Volcanoes and Igneous Activity Earth - Chapter 4

Volcanoes and Igneous Activity Earth - Chapter 4

Earth: An Introduction to Physical Geology, 10e Tarbuck & Lutgens 2011 Pearson Education, Inc. Metamorphism and Metamorphic Rocks Earth, 10e - Chapter 8

Stan Hatfield Southwestern Illinois College 2011 Pearson Education, Inc. Metamorphism 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 2011 Pearson Education, Inc. Metamorphism Metamorphism progresses incrementally

from low grade to high grade. During metamorphism, the rock must remain essentially solid. Metamorphic settings Contact or thermal metamorphismdriven by a rise in temperature within the host rock 2011 Pearson Education, Inc.

Metamorphism Metamorphic settings Hydrothermal metamorphismchemical 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. 2011 Pearson Education, Inc. Agents of Metamorphism Heat Most important agent Recrystallization results in new, stable

minerals. Two sources of heat: 1. Contact metamorphismheat from magma 2. An increase in temperature with depth geothermal gradient 2011 Pearson Education, Inc. Agents of Metamorphism

Pressure and differential 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. 2011 Pearson Education, Inc.

Pressure in Metamorphism 2011 Pearson Education, Inc. Agents of Metamorphism Chemically active fluids Mainly water Enhances migration of ions

Aids in recrystallization of existing minerals 2011 Pearson Education, Inc. Agents of Metamorphism Chemically active fluids Sources of fluids Pore spaces of sedimentary rocks

Fractures in igneous rocks Hydrated minerals such as clays and micas 2011 Pearson Education, Inc. Agents of Metamorphism The importance of parent rock Most metamorphic rocks have the same overall chemical composition as the original

parent rock. To a large extent, mineral makeup determines the degree to which each metamorphic agent will cause change. 2011 Pearson Education, Inc. Metamorphic Textures Texturesize, shape, and arrangement of

mineral grains Foliationany planar arrangement of mineral grains or structural features within a rock Examples of foliation Parallel alignment of platy and/or elongated minerals 2011 Pearson Education, Inc.

Metamorphic Textures Foliation Examples of foliation Parallel alignment of flattened mineral grains and pebbles Compositional banding Slaty cleavage where rocks can be easily split into thin, tabular sheets

2011 Pearson Education, Inc. Metamorphic Textures 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 2011 Pearson Education, Inc. Foliation Resulting from Directed Stress

2011 Pearson Education, Inc. Metamorphic Textures Foliated textures Rock or slaty cleavage Closely spaced planar surfaces along which rocks split Develops in a number of ways

2011 Pearson Education, Inc. Slaty Cleavage 2011 Pearson Education, Inc. Metamorphic Textures Foliated textures Schistosity

Platy minerals are discernible with the unaided eye. Exhibit a planar or layered structure Rocks having this texture are referred to as schist. 2011 Pearson Education, Inc. Metamorphic Textures

Foliated textures Gneissic During higher grades of metamorphism, ion migration results in the segregation of minerals. Gneissic rocks exhibit a distinctive banded appearance. 2011 Pearson Education, Inc.

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 2011 Pearson Education, Inc. Common Metamorphic Rocks Foliated rocks Slate

Very fine-grained Excellent rock cleavage Most often generated from low-grade metamorphism of shale, mudstone, or siltstone 2011 Pearson Education, Inc. Common Metamorphic Rocks Foliated rocks

Phyllite Gradational 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

2011 Pearson Education, Inc. Phyllite 2011 Pearson Education, Inc. Common Metamorphic 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). 2011 Pearson Education, Inc. Mica Schist

2011 Pearson Education, Inc. Common Metamorphic Rocks Foliated rocks Gneiss Medium- to coarse-grained Banded appearance High-grade metamorphism

Often composed of light-colored, feldspar-rich layers with bands of dark ferromagnesian minerals 2011 Pearson Education, Inc. Gneiss 2011 Pearson Education, Inc.

Classifying Metamorphic Rocks 2011 Pearson Education, Inc. Common Metamorphic Rocks 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 2011 Pearson Education, Inc. Marble

2011 Pearson Education, Inc. Common Metamorphic Rocks Nonfoliated rocks Quartzite Formed from a parent rock of quartz-rich sandstone Quartz grains are fused together.

2011 Pearson Education, Inc. Quartzite 2011 Pearson Education, Inc. Metamorphic Environments Contact or thermal metamorphism

Result from a rise in temperature when magma invades a host rock The zone of alteration (aureole) forms in the rock surrounding the magma. Most easily recognized when it occurs at or near Earths surface. 2011 Pearson Education, Inc.

Contact Metamorphism 2011 Pearson Education, Inc. Metamorphic Environments Hydrothermal metamorphism Chemical alteration caused when hot, ionrich fluids circulate through fissures and cracks that develop in rock Most widespread along the axis of the midocean ridge system

2011 Pearson Education, Inc. Hydrothermal Metamorphism 2011 Pearson Education, Inc. Metamorphic Environments Regional metamorphism

Produces the greatest quantity of metamorphic rock Associated with mountain building 2011 Pearson Education, Inc. Regional Metamorphism 2011 Pearson Education, Inc.

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. 2011 Pearson Education, Inc. Metamorphism Along a Fault Zone 2011 Pearson Education, Inc.

Metamorphic Environments Other metamorphic environments Impact metamorphism Occurs when high-speed projectiles, called meteorites, strike Earths surface Rocks are called impactiles. 2011 Pearson Education, Inc.

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 highgrade metamorphism.

2011 Pearson Education, Inc. Metamorphic Zones Index minerals and metamorphic 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 Facies Metamorphic rocks that contain the same assemblage of minerals 2011 Pearson Education, Inc. Metamorphic Zones in New England

2011 Pearson Education, Inc. Metamorphism and Plate Tectonics Most metamorphism occurs along convergent plate boundaries Compressional stresses deform the edges of the plate.

Formation of Earths major mountain belts, including the Alps, the Himalayas, and the Appalachians 2011 Pearson Education, Inc. Metamorphism and Plate Tectonics Large-scale metamorphism also occurs

along subduction zones at convergent boundaries. Several metamorphic environments exist here. Important site of magma generation 2011 Pearson Education, Inc. Metamorphism and

Plate Tectonics 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

2011 Pearson Education, Inc. Metamorphic Facies and Plate Tectonics 2011 Pearson Education, Inc. End of Chapter 8

2011 Pearson Education, Inc.

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