Metamorphic Rocks

• 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

Foliation

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

Plastic Deformation

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

Contact Metamorphism

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

Regional Metamorphism

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

Rock Cleavage

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

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.

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.

Other Metamorphic Rock Features

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.

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.

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.

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.

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.

• 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.