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Essential Science for Teachers: Earth and Space Science

When Continents Collide When Continents Collide | A Closer Look

A Closer Look

Look for the following topics in the video, indicated by the onscreen icon, and click below to learn more.

Metamorphic Rocks

Mountains

A Closer Look: Metamorphic Rocks

What are metamorphic rocks?

Metamorphic rock (left) compared with
sedimentary rock (right).

Metamorphic rock is rock that has physically and chemically changed, or “morphed,” into new rock. The word “metamorphic” has its origins in classical Greek and means “to change form.” Rock of any type (sedimentary, igneous, or metamorphic) that is subjected to high pressure, high temperatures, and/or reactions with chemical solutions can be converted to metamorphic rock. This transformation can involve changes in a rock’s texture (grain size and shape), fabric (how the grains are oriented relative to one another), chemical composition, and mineral content. The rock remains solid as these changes occur. Through the process of metamorphism, the original rock, or protolith, changes into a new metamorphic rock.

How are these high pressures and temperatures generated?

The answer lies in the processes of plate tectonics. Plates that move against each other produce huge forces that create high pressures and temperatures that can deform rock by bending or breaking it. Rock can also be buried and metamorphosed when plates collide. Temperatures and pressure within the Earth increase with depth, so that rock deep in the crust will experience extreme heat and pressure. Rock can also be subjected to high temperatures in regions of volcanism as well as in places beneath the Earth where magma intrudes into the rock above it.

What types of metamorphism exist?

Regional Metamorphism: Many metamorphic rocks form by regional metamorphism, named for the large areas of the crust that are affected. Regional metamorphism usually results from mountain building processes, which are caused by the collision of tectonic plates. These collisions compress and thicken the crust and cause considerable rock deformation.

High-Pressure Metamorphism: Some metamorphic rock forms at high pressures but at temperatures that are relatively low. This type of metamorphism occurs at subduction zones. Here, high pressures result when one plate is submerged under the mantle. Temperatures remain relatively low because the crust that forms the upper part of the subducting plate is cool, having been close to the Earth’s surface. As the plate subducts, it actually cools the mantle. The subducting plate reaches high pressures faster than it heats to high temperatures, and this pressure is enough to cause metamorphism.

High-Temperature Metamorphism: Some metamorphic rock forms at high temperatures but without high pressures. This occurs near hot intrusions of magma from the mantle into the crust. Rock that is in contact with these intrusions undergoes contact, or thermal, metamorphism. This heat causes minerals to react with each other, which produces new minerals.

Hydrothermal Metamorphism: This process is associated with contact metamorphism. When very large masses of magma — called plutons — intrude from the mantle into the crust, a great amount of heat is generated. This huge body of hot magma creates a heat source that can cause fluids in the crust to circulate. Chemical reactions occur as a result of this circulation. This type of metamorphism is common near mid-oceanic ridges and around large plutonic intrusions in the crust.

A Closer Look: Mountains

What generalizations can be made about mountain building?

Mountain

The cause of all mountain building is related to the movement of tectonic plates. Convergent plate boundaries, in particular, are places where mountains form. At these boundaries, one plate meets another. There are two types of convergent plate boundaries and mountains form differently at each.

Mountain Building Related To Subduction Zones

Subduction zones are places where the edge of one plate is forced under the edge of another plate. At subduction zones, mountains can be formed in two ways:

1) Pieces of buoyant lithosphere (the crust fused to the upper part of the mantle) riding on top of the downgoing plate may eventually be brought to the convergent boundary. Examples of buoyant lithosphere with continental crust include small continental fragments and island arcs (an arc-shaped formation of volcanoes built up from the sea floor). Buoyant lithosphere with oceanic crust includes oceanic plateaus (broad regions of thick oceanic crust). Regardless of type, buoyant lithosphere cannot be subducted when it is of the same density or less dense than the material with which it is colliding. As it merges with the overriding slab, the buoyant lithosphere attaches, or “accretes,” itself to the slab’s edge. Over time, a type of fold-thrust mountain belt can be created in which folded mountains appear as rock is pushed upward. The Coast Range of California and the Sierra Nevadas are two parallel mountain ranges formed at least partly in this way.

2) When an oceanic plate subducts, it brings with it materials, like water, that can induce melting in the mantle. This melting can lead to volcanism and the creation of mountain ranges. Examples of mountain ranges created by volcanism at subduction zones include the Andes Mountains in South America and the Cascade Mountains in the western United States.

Mountain Building Related To Continental Collision

In Session 5, we focused on mountain building that occurs where the once oceanic lithosphere between two continents completely subducts and the continental crust riding atop each plate collides. Continental collision is a special case of convergence. One continent may slide a short distance under the other, but continental crust never subducts. The two continents essentially weld together. Intense compression gradually squeezes rock upwards (and downwards) deforming and thickening the crust to create mountains. This is how the Appalachian Mountains formed and how the Himalaya Mountains are forming today. In the Appalachians, geologists can trace back hundreds of millions of years to identify three distinct collisions that each helped to create the mountains we see today. As a result, the Appalachians are an ideal tectonic setting to study metamorphic rocks.

Series Directory

Essential Science for Teachers: Earth and Space Science

Credits

Produced by Harvard-Smithsonian Center for Astrophysics. 2004.
  • Closed Captioning
  • ISBN: 1-57680-742-8

Sessions