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Lecture 8

GEO 101 Lecture 8: Geology Chapter 7

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Geological Sciences
GEO 101
William Lambert

Geology Chapter 7 • Smith Hall- constructed 1910 - Named after Eugene Allen Smith (1841-1927) - Alabama native - Fought in Civil War - Witnessed campus burning - Became state geologist - He believed Alabama’s natural resources (coal, etc.) could help struggling Alabama economy following the war - Smith was on the 1917 UA Football Team - He was a geologist, professor, naturalist, violinist, and sports enthusiast • The Rock Cycle- illustrates the different processes and paths as Earth materials change both on the surface and inside the Earth • Metamorphic rock- rocks that form when preexisting rock changes into new rock as the result of an increase in pressure and temperature and/or shearing under elevated temperatures (hot water can also play a role) - Metamorphism occurs without the rock first becoming a melt or a sediment - Example: igneous rock (granite) + high heat and pressure = metamorphic rock (gneiss) • Protolith- the original rock from which a metamorphic rock formed. One example is granite is the protolith for gneiss • James Hutton “father of Modern Geology” - Lived in Scotland - Developed the principle of uniformitarianism - Was puzzled by certain rocks that seemed to have been formed as other rocks do, but had been distorted in structure • Solid state change due to change in preexisting rock’s environment: 1. Temperature change 2. Pressure change 3. Compression and shear 4. Hydrothermal fluid 5. Or a combination of two or more • Environmental change produces new minerals that did not occur in protolith and/or produces a new texture • Changes to protolith environment: - Increase temperature- addition of heat helps to break chemical bonds, allowing atoms to move and form new bonds (new minerals) - Increase pressure- tremendous pressures can cause atoms to pack more closely together, sometimes forming new minerals - Increase pressure and temperature- certain minerals are stable at specific temperatures and pressure. Changing these variables promote new mineral formation • Compression and Shear - Compression- a push or squeezing felt by a body - Shear- when one part of a material moves sideways, relative to another • Results of Compression and Shear on mineral grains in rocks: - Equant grains- roughly same dimensions in all directions - Inequant grains- dimensions are not the same in all directions - Preferred orientation- alignment of inequant grains - Two things can happen: 1. Mineral grains change shape 2. Mineral grains change orientation • Exposure to hydrothermal fluid (hot water) - Metasomatism- the process of changing a rock’s chemical composition by reactions with hydrothermal fluids • Distinguishing feature of metamorphic rocks: 1. Metamorphic texture- the arrangement of grains formed as result of metamorphism 2. Metamorphic minerals- specific new minerals grow only under metamorphic temperatures and pressures 3. Metamorphic foliation- parallel alignment of platy minerals (such as mica) and/or the presence of alternating light-colored and dark-colored bands • Metamorphic Processes 1. Recrystallization- changes the shape and size of grains without changing the mineral chemistry, grains tend to get bigger (but not always) 2. Phase change- changes one mineral to another mineral of the same composition, but different crystalline structure (atoms rearrange) 3. Metamorphic reaction (neocrystallization)- results in new minerals that differ (different chemical composition) from those of the protolith 4. Pressure solution- in the presence of water, grains are dissolved on the sides undergoing more pressure and precipitate new mineral where the pressure is lower (water helps atoms move easier/quicker) 5. Plastic deformation- grains change shape (texture) without breaking due to compression at high heat and soft plastic-like nature of rock • Two fundamental classes of metamorphic
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