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.

• Foliation - Laminated structure in a metamorphic rock resulting from the parallel alignment of sheet-like minerals (usually micas).

Examples:

• Slate - Mica flakes are microscopic in size. Derived from the regional metamorphism of shale.


Slate. Relict sedimentary bedding (vertical) can be seen in this sample. Photo courtesy of Pamela Gore.

• Phyllite - Mica flakes are very fine-grained; other minerals such as garnet may also be present. Derived from the regional metamorphism of shale.


Phyllite. Photo courtesy of Pamela Gore.

• Schist - Mica flakes are visible to the unaided eye. Named for the most conspicuous minerals present, as in mica schists, chlorite schists, etc.
Derived from the regional metamorphism of shales or fine-grained volcanic rocks.


Muscovite schist. Photo courtesy of Pamela Gore.

• Gneiss - Coarse-grained rock with minerals segregated into light and dark layers or bands. Derived from the regional metamorphism of high-silica igneous rocks, and muddy sandstones.


Gneiss. Photo courtesy of Pamela Gore.

• Non-foliated or granular metamorphic rocks are those which are composed of equidimensional grains such as quartz or calcite. There is no preferred orientation. The grains form a mosaic.

  Examples:

  • Marble - Composed of finely- to coarsely-crystalline calcite or dolomite. Derived from the metamorphism of limestone or dolostone.
  • Quartzite - Composed of finely- to coarsely-crystalline quartz. Derived from the metamorphism of quartz sandstone.
  • Greenstone - Contains iron- and magnesium-rich green minerals such as chlorite and epidote. Fine-grained texture. Derived from the low-grade metamorphism of basalt.
  • Hornfels - Very hard, fine-grained rock. Derived from the contact metamorphism of shale and other fine-grained rocks.

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.

• Chlorite and muscovite form at relatively low temperatures.
• Biotite and garnet form at higher temperatures and pressures.
• Staurolite and kyanite form at intermediate to high temperatures and pressures.
• Sillimanite forms at the highest temperatures and pressures.

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

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