1. Define metamorphism and metamorphic rock, and understand why metamorphism is
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1. Define metamorphism and metamorphic rock, and understand why metamorphism is different from weathering or melting. Metamorphic rocks: Formed by subjecting pre-existing rocks to increased pressure (P) Metamorphism: transformation of pre-existing rock to a metamorphic rock through pressure and temperature relationships. Weathering is the physical break up and/or chemically change of rock exposed to air and water. Melting is the physical process that results in the phase transition of a substance from a solid to a liquid. This occurs due to a change in internal energy by the application of heat or pressure Essentially, metamorphism means to change form. Metamorphism is different from weathering or melting because metamorphism is the overall process that occured for the rock to change in shape, size, color, texture, composition, etc. Melting and weathering where the conditions that created the change within the rock. 2. Explain processes that change the mineral assemblage and texture of rock during metamorphism 3. Describe the conditions and geologic phenomena in the Earth that cause metamorphismExplanation / Answer
2.
The main causes of changes in mineral assemblage and texture that result from subjecting a rock to conditions such pressures, temperatures, and chemical environments different from those under which the rock originally formed.
3. Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids. Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature. When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism. The conditions and geologic phenomena in the earth that cause metamorphism are described in below:
Temperature : Temperature increases with depth in the Earth along the Geothermal Gradient. Thus higher temperature can occur by burial of rock. Temperature can also increase due to igneous intrusion.
Pressure: Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth. Pressure is defined as a force acting equally from all directions. It is a type of stress, called hydrostatic stress, or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. There are two kinds of differential stress. Normal stress causes objects to be compressed in the direction of maximum principal stress and extended in the direction of minimal stress. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. Shear stress causes objects to be smeared out in the direction of applied stress. Differential stress if acting on a rocks can have a profound affect on the appearance or texture of the rock. Rounded grains can become flattened in the direction of maximum stress. Minerals that crystallize or grow in the differential stress field can have a preferred orientation. This is especially true of the sheet silicate minerals (the micas: biotite and muscovite, chlorite, talc, and serpentine).
Fluid Phase.- Any existing open space between mineral grains in a rock can potentially contain a fluid. This fluid is mostly H2O, but contains dissolved ions. The fluid phase is important because chemical reactions that involve changing a solid mineral into a new solid mineral can be greatly speeded up by having dissolved ions transported by the fluid. If chemical alteration of the rock takes place as a result of these fluids, the process is called metasomatism
Time - Because metamorphism involves changing the rock while it is solid, metamorphic change is a slow process. During metamorphism, several processes are at work. Recrystallization causes changes in minerals size and shape. Chemical reactions occur between the minerals to form new sets of minerals that are more stable at the pressure and temperature of the environment, and new minerals form as a result of polymorphic phase transformations (recall that polymorphs are compounds with the same chemical formula, but different crystal structures.
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