The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin


Chapter 4 - page 9

Rocks and Minerals: Documents that Record Earth's History


Metamorphic Rocks

The word metamorphic means "changed form."
Metamorphism causes changes in the texture and mineralogy of other rocks.

Metamorphism is caused by:

  1. High temperatures.
  2. High pressures.
  3. Chemical reactions caused by solutions and hot gases.


Types of Metamorphism

  1. Contact metamorphism - Alteration of rock by heat adjacent to hot molten lava or magma.
    Economically important as setting for metallic ores (gold, silver, copper, lead, zinc, etc.)
  2. Regional or dynamothermal metamorphism - Alteration of rock over a large area by heat and pressure due to deep burial or tectonic processes.


Types of Metamorphic Rocks

Metamorphic rocks may be separated into two groups on the basis of texture.


Metamorphic Index Minerals

Certain minerals form during metamorphism, under specific pressure and temperature conditions. These minerals can be used as a guide to metamorphic pressures and temperatures. They are called metamorphic index minerals.

From studies of minerals in metamorphic rocks it is possible to determine the conditions under which the rocks formed.

metamorphic index minerals
Diagram showing the metamorphic index minerals which form with increasing temperature and pressure as a shale is metamorphosed.


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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

September 11-13, 2005