Metamorphic Rock Formation Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 1, 2011

Hydrothermal Metamorphism

This type of metamorphism is common with mid-ocean ridges where the crust is spreading and growing as a result of the outpouring of hot lava. The ocean water that bubbles through the hot, fractured basalts of the ridge margins becomes heated, causing chemical reactions between the surrounding ridge rock and seawater. These chemical changes produce metamorphosed basalt.

Hydrothermal metamorphism can also take place on land, when fluids from igneous rock intrusions percolate through surrounding country rock, causing a regional metamorphism .


Higher temperature and pressure metamorphic boundaries mark the lower limits of magma production. With a good amount of water, magma formation starts at a lower temperature. When there is little water, magma doesn’t form until higher temperatures are reached. This allows different types of metamorphic rock (schists, gneisses, and amphibolites) to form in different areas depending on the amount of fluid present.

Different types of layering are also possible depending on fluid intrusion, as well as temperature and pressure factors. When there is a variety of metamorphic rock types in an area, geologists find that a combination (mixed) rock has formed. Alternating layers of granite and schist form a mixed rock called migmatite .

A combination metamorphic rock type that contains both igneous and metamorphic rock is known as migmatite.

Practice problems of this concept can be found at: Metamorphic Rock Practice Test

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