Metamorphic Rocks

Metamorphic rock

Metamorphic rock marble

An example of low-grade metamorphism is shale becoming slate. Metamorphic Rocks Metamorphic rocks are those that have changed their form due to external environmental issues. Metamorphic rocks are produced from preexisting igneous, sedimentary, or even other metamorphic rocks. Metamorphism takes place where preexisting rock is subjected to temperatures and pressures unlike those in which it formed. There are different degrees of metamorphism which can be seen in the rock’s texture and in its mineralogy. An example of low-grade metamorphism is shale becoming slate. In high-grade metamorphism, obliteration of the rock’s original features occurs.

Metamorphic Rocks There are two basic types of rock metamorphism: thermal or contact metamorphism regional metamorphism

Metamorphic Rocks Contact metamorphism is a type of metamorphism in which the mineralogy and texture of a body of rock are changed by exposure to the pressure and extreme temperature associated with a body of intruding magma. Contact metamorphism often results in the formation of valuable minerals, such as garnet and emery, through the interaction of the hot magma with adjacent rock.

Metamorphic Rocks During mountain-building, great quantities of rock are subjected to directed pressures and high temperatures associated with large-scale deformation called regional metamorphism. Regional metamorphism is a type of metamorphism in which the mineralogy and texture of rocks are changed over a wide area by deep burial and heating associated with the large-scale forces of plate tectonics. In regional metamorphism, rocks that form closer to the margin of the tectonic plates, where the heat and pressure are greatest, often differ in their minerals and texture from those that form farther away.

What Drives Metamorphism? The driving agents of metamorphism include: heat pressure (stress) from burial (confining pressure) or from differential stress during mountain building chemically active fluids (mainly water and other volatiles)

Pressure as a Metamorphic Agent

Metamorphic Textures The degree of metamorphism is reflected in the rock’s texture and mineral composition. When rocks are subjected to low-grade metamorphism, they become more compact and more dense (like slate.) With high-grade metamorphism, stress causes certain minerals to re-crystallize. Consequently, many metamorphic rocks consist of visible crystals, much like coarse-grained igneous rocks.

Metamorphic Textures The crystals of some minerals will recrystallize with a preferred orientation, essentially perpendicular to the direction of the compression force. The resulting mineral alignment usually gives the rock a layered or banded appearance termed foliated texture. Examples: schist, gneiss

Some metamorphic rocks have a nonfoliated texture. Metamorphic Textures Some metamorphic rocks have a nonfoliated texture. Metamorphic rocks composed of only one mineral that forms equidimensional crystals are, as a rule, not visibly foliated (examples: quartzite and marble.)

Common Metamorphic Rocks

Classification of Metamorphic Rocks

The Launch Pad Tuesday, 2/28/17 1/3 Identify the rocks. shale slate

The Launch Pad Tuesday, 2/28/17 2/3 Identify the rock. gneiss

The Launch Pad Tuesday, 2/28/17 3/3 Identify the rock. marble