Essay: Metamorphism Physical Geology Metamorphism: Causes, Manifestations

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Metamorphism

Physical geology

Metamorphism: Causes, manifestations, and varieties

Metamorphism: Causes, manifestations, and varieties

Metamorphic rocks are the result of mineralogical and structural changes to the integrity of existing rock structures. The appearance of metamorphic rocks is as varied as the temperature, pressure, or chemical changes that produce the rock. Both igneous and sedimentary rocks can be changed into metamorphic rocks when subjected to physical and chemical conditions differing from those under which the rocks were originally formed (Jessey & Tarman 2010). "The term metamorphism means to change. Most of us think of the metamorphosis that occurs when a caterpillar becomes a butterfly. While not as dramatic, similar changes can occur in rocks. Rocks will alter their form and appearance to suit new conditions. Unfortunately, metamorphism is a slow process that occurs deep within the Earth. We cannot directly observe the process, but we can see the end result" (Jessey & Tarman 2010).

Metamorphism can be caused by the heat generated when the earth's sliding plates or the compressive force of gravity creates friction. Radioactivity can also be heat-generating. Other structural sources of heat include pressure from lithostatic rock burial as the result of plate tectonics. Chemically active fluids, including water which "circulates in response to heat generated by cooling magmas" can cause ion exchanges between the liquid solution and the rock through which the liquid is traveling and also generate heat (Jessey 2010). Metamorphism "occurs at temperatures and pressures higher than 200oC and 300 MPa" (Mega Pascals) and while rocks subjected to lower temperatures can change in their appearance this is technically not considered metamorphic activity (Nelson 2004). For example, "diagenesis is also a change in form that occurs in sedimentary rocks. In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200oC and pressures below about 300 MPa…equivalent to about 3 kilobars of pressure (1kb = 100 MPa)" (Nelson 2004). So-called low-grade metamorphism occurs at temperatures between 200 -320oC at relatively low pressure while high-grade metamorphism occurs at temperatures above 320oC at high pressure. Low grade metamorphic rocks have abundance of hydrous minerals vs. high-grade rocks that do not, as the level of pressure limits fluid circulation (Nelson 2004).

Metamorphism is classified according to certain types, Contact or thermal metamorphism is the "alteration of rocks at or near the contact of a cooling pluton," in a very narrow area of a rock's surface (Jessey 2010). A pluton is an igneous rock formation like a dike or a batholith. The metamorphosis of the pluton is caused by heat and circulating fluids rather than pressure. The heat produces a series of 'zones' that are "characterized by the presence of one or more diagnostic mineral," a mineral that 'tells' the geologist about the kind of change that has occurred (Jessey 2010). A common visual example of contact metamorphism can be seen in the series of concentric effects "produced adjacent to igneous intrusions where several metamorphic zones represented by changing mineral assemblages reflect the temperature gradient from the high-temperature intrusion to the low-temperature host rocks…Because the volume affected is small, the pressure is near constant. Resulting rocks have equidimensional grains because of a lack of stress and are usually fine-grained due to the short duration of metamorphism" (Jessey & Tarman 2010). The dull appearance of slate rock is an example of such a phenomenon. The original temperature of the rock, the magma, conductivity, diffusion, crystallization of the magma, the rate of change in the temperature of the rock of surface or other changes in the environment, fluid transport, and radioactivity all affect the appearance of the rock produced by contact metamorphism (Contact, 2010, Pomona).

In contrast to the relatively limited expanse of contact metamorphosis, regional or diothermal… [END OF PREVIEW]

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