GEOL 101 Metamorphic Rocks
GEOL 101 Metamorphic Rocks GEOL 101
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This 4 page Class Notes was uploaded by Victoria Williams on Wednesday February 24, 2016. The Class Notes belongs to GEOL 101 at George Mason University taught by Mark Uhen in Fall 2016. Since its upload, it has received 36 views. For similar materials see Introductory Geology in Geology at George Mason University.
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Date Created: 02/24/16
Metamorphic Rocks Fun fact: Hawaii has NO metamorphic rocks! Only Igneous. Metamorphic rocks are rocks that are produced from preexisting rocks by applications of heat and/or pressure. Metamorphism – The process that leads to changes in the minerology, texture, and sometimes the chemical composition of the rocks. Different kinds and amounts of pressure and heat can affect the outcome. These rocks are in part the composition of the parent rock. Start with shale, a rock made of clay sized particles, and add heat and pressure to get slate. Rock cycle revisited o Sedimentary rocks with added heat and pressure metamorphizes the rock into a metamorphic rock. But with enough heat, the rock will melt and become igneous. Metamorphic Grade o Metamorphism often progresses incrementally leading to slight changes (low grade) to dramatic changes (high grade). Low heat, low pressure is low grade. High heat and high pressure is high grade. Metamorphic Agents: o Heat can cause recrystallization of mineral grains and may increase the temperature to a point where a particular mineral s no longer stable. Some crystals will switch out for other types of crystals as their compositions change. o Pressure – Buried rocks are subjected to confining pressure, which is EQUAL in all directions. This pressure increases with depth. If one side of the pressure is taken away, it’s kind of like your toothpaste. The cap keeps the equal pressure, take that away and the paste will squeeze out and move. Rocks may also be subjected to differential stress, which is NOT equal in all directions, but instead the one force is greater than others in one direction. o Behavior under pressure – Rocks are generally brittle at the surface, but ductile under heat and pressure. Brittle rocks will fracture under stress, these are cooler rocks. Ductile rocks are put under pressure and heat so they can be squished and bent like silly putty, causing folds in the rocks. The Geothermal Gradient affects the position metamorphic environments. This is the change in temperature based on depth. The deeper you go, the hotter it will get. The gradient goes closer to the surface where there are active magmatic chambers and the gradient will subduct with the colder oceanic plate going under the continental plate. Metamorphic Fluids o Metamorphism due to the contact with heated fluids, which is called metasomatism. Most often, the fluid is water. Around a magmatic chamber there can be a metamorphic halo caused by the superhot water. If the water is squeezed out on a fault to the surface, a geyser will form. Grain Deformation – A spherical grain of a rock (like quartz) can be deformed through slippage along crystal structures that causes the grains to elongate and squish. Metamorphic Textures – o Foliation is any planar arrangement of mineral grains or structural features in a rock. Three foliation types: slatey cleavage (low grade), schistosity (intermediate grade), gneissic texture (high grade). Slatey Cleavage – Excellent splitting of a rock along closely spaced planar surfaces. They are usually perfectly flat and are a partly metamorphized shale. Schistosity – Platy minerals are visible to the naked eye and exhibit a planar structure. Gneissic Texture – A separation of minerals into bands of similar minerals. They are usually light and dark bands/stripes on the rock. Based on the size of the grains/minerals in the rock will determine which minerals sort with which. This is the most metamorphized a rock can get before melting into an igneous rock. Porphyroblastic Texture – When gems form within rocks, this is another tool to identify the grades. Foliation sometimes doesn’t occur if the metamorphic rock is only made up of one type of rock. Also, if only heat is applied there won’t be foliation. An example of this is metamorphosed limestone, better known as marble. Marble usually has a sugary texture. o These different textures are caused by pressure, when minerals are scattered in all directions in a normal rock and then when put under pressure, the mineral pieces are set into one direction. o Classification (this table is in your book). First, you organize the rock into the type of foliation present. If it’s nonfoliated, find out what it’s made out of (quartz, limestone, etc.) Types of metamorphism – o Little pressure, some heat – Hyrdothermal o Little pressure, high heat – Contact metamorphism o Lowhigh pressure, low and high heat – Regional (low and high grade) o Low/Midhigh pressure, low and high heat – Subduction zone metamorphism (more pressure than regional) Contact Metamorphism – Heat only, no pressure. For example, a group of sedimentary rock coming in close contact with a magma chamber so the sedimentary rock bakes, creating a metamorphic aureole around it. o Shale getting close to the heat, turns into Hornfels (no pressure) o Quartz sandstone getting close to the heat, turns into Quartzite (pressure may be included) o Limestone getting close to the heat, turns to Marble. (pressure may be included) Hydrothermal Metamorphism – Occurs at shallow crustal depths in regions where geysers and hot springs are active. Where the water is traveling through cracks to get to the surface, the rock around the cracks are metamorphosed. This also happens at the mid ocean ridges where the water saturates the hot new crust. Spit out of ‘black smokers’ in the water, spewing hot mineralrich water. Regional Metamorphism – When heat from tectonic boundaries effects a full region. Fault Zone Metamorphism – Fault breccias form along faults due to the grinding action of the blocks on either side of the fault. At the faults, the rocks are brittle and they break apart at the fault line to create these fault breccias. At depth, the same fault will have more ductile rock moving around. Impact metamorphism – Metamorphism due to impacts of meteorites is called impact metamorphism. The products of these impacts are called impactiles (fragments of rock, tiny diamonds, “shocked” quartz, and tectites – glassy droplets). Rock is vaporized and ultraheated by the sudden heat and pressure from the celestial body impact. Metamorphic Mineralogy – Low > Intermediate > High Graded (chart should be in book) Chlorite is formed first, then muscovite, biotite, garnet is only in intermediate and high, staurolite is mainly in intermediate but a bit in high, then sillimanite mainly forms in high grade. Quartz forms pretty much all grades.
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