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
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