Earth Science 3.4

Metamorphic Rock

Metamorphism

Recall that metamorphic rocks form when existing rocks are changed by heat and pressure.

Metamorphism means

“ to change form”.

Rocks produced during metamorphism often look much different from the original parent rocks.

Formation of Metamorphic Rocks

Most metamorphic changes occur at elevated temperatures and pressures.

These conditions are found a few kilometers below Earth’s surface and extend into the upper mantle.

Most metamorphism occurs in one of two settings

- Contact Metamorphism- Regional Metamorphism

Contact Metamorphism

During contact metamorphism, hot magma moves into rocks.

Contact metamorphism often produces what is called low-grade metamorphism, changes to rocks are minor

Marble, like the marble used in Michelangelo’s sculpture of David, is a common contact metamorphic rock.

Marble often forms when magma intrudes on limestone.

marble

Regional Metamorphism

During mountain building, large areas of rocks are

subjected to extreme pressures and temperatures.

The intense changes produced during these processes are described as high-grade metamorphism.

Regional metamorphism results in large-scale deformation and high grade metamorphism.

Agents of Metamorphism The agents of

metamorphism are:

HeatPressureHydrothermal

solutions

During metamorphism, rocks are usually subjected to all three at the same time.

Heat The most important agent is

heat.

Heat provides the energy needed to drive chemical reactions.

Some of these chemical reactions cause minerals to recrystallize; changing their structure.

Other reactions cause entirely new minerals to form.

Heat

The heat for metamorphism comes from two sources mainly:

○ Magma

○ Change in temperature with depth

Heat Magma, molten rock deep within

the Earth, causes metamorphism to happen when it comes in contact with other cooler rock.

Magma basically “bakes” whatever rocks it comes in contact with.

Heat also comes from the gradual increase in temperature within the crust.

In the upper crust, this increase averages between 20 degrees centigrade and 30 degrees centigrade per kilometer of change.

Pressure or Stress

Pressure, like heat, also increases with depth.

Like the water pressure you feel underwater when you swim in a pool, the pressure on rocks in the Earth is applied in all directions.

Pressure or Stress

Pressure on rocks causes the spaces between the mineral grains to close.

The result is more compact rock with greater density.

This pressure may also cause minerals to recrystallize into entirely new minerals.

Pressure or Stress

Increases in temperature and pressure cause rocks to flow rather than fracture.

Under these conditions, mineral grains tend to flatten and elongate.

Pressure or Stress

During mountain building, horizontal forces caused by the collision of plates metamorphose large parts of the Earth’s crust.

This often produces intricate deep folds in the rocks.

Reactions in Solutions

Water solutions containing other substances that readily change to gases at the surface play an important role in metamorphism.

Solutions that surround mineral grains aid in recrystallization by making it easier for ions to move.

Reactions in Solutions

When these hot water-based solutions escape from a mass of magma, they are called hydrothermal solutions.

As a result of contact with hydrothermal solutions, a change in a rock’s overall composition may occur.

Classification of Metamorphic Rock

Like igneous rocks, metamorphic rocks can be classified by texture and composition into two categories:

Foliated

Nonfoliated

Foliated

When rocks undergo contact metamorphism, they become more compact and thus more dense.

Under more extreme conditions, certain minerals will recrystallize.

Foliated

Some minerals recrystallize with a preferred orientation, which is at right angles to the direction of force.

This alignment gives the rock a layered or striped appearance which we call a foliated appearance.

Nonfoliated Rocks

A metamorphic rock that does not have a banded texture is called a nonfoliated metamorphic rock.

Most nonfoliated rocks contain only one mineral.

Nonfoliated Rocks

Marble, for example, is a nonfoliated rock made of calcite.

When it’s parent rock, limestone, is metamorphosed, the calcite crystals combine to form the larger interlocking crystals seen in marble.