A. When studying metamorphic rocks, we have to consider that the rock was originally something else before it suffered metamorphism. We refer to that something else as the parent rock (or protolith).

B. Factors that control metamorphism

1. Composition

a. Usually a parent rock will metamorphose into a rock that has approximately the same composition as the original parent rock.

b. Example: a basalt metamorphoses to a mineralogy that collectively has about 50% silica and high oxides of iron, magnesium, calcium, and aluminum found in the original rock.

c. Example: a limestone composed mostly of calcite metamorphoses to a calcite rich marble rather than a quartzite.

2. Pressure

a. Refers to pressure applied equally on all sides of a body (ex., water pressure on a diver's suit)

b. Pressure on rocks comes from the burial of overlying material and sometimes from two tectonic plates colliding together.

c. Some minerals are more stable at higher pressures than at low pressures. Ex: graphite (low P) and diamond (hi P)

3. Heat

a. As with pressure, some minerals are more stable at higher temperatures than at lower temperatures.

b. Sources of heat include

i. geothermal gradient

ii. igneous intrusion

4. Hot fluids often trigger chemical reactions during metamorphism.

C. How do we know if a rock is metamorphic? By looking at its texture.

1. Foliation occurs in metamorphism when pressure is high enough in one orientation so as to flatten the parent rock material or to induce the platy minerals to crystallize in an orientation perpendicular to the direction of the highest pressure.

2. There are three main types of foliation

a. Slaty cleavage -- found in a rock that splits easily along flat, parallel planes in response to the parallel alignment of platy, microscopic minerals.

b. Schistose texture is identified by its visible platy or needle-shaped minerals generally aligned parallel to a plane.

c. Gneissic (pronounced nice - ick) texture occurs in rocks that become very plastic, and the minerals segregate into separate layers, like a sandwich. Layers are usually distinguished by dark and light layers.

3. Not all metamorphic rocks have foliation. In fact, metamorphic rocks are distinguished by whether they are foliated or non-foliated.

a. Foliated metamorphic rocks are usually given names that indicate their mineralogy and texture, for example, a mica schist or amphibole schist. Also, slate, or amphibole quartz gneiss.

b. Non-foliated metamorphic rocks include quartzites and marbles. Non-foliated metamorphic rocks are usually very crystalline or sugary in appearance.

A. Contact metamorphism occurs when high temperature is the dominant factor. Pressure is not a great factor.

1. When magma intrudes cooler country rock (country rock is the rock into which magma intrudes), the margins of the magma chamber are baked.

2. The baked margin around the intrusion is called the metamorphic aureole.

3. Contact metamorphism usually produces non-foliated metamorphic rocks.

B. Regional metamorphism occurs under conditions of high pressure and temperature.

1. This type of metamorphism occurs in the most intensely deformed parts of mountain ranges, usually at deeply buried cores of mountains.

2. Temperatures and pressures may vary in regional metamorphism. As a result, the same parent rock material may metamorphose into different metamorphic rocks, depending on the temperature and pressure.

Last update 2/23/2005
Webpage designed by Hiram Jackson.
Contact Geology webmanager, Hiram Jackson, at jacksoh@crc.losrios.edu