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Metamorphic Environments

Control or Thermal Metamorphism is a type of metamorphism that takes place next to igneous intrusions resulting from high temperatures. Metamorphism is restricted only to the small area of the intrusion called the contact aureole. Rocks that are produced through this process are often fine-grained and have no foliation. These rocks are called hornfels (Nelson, 2003). Hydrothermal Metamorphism is a type of metamorphism in which metamorphic rocks are produced at high temperatures but with only moderate pressure.

This type of metamorphism is common in rocks lacking hydrous minerals. This metamorphism often results in rich ore deposits (Nelson, 2003). Regional metamorphism is a type of metamorphism that often occurs in large areas. Rocks that are formed through this type of metamorphism are strongly foliated. Compressional stresses are commonly formed by the differential stress such as continental mass colliding to each other. Rocks that are born through this process are subjected to compressive stress which causes the folding of rock and thickening of the crust.

This tends to push the rocks to deeper levels and subject them to higher temperature and pressure as it (what is β€˜it’ here? ) goes down (Nelson, 2003). (2) List and describe the three main types of unconformities (Angular unconformity, disconformity & nonconformity) Angular unconformity – beds below the unconformity tend to have different attitudes and are truncated the unconformity, unconformity is simply the layer that separates rocks above and below also called the gap in the rock records. (Babaie, 2009).

Disconformity – beds below and above the unconformity tend to be parallel, and hiatus is present because of non-deposition and erosion (Babaie, 2009). Nonconformity – strata are deposited on older crystalline basement rocks (Babaie, 2009). (3) In a rock sample containing radioactive elements, will the decay rate or the half life of these radioactive elements slow down over geologic time? The decay rate of a radioactive element does not change or slow down over geologic time. They are always constant and in an exponential function.

That is why half-life or decay rate of elements are used as a tool for scientists to know the age of rocks (Prentice, 2002). (4) If a scientist collected gneiss and decided to determine its age using radioactive elements of Potassium stored in the rock. Would the date derived from the laboratory analysis be the date of formation of the precursor rock or the date of metamorphism of the precursor rock? Support your answer. Potassium-argon method of radioactive dating is often used because argon is gas and is not included when the solidification process of a rock starts to occur.

If potassium is present in the rock, then certainly, there would be some traces of argon in the rock since argon is a decay of potassium. The date that would be derived in using the potassium stored in the rocks would be the date of the precursor rock, as the potassium-argon method of radioactive dating can give an age estimate which is often the last molten time of the rock (Nave, 2000). References Babaie, H. A. (2009). Primary Structures. Department of Geosciences, Georgia State University. Retrieved February 20, 2009 from http://www2. gsu.

edu/~geohab/courses/geol4013/lectures/0_primary_structures. ppt. Nave, C. R. (2000). Clocks in the Rocks. HyperPhysics. Retrieved February 20, 2009 from http://hyperphysics. phy-astr. gsu. edu/hbase/nuclear/clkroc. html. Nelson, S. A. (2003). Types of Metamorphism. Department of Earth & Environmental Sciences, Tulane University. Retrieved February 20, 2009 from http://www. tulane. edu/~sanelson/geol212/typesmetamorph. htm. Prentice, D. A. (2002). Lesson 6: Radioactive Dating and Geologic Time. Retrieved February 20, 2009 from http://74. 185. 192. 97/pubs/ssless06. pdf.

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