Metamorphism Physical Geology Metamorphism: Causes,

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 metamorphism occurs over the span of a large area in response to both increased temperature and pressure (Jessey 2010). The effects of liquids are not a factor in regional metamorphism as fluids cannot circulate due to the effects of pressure upon the rocks, which seal in the circulation of moisture. Regional metamorphism occurs in areas of heavy tectonic activity, near the places where the earth's plates rub up against one another.

"Regional metamorphism can be subdivided into different pressure-temperature conditions based on observed sequences of mineral assemblages. It may include an extreme condition, where partial melting occurs" (Jessey & Tarman 2010). Regional metamorphism is commonly found in mountain regions (hence the name regional metamorphism), consisting of foliated rocks developed under medium to high temperatures. "The accompanying pressures vary from low to high. Geothermal gradients, which are likewise moderate to high, produce Buchan and Barrovian Facies series.

Because the pressures of Buchan and Barrovian Facies series are commonly higher than are those of Contact Facies Series, they may contain different sequences of minerals" in their observed composition (Regional, 2010, Pomona). For example, the "Buchan Facies Series of regional metamorphism is characterized by the presence of andalusite, and sometimes cordierite, in intermediate grade mineral assemblages indicating that the conditions of metamorphism were at lower pressure and along a higher metamorphic field gradient than that recorded in Barrovian Facies Series metamorphic rocks" (Nelson 2003).

For a Buchan Facies "the critical sequence of aluminum silicates is kaolinite -->pyrophyllite -->andalusite --> sillimanite" (Regional, 2010, Pomona). For a Barrovian Facies Series, "the resulting aluminum silicate mineral sequence is kaolinite -->pyrophyllite -->kyanite --> sillimanite…the presence of either andalusite or kyanite on metamorphosed shales and siltstones at the middle grades of metamorphism is one feature that distinguishes these facies series from one another" (Regional, 2010, Pomona).

Other, less common types of metamorphism include dynamic metamorphism, which occurs only along faults zones due to pressure, and is not due to heat or fluids. "Textures produced by such adjustments range from breccias composed of angular, shattered rock fragments to very fine-grained, granulated or powdered rocks with obvious foliation and lineation termed mylonites" (Jessey & Tarman 2010). Rocks produced by metamorphism are usually classified into two distinct types, based upon the.