Metamorphic Rocks

Metamorphic Rocks

Middle School

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Learning Objectives

• Define metamorphic rocks and how they are formed by heat and pressure.

• Differentiate between foliated and non-foliated rocks based on their texture.

• Identify parent rocks of metamorphic rocks like marble, quartzite, and gneiss.

• Compare the two main causes of metamorphism: regional and contact metamorphism.

Key Vocabulary

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Foliation

The appearance of parallel layers in metamorphic rocks, caused by uneven pressure during their formation.

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Plastic Deformation

A permanent change in the shape of rocks by bending and folding under high heat and pressure.

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Contact Metamorphism

Rock transformation caused by close contact with a significant heat source, like a magma chamber.

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Regional Metamorphism

Large-scale metamorphism due to immense stress and pressure from tectonic plate collisions.

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Rock Cleavage

The tendency of a metamorphic rock to split or break along a single, flat plane from intense pressure.

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What Are Metamorphic Rocks?

• Metamorphic rocks form from other rocks under intense heat and pressure.

• ​These rocks do not melt, but their properties and appearance change.

• This pressure can cause rocks to bend and fold, a process called plastic deformation.

• This bending and folding is a key part of how mountain ranges form.

Multiple Choice

What is the process called when rocks permanently change shape by bending and folding under heat and pressure?

• Plastic Deformation

• Melting

• Crystallization

• Erosion

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Classification of Metamorphic Rocks

Foliated Rocks

• ​These rocks contain parallel layers of flat and elongated minerals.

• ​​This layering is called foliation and can show different colored bands.

• ​Foliation develops from an uneven distribution of pressure during metamorphism.

Non-Foliated Rocks

• ​These rocks have mineral grains with a random, interlocking texture.

• ​​The crystals do not line up in layers, so there is no banding.

• ​The block-like crystals are generally all about the same size and color.

Multiple Choice

What is the main difference between foliated and non-foliated metamorphic rocks?

• Foliated rocks have layers, while non-foliated rocks have a random texture.

• Foliated rocks are made of magma, while non-foliated rocks are not.

• Foliated rocks are always dark, while non-foliated rocks are light.

• Foliated rocks are found on the surface, while non-foliated rocks are not.

Other Metamorphic Rock Features

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Foliation

• ​Foliation is the appearance of bands or layers within a metamorphic rock.

• ​​This layering effect can be seen in rocks such as gneiss.

• ​Non-foliated rocks appear as a single, solid color without any banding.

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Rock Cleavage

• ​Rock cleavage is when a rock splits along a single flat plane.

• ​​This is caused by intense, directional pressure during the metamorphic process.

• ​It results in smooth, flat surfaces where the rock has broken apart.

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Lineation

• ​Lineation occurs when minerals inside the rock become aligned together.

• ​​All the aligned minerals point in the very same direction.

• ​This alignment is also a result of pressure during metamorphism.

Multiple Choice

Which feature is characterized by minerals aligning in the same direction within a metamorphic rock?

• Lineation

• Foliation

• Rock Cleavage

• Gneissic Banding

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Examples of Metamorphism

• Shale becomes slate, schist, or gneiss depending on the grade of metamorphism.

• The sedimentary rock sandstone changes into the metamorphic rock quartzite.

• Limestone, another sedimentary rock, is transformed into marble through metamorphism.

• Igneous rocks like basalt can become greenstone, and granite can become gneiss.

Multiple Choice

Which metamorphic rock is formed from the parent rock limestone?

• Marble

• Quartzite

• Gneiss

• Slate

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Major Causes of Metamorphism

Contact Metamorphism

• This occurs when rock comes into direct contact with a powerful heat source, like a magma chamber.

• The intense heat from the magma effectively “bakes” the surrounding rock, causing its composition to change.

• This process is localized, affecting a relatively small area of rock around the heat source.

Regional Metamorphism

• This process happens on a much larger scale, affecting vast areas of rock.

• It is caused by the immense stress and pressure from the collision of tectonic plates.

• Regional metamorphism is powerful enough to create entire mountain ranges made of metamorphic rock.

Multiple Choice

What is the primary cause of regional metamorphism?

• Stress and pressure from tectonic collisions

• Intense heat from a nearby magma chamber

• The weight of overlying sedimentary layers

• Chemical reactions with hot fluids

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Common Misconceptions

Misconception	Correction
Metamorphic rocks must melt to reform.	They transform from heat and pressure while solid, without melting.
Rock layers in metamorphic and sedimentary rocks are the same.	Metamorphic bands are from mineral re-alignment, not sediment deposition.
All metamorphic rocks have layers.	Only foliated rocks have layers; non-foliated rocks do not.

Multiple Choice

How does the texture of slate differ from its parent rock, shale, as a result of low-grade metamorphism?

• Slate develops rock cleavage, allowing it to split into flat planes.

• Slate becomes granular with interlocking crystals.

• Slate develops visible bands of different colored minerals.

• Slate becomes much softer and more crumbly.

Multiple Choice

Choose the correct statement for the process of Metamorphism.

• Existing rocks are transformed by intense heat and pressure deep within the Earth's crust.

• Limestone turns to marble.

• Sandstone turns to quartzite.

• All are correct.

Multiple Choice

A geologist finds a large area of gneiss and schist. Based on these rocks, what can the geologist infer about the geologic history of this area?

• The area was likely the site of a major mountain-building event from a continental collision.

• The area was once a shallow sea where limestone was deposited.

• A volcano erupted, and the lava cooled very slowly.

• A nearby magma chamber baked the surrounding sandstone.

Multiple Choice

Predict what would happen to a piece of granite that is subjected to the immense, directional pressure of a continental collision.

• It would likely become foliated and transform into gneiss.

• It would melt completely into magma.

• It would transform into quartzite.

• It would develop a random, interlocking texture and become marble.

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Summary

• Metamorphic rocks are formed from existing rocks by heat and pressure.

• They are classified as foliated (layered) or non-foliated (not layered).

• Limestone becomes marble, and sandstone turns into quartzite.

• Contact metamorphism is caused by heat; regional metamorphism by pressure.

Poll

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