4.13 Metamorphic Rocks

Can you decipher the history of this rock?

The rock in this photo is a banded gneiss. The bands are of different composition, more felsic and more mafic, that separated as a result of heat and pressure. The waviness of the bands also shows how the rock was hot enough to alter but not to melt all the way.

Metamorphism

Any type of rock – igneous, sedimentary, or metamorphic — can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of different chemical composition. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Hornfels is shown in the table for the "Metamorphic Rock Classification" concept.

Texture

Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (Figure below). Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated. Quartzite and marble, shown in the concept "Metamorphic Rock Classification," are non-foliated.

Types of Metamorphism

The two main types of metamorphism are both related to heat within Earth:

1. Regional metamorphism: Changes in enormous quantities of rock over a wide area caused by the extreme pressure from overlying rock or from compression caused by geologic processes. Deep burial exposes the rock to high temperatures.
2. Contact metamorphism: Changes in a rock that is in contact with magma. The changes occur because of the magma’s extreme heat.

Summary

• Any type of rock - igneous, sedimentary or metamorphic - can become a metamorphic rock.
• Foliated rocks form when rocks being metamorphosed are exposed to pressure in one direction.
• Regional metamorphism occurs over a large area but contact metamorphism occurs when a rock is altered by a nearby magma.

Review

1. Why do changes in temperature or pressure cause rocks to change?
2. What are the similarities and differences in conditions that cause regional versus contact metamorphism?
3. What causes foliation in a metamorphic rock? Under what circumstances would you expect this to happen?