Which Of The Following Forces Applies Stress Or Pressure That Is Equal In All Directions?

GEY 101 - Introductory Geology: Exploring Planet Earth
Metamorphic Rocks

Metamorphism -- The transition of one rock into another by temperatures and/or pressures unlike those in which it formed

Metamorphic rocks are produced from:

Igneous rocks
Sedimentary rocks
Other metamorphic rocks

Metamorphism progresses incrementally from low-grade to high-grade

During metamorphism the rock must remain essentially solid

Metamorphic settings

Contact or thermal metamorphism �- driven by a rise in temperature within the host rock
Hydrothermal metamorphism �- chemical alterations from hot, ion-rich water
Regional metamorphism --
- Occurs during mountain building
- Produces the greatest volume of metamorphic rock
- Rocks usually display zones of contact and/or hydrothermal metamorphism

Agents of metamorphism

Heat
- The most important agent
- Recrystallization results in new, stable minerals
- Two sources of heat
  - Contact metamorphism � heat from magma
  - An increase in temperature with depth due to the geothermal gradient
Pressure (stress)
- Increases with depth
- Confining pressure applies forces equally in all directions
- Rocks may also be subjected to differential stress which is unequal in different directions
- Brittle � tend to fracture when subjected to differential stress; low T
- Ductile � mineral grains tends to flatten and elongate when subjected to differential stress; high T
Chemically active fluids
- Mainly water with other volatile components
- Enhances migration of ions
- Aids in recrystallization of existing minerals
- Sources of fluids
  - Pore spaces of sedimentary rocks
  - Fractures in igneous rocks
  - Hydrated minerals such as clays and micas

The importance of parent rock

Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed
Mineral makeup determines, to a large extent, the degree to which each metamorphic agent will cause change

Metamorphic textures

Texture refers to the size, shape, and arrangement of grains within a rock
Foliation � any planar arrangement of mineral grains or structural features within a rock
- Examples of foliation
  - Parallel alignment of platy and/or elongated minerals
  - Parallel alignment of flattened mineral grains and pebbles
  - Compositional banding
  - Slaty cleavage where rocks can be easily split into thin, tabular sheets
- Foliation can form in various ways including
  - Rotation of platy and/or elongated minerals
  - Recrystallization of minerals in the direction of preferred orientation
  - Changing the shape of equidimensional grains into elongated shapes that are aligned
Foliated textures
- Rock or slaty cleavage
  - Closely spaced planar surfaces along which rocks split
  - Can develop in a number of ways depending on metamorphic conditions and parent rock
- Schistosity
  - Platy minerals are discernible with the unaided eye and exhibit a planar or layered structure
  - Rocks having this texture are referred to as schist
- Gneissic
  - During higher grades of metamorphism, ion migration results in the segregation of minerals
  - Gneissic rocks exhibit a distinctive banded appearance
- Other metamorphic textures
  - Those metamorphic rocks that lack foliation are referred to as nonfoliated
    - Develop in environments where deformation is minimal
    - Typically composed of minerals that exhibit equidimensional crystals
  - Porphyroblastic textures
    - Large grains, called porphyroblasts, surrounded by a fine-grained matrix of other minerals
    - Porphyroblasts are typically garnet, staurolite, and/or andalusite

Common metamorphic rocks

Foliated rocks

Slate
- Very fine-grained
- Excellent rock cleavage
- Most often generated from low-grade metamorphism of shale, mudstone, or siltstone
Phyllite
- Gradation in the degree of metamorphism between slate and schist
- Platy minerals not large enough to be identified with the unaided eye
- Glossy sheen and wavy surfaces
- Exhibits rock cleavage
- Composed mainly of fine crystals of muscovite and/or chlorite
Schist
- Medium- to coarse-grained
- Platy minerals predominate
- Commonly include the micas
- The term schist describes the texture
- To indicate composition, mineral names are used (such as mica schist)
Gneiss
- Medium- to coarse-grained
- Banded appearance
- High-grade metamorphism
- Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian minerals

Nonfoliated rocks

Marble
- Coarse, crystalline
- Parent rock was limestone or dolostone
- Composed essentially of calcite or dolomite crystals
- Used as a decorative and monument stone
- Exhibits a variety of colors
Quartzite
- Formed from a parent rock of quartz-rich sandstone
- Quartz grains are fused together

Metamorphic environments

Contact or thermal metamorphism
- Occurs due to a rise in temperature when magma invades a host rock
- A zone of alteration called an aureole forms in the rock surrounding the magma
- Most easily recognized when it occurs at the surface, or in a near-surface environment
Hydrothermal metamorphism
- Chemical alteration caused when hot, ion-rich fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock
- Most widespread along the axis of the mid-ocean ridge system
Regional metamorphism
- Produces the greatest quantity of metamorphic rock
- Associated with mountain building
Burial metamorphism
- Associated with very thick sedimentary strata
- Required depth varies from one location to another depending on the prevailing geothermal gradient
Metamorphism along fault zones
- Occurs at depth and high temperatures
- Pre-existing minerals deform by ductile flow
Impact metamorphism
- Occurs when high speed projectiles called meteorites strike Earth�s surface
- Products are called impactites

Metamorphic zones

Systematic variations in the mineralogy and often the textures of metamorphic rocks are related to the variations in the degree of metamorphism
Index minerals and metamorphic grade
- Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism
- Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form
- Migmatites
  - Highest grades of metamorphism that is transitional to igneous rocks
  - Contain light bands of igneous components along with areas of unmelted metamorphic rock

Metamorphism and plate tectonics

Most metamorphism occurs along convergent plate boundaries
- Compressional stresses deform the edges of the plate
- Formation of the Earth�s major mountain belts including the Alps, Himalayas, and Appalachians
Large-scale metamorphism also occurs along subduction zones at convergent boundaries
- Several metamorphic environments exist here
- Important site of magma generation
Metamorphism at subduction zones
- Mountainous terrains along subduction zones exhibit distinct linear belts of metamorphic rocks
- High-pressure, low-temperature zones nearest the trench
- High-temperature, low-pressure zones further inland in the region of igneous activity

Additional Study Guide

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