Geology 111G/Lecture 10

Metamorphic Rocks

Metamorphism

Types of Pressure

Sources of Fluid

Types of Metamorphism

I. Metamorphic Rocks: third major class of rocks, which have undergone changes in mineralogy, texture, and or chemical composition.

II. Metamorphism: a set of processes by which rocks undergo these changes. Usually involves heat, pressure and often fluids. The pressure and heat are the result of three factors:

A. Earth's internal heat. Results in recrystallization and dehydration of minerals.

B. Pressure. Also known as stress. There are two types:

1. Confining (lithostatic) pressure. Stress is equal or uniform in all directions, caused by burial. This is the weight of the overlying rock. Results in volume reduction and conversion to minerals of increased density.

2. Directed pressure. Horizontal stresses or pressure developed by movement of the plates or gravity. Dominant stress operates in a specific direction. Reorients minerals, favoring planar or linear fabric in rock.

a. Foliation: planar fabric in rock resulting from alignment of platy or fibrous minerals. If the foliation is weak, it is termed slaty cleavage.

3. Grade of a metamorphic rock: as temperature and pressure rise, rocks are considered according to those conditions.

a. Low-grade rocks: formed at shallow crustal conditions.

b. High-grade rocks: formed at deep crustal conditions.

C. Fluids. Solutions rich in various ions may react with existing rocks, causing changes in chemical composition, as well as mineralogy. Metasomatism: Alteration of rocks by fluid transport of elements into or out of the rocks. Sources of fluids include:

1. Ground water or formation water present in the pore spaces of the rocks.

2. Solutions from dehydration reactions as minerals lose their structural water, i.e., the reaction amphibole to pyroxene.

3. Hydrothermal fluids derived from cooling igneous rocks or recirculating fluids that have been buried very deeply.

III. Types of metamorphism. Metamorphism takes place under a variety of temperature and pressure conditions that have been determined in the lab and through field studies.

A. Regional metamorphism. High temperature and high pressure are imposed over large volumes of the crust. This takes place in the vicinity of volcanic arcs, subduction zones, or collisional mountain belts where two continental plates converge. Tends to obliterate original features of igneous or sedimentary rock as new minerals grow. Pressure is commonly directed.

B. Burial metamorphism. Caused by widespread burial of sedimentary and volcanic rocks. By definition diagenesis takes place up to 200° C. and metamorphism takes place above that. Pressure here is dominantly lithostatic.

C. Contact metamorphism. Transformation of rocks at or near igneous intrusions. These zones or aureoles can be wide or narrow depending upon the size of the pluton and its temperature.

D. Hydrothermal metamorphism. Typically refers to interaction of hot rocks with sea water, usually near mid-ocean ridges or where basalts come into contact with sea water.