Metamorphic Rocks

Metamorphism occurs when any previously existing rock, the parent rock, is buried in the earth under layers of other rock. The deeper the rock is buried the hotter it gets, and the higher the pressure becomes. Eventually, rock must adjust to the new conditions, whether it is baked, or squeezed, or both, and in the process becomes a metamorphic rock.

Temperature The temperature is going up because the center of the earth is molten, and the closer we get to that heat the hotter it gets. This rise in temperature with depth is called the geothermal gradient.

Pressure The pressure is going up because,... well, imagine having a pile of rocks ten miles thick sitting on your head. This is hydrostatic pressure. It is equal in all directions. But, there is also directed pressure, squeezing pressure that happens when two continents collide at subduction zones of plate boundaries.

Metamorphism - The mineralogical and textural changes that occur in a rock in response to changes in temperature and pressure, short of melting

Two Types of Metamorphic Rocks There are two basic types of metamorphic rocks –Foliated Rocks can be observed in the field by parallel bands within the rocks or it can often be observed on a smaller scale in the hand specimen. –Non Foliated Rocks as the name implies, does not have any parallel orientation of the grains within the metamorphic rock. Non foliated rocks have recrystallized without producing parallel structures.

Contact Metamorphism When magma intrudes into host rock, localized contact metamorphism occurs along the contact between the pre-existing rock mass and the cooling magma. Contact metamorphism occurs at shallower levels of the crust, where the pressure is relatively low. Contact metamorphism commonly produces fine-grained rocks that lack foliation.

Regional Metamorphism Regional metamorphism occurs over broad areas of the crust. This occurs in deep basins where sediments or sedimentary rocks have accumulated. At a depth of about 10 kilometers, the confining pressure of the overlying material combined with geothermal heat is great enough to metamorphose rocks. Because the compression does not impose a directed pressure, metamorphic rocks formed from burial are non foliated and lack any banding of minerals.

Formation of Foliated Rocks Along the boundary of tectonic plates where collision or subduction is occurring, directed pressure is exerted on rock. Under great pressure, mineral grains may be smeared or partially melted and recrystallized into bands aligned perpendicular to the direction of greatest pressure. This creates foliated metamorphic rocks with minerals in distinct bands.

Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of amphibole and plagioclase, usually with very little quartz.

Gneiss is foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals.

Hornfels is a fine-grained nonfoliated metamorphic rock with no specific composition. It is produced by contact metamorphism. Hornfels is a rock that was "baked" while near a heat source such as a magma chamber, sill or dike.

Marble is a non-foliated metamorphic rock that is produced from the metamorphism of limestone. It is composed primarily of calcium carbonate.

Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz.

Schist is metamorphic rock with well developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. The specimen shown above is a "muscovite schist" because it contains a significant amount of muscovite mica.