9.49 Metamorphic Rock Definition & Facts

Metamorphic rocks are rocks formed from other types of existing rock due to changes in heat or pressure. Learn about the definition and examples of metamorphic rocks and discover facts about how these rocks are formed.

What Are Metamorphic Rocks?

Rocks are divided into three main categories: igneous, sedimentary, and metamorphic. Metamorphic rocks are formed when there is a change in the heat or the pressure of their environment. Any rock, including igneous and sedimentary rocks, can become metamorphic rocks when the temperature or pressure changes. The word metamorphic means to change form, so that’s what metamorphic rocks do!

How Do Metamorphic Rocks Form?

A few different events can change the heat and pressure and cause rocks to become metamorphic rocks: pressure deep inside the earth, continental collisions (when continents shift and press against one another), or heat from magma and lava in volcanoes. Metamorphic rocks are often found in mountainous areas.

There are two types of metamorphism: contact metamorphism and regional metamorphism. Contact metamorphism, also called thermal metamorphism, is when magma changes broken rocks near the surface of the earth. This diagram shows how contact metamorphism takes place.

This diagram shows how contact metamorphism takes place.
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Regional metamorphism is when magma changes rocks deep inside the earth. Regional metamorphism causes rocks to form that are divided into different layers through a process called foliation.

1. Main points:

Metamorphic rocks are formed from igneous, sedimentary, or other metamorphic rocks due to changes in heat or pressure. This transformation can occur through contact metamorphism, where magma alters rocks near the Earth’s surface, or regional metamorphism, which affects rocks deep within the Earth, often resulting in layered rocks through foliation.

2. Questions:

• Content Analysis: What causes rocks to become metamorphic?
• Thematic Analysis: What are the differences between contact and regional metamorphism?
• Socio-cultural Analysis: How does the study of metamorphic rocks contribute to our understanding of geological processes?

3. Further Discussion:

• What might be the implications of finding metamorphic rocks in a particular geographic area?

4. Answers:

• Content Analysis: "Metamorphic rocks are formed when there is a change in the heat or the pressure of their environment."
• Thematic Analysis: "Contact metamorphism occurs when magma changes rocks near the surface… Regional metamorphism happens deep inside the earth and causes rocks to form layers through foliation."
• Socio-cultural Analysis: The study of metamorphic rocks helps geologists understand the history of Earth’s crust, including past geological events like continental collisions and volcanic activities, contributing significantly to the field of geology.

Metamorphic Rocks Examples

• Slate is a type of metamorphic rock that was first part of a lake or ocean bottom. Slate is a dark, smooth rock. Slate can turn into another kind of metamorphic rock called schist.
• Marble is another kind of metamorphic rock that comes from limestone. Limestone turns into marble when it’s subjected to increased heat and pressure. Marble is very strong, but lemon juice and other acids can cause it to wear away.
• Quartzite is a rock that looks like it has crystals in it. Quartzite comes from sandstone.
• Gneiss is a rock that has lots of different colors. It comes from shale. Gneiss is a very common rock, and you could see it in lots of different places, from Germany to Scotland to the United States.

Metamorphic Rocks Fun Facts

• Because they come from rocks, metamorphic rocks often contain fossils, or the remains of plants and animals from long ago.
• Did you know that metamorphic rocks can become new metamorphic rocks? Rocks can change lots of times.
• The Taj Mahal is a special building in India. It was built entirely of marble, which is a metamorphic rock.
• Most of the earth’s crust is made up of metamorphic rock.

1. Main points:

Metamorphic rocks, such as slate, marble, quartzite, and gneiss, are formed from other rock types under heat and pressure. These rocks can contain fossils and undergo multiple transformations. Notable structures like the Taj Mahal are made from metamorphic rocks, and a significant portion of the Earth’s crust is composed of these rocks.

2. Questions:

• Content Analysis: What are some examples of metamorphic rocks and their origins?
• Thematic Analysis: How do the properties of different metamorphic rocks vary?
• Socio-cultural Analysis: What is the significance of metamorphic rocks in human history and architecture?

3. Further Discussion:

• How does the presence of fossils in metamorphic rocks help in understanding Earth’s history?

4. Answers:

• Content Analysis: "Slate… comes from a lake or ocean bottom… Marble comes from limestone… Quartzite comes from sandstone… Gneiss comes from shale."
• Thematic Analysis: "Slate is dark and smooth… Marble is strong but can wear away with acid… Quartzite looks like it has crystals… Gneiss has lots of different colors."
• Socio-cultural Analysis: "The Taj Mahal… was built entirely of marble, which is a metamorphic rock… Most of the earth’s crust is made up of metamorphic rock."

Lesson Summary

All right, let’s take a moment to review what we’ve learned. Metamorphic rocks are formed when rocks are subjected to an increase in pressure or heat, usually from pressure under the earth’s surface or from magma and lava from volcanoes. This is how metamorphism is separated into two different categories:

• Contact metamorphism, also called thermal metamorphism, which is when magma changes broken rocks near the surface of the earth.
• Regional metamorphism, which is when magma changes rocks deep inside the earth.

Slate, a rock from lakes or oceans, and marble are examples of metamorphic rocks. Fossils, or remains of plants and animals, are often found in metamorphic rocks. Now you should be able to point out the next marble statue you see as being made of metamorphic rock. Just make sure you let everyone know that you don’t think that means the statue is going to start moving!