Metamorphic Rocks

Metamorphic rocks are igneous, sedimentary, or other metamorphic rocks that have undergone extreme heat and/or pressure.

The extreme heat and/or pressure has caused the original rock to change form (metamorphosis).

The “new” rock is usually much different from its original form.

Figure 8.2

Types of Metamorphism

There are 2 types of metamorphism:

1. Regional Metamorphism

2. Contact Metamorphism

Notice the foliation

Regional Metamorphism

Occurs when large areas of rock are under intense heat AND pressure.

Most often happens during mountain building events.

It is important to keep in mind that the heat and pressure are VERY intense and this happens over VERY LARGE areas.

Regional Metamorphism of Shale

As you know, shale is a sedimentary rock.

During metamorphism, the pressure squeezes the mineral crystals closer together.

Heat then rearranges the mineral crystals.

SlateMetamorphic

Rock

ShaleSedimentary

Rock

Regional Metamorphism of Shale

The first level of metamorphism of shale results in the metamorphic rock slate.

This “new” rock slate shows low-grade foliation – layers.

Regional Metamorphism of Shale

Regional Metamorphism of Shale

Foliation is:

Parallel alignment or orientation of platy minerals.

Occurs during directed stress.

Foliation

Before heat and pressure is applied – mineral crystals are random

After heat and pressure, mineral crystals are aligned into layers - FOLIATION

Can you see the foliation?

Textural Changes

Foliated Textures: Elongate minerals and structural features are forced into parallel alignment by directed (differential) stress.

In your notes fill it in next to picture!

Regional Metamorphism of Shale

If further heat and pressure is applied to slate, the second level of metamorphism will result in the rock phyllite.– Phyllite is shiny and has medium foliation.

If more heat and pressure are applied to phyllite it will result in a third level of metamorphism and the rock schist.– Schist has medium to high foliation and is very

shiny.

schist –

Metamorphic

Forms from-

Phyllite –Metamorphic

Which forms from-

slate-

Which is-Low Grade Metamorphism of shale

Regional Metamorphism of Shale

Finally, if more heat and pressure is applied to schist, it results in the highest grade of metamorphism and the rock gneiss.

– Gneiss shows mineral banding (high foliation).

– So much heat and pressure has been applied that the minerals have aligned themselves into bands of colors.

Banding-

Shale (Sedimentary rock)

Slate (low grade metamorphism)

Phyllite (low to medium grade metamorphism)

Schist (medium to high grade metamporphism)

Gneiss (very high grade metamorphism – shows banding)

Gneiss Rock In Ireland

MetaconglomerateMetamorphic

Form ofConglomerate-

shows theflattened,StretchedPebblesCharacteristicof this rock. There is less space between

the pebbles also…

metaconglomerate –

Metamorphic-

Look at how close

the pebbles are-

Conglomerate-

Sedimentary-

More Space between the pebbles

Contact Metamorphism

• Contact metamorphism occurs on a much smaller level and ONLY includes heat. NO PRESSURE.

• This happens when hot magma comes in direct contact with existing rock and the heat changes the rocks form.

• Contact metamorphism is not strong enough to create foliation.

Marble -

Metamorphic Rock

Limestone –

a parent rock for marble

Sedimentary Rock

Contact Zone

Morph = Change

When Identifying Look for –

DistortionsBanding

Aligned Minerals (Foliation)

Hornfels

Schist

Gneiss