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Chapter 8 Metamorphism and Metamorphic Rocks: 

Chapter 8 Metamorphism and 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 remains essentially solid. It does not melt.

Metamorphic Settings: 

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


Figure 8.2

Agents of metamorphism: 

Agents of metamorphism Heat 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


Figure 8.3

Agents of metamorphism: 

Agents of metamorphism Pressure and differential stress Pressure 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

Pressure in metamorphism: 

Pressure in metamorphism Figure 8.4


Figure 8.6


Figure 8.5

Agents of metamorphism: 

Agents of metamorphism Chemically active fluids Mainly water with other volatile components Fluids enhance reactions and migration of ions Aids in recrystallization of existing minerals Sources of fluids Trapped in pore spaces of sedimentary rocks Fractures in igneous rocks Alteration of hydrated minerals such as clays, gypsum, and micas

Agents of metamorphism: 

Agents of metamorphism 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: 

Metamorphic textures Texture refers to the size, shape, and arrangement of mineral grains Foliation – any planar arrangement of mineral grains or structural features within a rock

Metamorphic textures: 

Metamorphic textures 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 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


Figure 8.7

Foliation resulting from directed stress: 

Foliation resulting from directed stress

Foliated Textures: 

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


Figure 8.8


Figure 8.9

Foliated Textures: 

Foliated Textures Gneissic During higher grades of metamorphism, ion migration results in the segregation of minerals Gneissic rocks exhibit a distinctive banded appearance

Other metamorphic textures: 

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


Figure 8.11


Figure 8.12

Foliated Rocks: 

Foliated Rocks Slate Very fine-grained Excellent rock cleavage Most often generated from low-grade metamorphism of shale, mudstone, or siltstone

Foliated Rocks: 

Foliated Rocks 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

Slate (left) and phyllite (right): 

Slate (left) and phyllite (right) Figure 8.14

Foliated Rocks: 

Foliated Rocks Schist Medium- to coarse-grained Platy minerals (mainly micas) predominate The term schist describes the texture To indicate composition, mineral names are used (such as mica schist)

Foliated Rocks: 

Foliated Rocks 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

Non-Foliated Rocks: 

Non-Foliated 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

Non Foliated Rocks: 

Non Foliated Rocks Quartzite Formed from a parent rock of quartz-rich sandstone Quartz grains are fused together

Metamorphic environments: 

Metamorphic environments Contact or thermal metamorphism Result from 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

Contact metamorphism: 

Contact metamorphism Figure 8.19


Figure 8.19C

Metamorphic environments: 

Metamorphic environments 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

Hydrothermal metamorphism: 

Hydrothermal metamorphism Figure 8.20

Metamorphic environments: 

Metamorphic environments Regional metamorphism Produces the greatest quantity of metamorphic rock Associated with mountain building


Figure 8.21

Other Metamorphic environments: 

Other Metamorphic environments Burial metamorphism Associated with very thick sedimentary strata Required depth varies 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


Figure 8.23

Metamorphic zones: 

Metamorphic zones Systematic variations in the mineralogy and 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


Figure 8.24


Figure 8.25

Index Minerals and Grade: 

Index Minerals and Grade 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


Metamorphic zones in New England Figure 8.26

Metamorphism and plate tectonics: 

Metamorphism and plate tectonics Most metamorphism occurs along convergent plate boundaries Compressional stresses deform the edges of the plate Formation of Earth’s major mountain belts including the Alps, Himalayas, and Appalachians

Metamorphism and Subduction Zones: 

Metamorphism and Subduction Zones Large-scale metamorphism occurs along subduction zones at convergent boundaries Several metamorphic environments exist here Important site of magma generation 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

Metamorphic environments and plate tectonics: 

Metamorphic environments and plate tectonics Figure 8.28

End of Chapter 8: 

End of Chapter 8

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