Metamorphic Rocks

Nonfoliated Metamorphic Rocks

A metamorphic rock that does not have a banded texture

How does heat affect metamorphic rocks?

Heat causes chemical reactions to take place. These chemical reactions may cause recrystallization of existing minerals or the formation of new minerals.

Nonfoliated Metamorphic Rocks 1

Most nonfoliated rocks contain only one mineral. Marble, for example, is a nonfoliated rock made of calcite. When its parent rock, limestone, is metamorphosed, the calcite crystals combine to form the larger interlocking crystals seen in marble. Quartzite and anthracite are other nonfoliated metamorphic rocks.

Pressure (Stress)

Pressure, like temperature, also increases with depth. Like the water pressure you might have experienced at the bottom of a swimming pool, pressure on rocks within Earth is applied in all directions. Pressure on rocks causes the spaces between mineral grains to close. The result is a more compact rock with a greater density. This pressure also may cause minerals to recrystallize into new minerals. Increases in temperature and pressure cause rocks to flow rather than fracture. Under these conditions, mineral grains tend to flatten and elongate.

What property distinguishes foliated metamorphic rocks, such as gneiss and schist, from nonfoliated metamorphic rocks?

a banded texture

Where do most metamorphic changes to rock occur?

a few kilometers below Earth's surface

Metamorphism

a very appropriate name for this process because it means to change form

What changes rocks during a process of regional metamorphism?

extreme pressures and heat from Earth's interior

In a metamorphic rock, the alignment of the crystals or minerals determines whether the rock is _____.

foliated or nonfoliated

Metamorphic rocks

form when existing rocks are changed by heat and pressure

Regional Metamorphism

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

When rocks undergo metamorphic changes, chemical reactions cause minerals to recrystallize or change form. Which agent provides the energy for these reactions?

heat

What are the three agents of metamorphism?

heat, pressure, and hydrothermal solutions

Heat 1

The most important agent of metamorphism is heat. Heat provides the energy needed to drive chemical reactions. Some of these reactions cause existing minerals to recrystallize. Other reactions cause new minerals to form. The heat for metamorphism comes mainly from two sources—magma and the change in temperature with depth. Magma essentially "bakes" any rocks that are in contact with it. Heat also comes from the gradual increase in temperature with depth. In the upper crust, this increase averages between 20°C and 30°C per kilometer.

Contact Metamorphism 1

Contact metamorphism often produces what is described as low-grade metamorphism. Such changes in rocks are minor. Marble, like that used to make the statue in the figure, is a common contact metamorphic rock. Marble often forms when magma intrudes into a limestone body.

Regional Metamorphism 1

During mountain building, large areas of rocks are subjected to extreme pressures and temperatures. The intense changes produced during this process are described as high-grade metamorphism.

Metamorphic Rocks 1

Metamorphic rocks form when existing rocks are changed by heat and pressure. Rocks produced during metamorphism often look much different from the original rocks, or parent rocks. The folds in the rocks are formed when the parent rocks were subjected to intense forces. These highly folded metamorphic rocks may also develop a different composition than the parent rocks had.

Formation of Metamorphic Rocks 1

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 or regional metamorphism.

Agents of Metamorphism 1

The agents of metamorphism are heat, pressure, and hydrothermal solutions. During metamorphism, rocks are usually subjected to all three of these agents at the same time. However, the effect of each agent varies greatly from one situation to another.

Foliated Metamorphic Rocks 2

Under more extreme conditions, certain minerals will recrystallize. Some minerals recrystallize with a preferred orientation, which is at right angles to the direction of the force. The resulting alignment usually gives the rock a layered or banded appearance.

Reactions in Solution

Water solutions containing other substances that readily change to gases at the surface play an important role in some types of metamorphism. Solutions that surround mineral grains aid in recrystallization by making it easier for ions to move. When solutions increase in temperature reactions among substances can occur at a faster rate. When solutions increase in temperature reactions among substances can occur at a faster rate. These hot fluids also promote recrystallization by dissolving original minerals and then depositing new ones. As a result of contact with hydrothermal solutions, a change in a rock's overall composition may occur.

Heat 2

When buried to a depth of about 8 kilometers, clay minerals are exposed to temperatures of 150°C to 200°C. These minerals become unstable and recrystallize to form new minerals that are stable at these temperatures, such as chlorite and muscovite. In contrast, silicate minerals are stable at these temperatures. Therefore, it takes higher temperatures to change silicate minerals.

Hydrothermal Solutions

When hot, water-based solutions escape from a mass of magma

Contact Metamorphism

When magma intrudes—forces its way into—rock

Foliated Metamorphic Rocks 1

When rocks undergo contact metamorphism, they become more compact and thus more dense. A common example is the metamorphic rock slate. Slate forms when shale is subjected to temperatures and pressures only slightly greater than those at which the shale formed. The pressure on the shale causes the microscopic clay minerals to become more compact. The increase in pressure also causes the clay minerals to align in a similar direction.

In undeformed strata, where are the rocks typically the densest?

in the lowest layers

Unlike regional metamorphism, contact metamorphism occurs when _____.

magma intrudes into existing rock