This lesson on metamorphic rocks, the third major category of rocks in the rock cycle, completes our description of earth materials (rocks and minerals). The information on igneous and sedimentary processes in previous chapters should help you understand metamorphic rocks, which form from pre-existing rocks.

After reading the chapter on weathering, you know how rocks are altered when exposed at Earth's surface. Metamorphism (a word from Latin and Greek that means literally "changing of form") also involves alterations, but the changes are due to deep burial, tectonic forces, and/or high temperature rather than surface conditions.

As you study this lesson, try to keep clearly in mind how the chemical composition of a rock and the temperature, pressure, and water present each contribute to the metamorphic process and the resultant metamorphic rock.

We also discuss the hydrothermally deposited rocks and minerals, which are usually found in association with both igneous and metamorphic rocks. Hydrothermal ore deposits, while not volumetrically significant, are of great importance to the world's supply of metals.

Because nearly all metamorphic rocks form deep within the earth's crust, they provide geologists with many clues about conditions at depth. Therefore, understanding metamorphism will help you when we consider geologic processes involving Earth's internal forces. Metamorphic rocks are a feature of the oldest exposed rocks of the continents and of major mountain belts. They are especially important in providing evidence of what happens during subduction and plate convergence.

Questions

What is metamorphic rock?

What is regional metamorphism?

The factors that cause regional metamorphism?

What are source of heat inside the earth crust?

How the heat affect the rock?

How the pressure affect the rock?

What is contact metamorphism? The main factor cause of contact metamorphism?

What is dynamic metamorphism? he main factor cause of dynamic metamorphism?

What is aureole?

What is protolith?

What is mylonite?

What is catalasite?

What is foliation?

What are different types of foliated texture?

What is the cause of foliation?

How are metamorphic rocks classified?

What are the principal agents of metamorphism?

What are different types of rock formed by increasing metamophism?

Write the list of metamorphic rocks in order from highest to lowest grade?

Write the names of non-foliated metamorphic rock?

Extra readings

1. Metamorphism changes the texture or mineralogy (or both) of its parent rock usually in response to high temperature and pressure within the earth's interior and under conditions that produce ductile (plastic) strain.

2. The chemical composition of the parent rock controls that of the metamorphic rock, although mineralogy may change. Minerals are stable within a particular temperature range, but that range varies with pressure and presence of other substances. The upper limits of metamorphism may overlap partial melting.

3. New metamorphic minerals crystallize under high confining pressure and tend to be denser than their low-pressure counterparts. Differential stress (compression and/or shearing) produces foliated textures described as slaty, schistose, or gneissic. Water triggers metamorphism and promotes new mineral formation. Metamorphism is a slow process taking place over millions of years.

4. The classification of metamorphic rocks is based on texture (foliated versus nonfoliated) and mineralogy (chemical composition is controlled by parent rock).

5. Metamorphism is either contact (high temperatures but low confining pressure) or regional (high-temperatures and high confining pressure). Metamorphic rocks produced in a contact aureole ("baked zone" ) include hornfels (shale parent rock), marble (limestone parent rock), and quartzite (quartz sandstone parent rock). All of these rocks have nonfoliated textures.

6. Regional metamorphism usually produces foliated rocks such as greenschist (basalt parent rock) and amphibole schist (basalt parent rock), although marble and quartzite also form, if their appropriate parent is present. Progressive metamorphism of shale can produce slate, phyllite, schist or gneiss as temperature and pressure increase. Partial melting produces migmatites. Retrogressive metamorphism reflect the effects of water movement that allows recrystallization under conditions below the peak of metamorphism.

7. Plate tectonics, particularly subduction, explain differential stress and temperature variations, which increase toward the continents because of rising magma from melting at depth.

8. Water plays an important role in metamorphism. Metasomatism involves hot water transporting ions from outside the rock, and form significant ore deposits. Hydrothermal rocks are formed by crystallization from hot water, most commonly quartz veins and disseminated ore deposits.

9. The water involved in metamorphism originates as either ground water or water trapped in descending oceanic crust in a subduction zone.

Technical terms

compressive stress

A stress due to a force pushing together on a body.

confining pressure

Pressure applied equally on all surfaces of a body; also called geostatic or lithostatic pressure.

contact metamorphism

Metamorphism under conditions in which high temperature is the dominant factor.

differential stress

When pressures on a body are not of equal strength in all directions.

ductile (plastic)

Capable of being molded and bent under stress.

foliation

Parallel alignment of textural and structural features of a rock.

gneiss

A metamorphic rock composed of light and dark layers or lenses.

gneissic

The texture of a metamorphic rock in which minerals are separated into light and dark layers or lenses.

hornfels

A fine-grained, unfoliated metamorphic rock.

hydrothermal rock

Rock deposited by precipitation of ions from solution in hot water.

isotherm

A line along which the temperature of rock (or other material) is the same.

marble

A coarse-grained rock composed of interlocking calcite (or dolomite) crystals.

metamorphic facies

Metamorphic rocks that contain the same set of pressure or temperature sensitive minerals are regarded as belonging to the same facies, implying that they formed under broadly similar pressure and temperature conditions.

metamorphic rock

A rock produced by metamorphism.

metamorphism

The transformation of preexisting rock into texturally or mineralogically distinct new rock as a result of high temperature, high pressure, or both, but without the rock melting in the process.

metasomatism

Metamorphism coupled with the introduction of ions from an external source.

migmatite

Mixed igneous and metamorphic rock.

parent rock

Original rock before being metamorphosed.

phyllite

A metamorphic rock in which clay minerals have recrystallized into microscopic micas, giving the rock a silky sheen.

quartzite

A rock composed of sand-sized grains of quartz that have been welded together during metamorphism.

regional metamorphism

Metamorphism that takes place at considerable depth underground.

schist

A metamorphic rock characterized by coarse-grained minerals oriented approximately parallel.

schistose

The texture of a rock in which visible platy or needle-shaped minerals have grown essentially parallel to each other under the influence of directed pressure.

shearing

Movement in which parts of a body slide relative to one another and parallel to the forces being exerted.

slate

A fine-grained rock that splits easily along flat, parallel planes.

slaty

Describing a rock that splits easily along nearly flat and parallel planes.

slaty cleavage

The ability of a rock to break along closely spaced parallel planes.

stress

A force acting on a body, or rock unit, that tends to change the size or shape of that body, or rock unit. Force per unit area within a body.

vein

Fracture in rock usually filled with late stage magmatic minerals and often containing metal ore.