Quartzite marble greenstone serpentinite lab 4

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quartzite, marble, greenstone, serpentinite
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Lab #4: Metamorphic Rocks 72 Classifying Metamorphic Rocks Follow these steps to help you identify metamorphic rocks. Use the chart below to name the rocks. Step 1: Determine whether the rock is foliated or non-foliated (Fig. 4, first column in the chart below). Step 2: Make observations about the texture. Look at the 3rd and 4th columns for suggestions. Are the minerals aligned (foliated) or it the texture random (granular)? Are the rocks shiny? Scaly? Banded? Step 3: Are the minerals big enough to see without a hand lens? Are the minerals big enough to identify? If so, identify them. Check the 4th column to see if they are listed. Step 4: Identify the rock. Look at the 1st column to estimate the metamorphic grade and the 2nd column to identify the protolith. Figure 8. Metamorphic rock classification chart. Non-Granular (Massive) (shiny but no crystals) (“scaly” texture) Often grayish-green , Aligned minerals,
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Lab #4: Metamorphic Rocks 73 Metamorphic Rocks and Isotherms It is important to understand how pressure and temperature change in the lithosphere. Since the center of the Earth is very hot (hotter than the surface of the sun), the temperature below Earth’s surface quickly rises as depth increases. The rate of temperature change with depth is called the geothermal gradient . The geothermal gradient varies depending where you are on the planet. In some places it is steeper (meaning it gets hotter faster), and in other places is shallower (meaning is doesn’t get hot very quickly with depth). Geothermal Gradient = Temperature (°C) Depth (km) Low grade metamorphism begins to take place at around 200 o C. How deep must a rock be in order to reach this temperature? Using a few data points and some clever math to calculate how heat flows through the Earth, we can establish lines of equal temperature, called isotherms (“iso” meaning the same, and “therm” meaning tem- perature). Isotherms can be flat or bend around cold or hot areas. In general, isotherms tend to be parallel to one another. However, isotherms (and the geothermal gradient) are much more complex where there are inconsisten- cies within the crust (e.g. a rising magma plume). See Fig. 9 below for a hypothetical isotherm diagram. Metamorphism and Plate Tectonics Pressure and temperature conditions vary in different tectonic settings. Also, different rocks types will be present in different tectonic environments, so the protoliths can vary as well. As you examine the rock samples in this lab, keep the following things in mind: The same parent rock can metamorphose into different metamorphic rocks depending on the grade. Typically, the grain size and the coarseness of the foliated texture increase with an increased grade. The type of minerals present in the rock are also clues to the pressure and temperature conditions. Many minerals have very restricted conditions where they are stable. If conditions exceed the stability limit, the mineral will break down and change into another mineral that is stable under the new conditions.
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