Metamorphic rocks are:
 
  1. Quartzite – Quartzite has both quartz and sandstone in it. It’s used for roofing, stair steps, and flooring.
 
  1. Marble – Marble includes both Calcite and Limestone. We use this today to make tables and countertops! It can also form caves when weathered.

    An image of a marble countertop in a kitchen.

    A marble countertop.

Other Great Resources:

Non-Foliated Metamorphic Rocks – Minerology4Kids: http://www.mineralogy4kids.org/rock-cycle/nonfoliated-metamorphic-rocks

Foliated Metamorphic Rocks – Minerology4Kids: http://www.mineralogy4kids.org/rock-cycle/foliated-metamorphic-rocks

Metamorphic Rocks – SimpleWikipedia: https://simple.wikipedia.org/wiki/Metamorphic_rock

Metamorphic Rock Facts: http://www.softschools.com/facts/rocks/metamorphic_rocks_facts/366/

Written by: Hannah Bertoch