Metamorphism PDF

# METAMORPHISM ## Definition of Metamorphism "Metamorphism" comes from the Greek: meta = after, morph = form, so metamorphism means the after form. - changing of one type of rock to another as a result of great heat, great pressure, and chemical change. - occurs at temperatures and pressures higher...

# METAMORPHISM ## Definition of Metamorphism "Metamorphism" comes from the Greek: meta = after, morph = form, so metamorphism means the after form. - changing of one type of rock to another as a result of great heat, great pressure, and chemical change. - occurs at temperatures and pressures higher than 200°C and 300 MPa. Rocks can be subjected to these higher temperatures and pressures as they become buried deeper in the Earth. Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction. - The upper limit of metamorphism occurs at the pressure and temperature of wet partial melting of the rock in question. Once melting begins, the process changes to an igneous process rather than a metamorphic process. ## Different Types of Metamorphism The 4 ways in which metamorphism takes place are: 1. **Regional Metamorphism** - occurs when rocks over a large region are exposed to great heat and pressure this is due to deep burial or to movements of rocks in the earth's crust. 2. **Contact Metamorphism** occurs when rocks are heated as they come into contact with magma or lava. 3. **Dynamic metamorphism** - takes place in the high-pressure conditions along faults. 4. **Hydrothermal metamorphism** ensues from contact with fluids heated by igneous rock. ## How metamorphism affects the components and texture of rocks? 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. During metamorphism the protolith undergoes changes in texture of the rock and the mineral make up of the rock. These changes take place mostly in the solid state and are caused by changes in physical or chemical conditions, which in turn can be caused by such things as burial, tectonic stress, heating by magma or interactions with fluids. ## 5 Factors that Control Metamorphism 1. **The mineral composition of the parent rock.** The critical feature of the parent rock is its mineral composition. This is because the stability of minerals-how they are influenced by changing conditions-is what determines which minerals form as metamorphism takes place. When a rock is subjected to increased temperatures and pressures, some minerals will undergo chemical reactions and turn into new minerals, while others might just change their size and shape. 2. **The temperature at which metamorphism takes place.** The temperature under which metamorphism occurs is a key variable in determining which metamorphic reactions happen. Minerals are stable over a specific range of temperatures. Quartz, for example, is stable from surface temperatures up to approximately 1800°C. If the pressure is higher, that upper limit will also be higher. If there is water present, it will be lower. Most other common minerals have upper limits between 150°C and 1000°C. 3. **The amount and type of pressure during metamorphism.** Pressure has implications for mineral stability, and therefore the mineral content of metamorphic rocks, but it also determines the texture of metamorphic rocks. When directed pressure (or directed stress) acts on a rock, it means the stress on the rock is much greater in one direction than another. 4. **The types of fluids (mostly water) that are present during metamorphism.** Water is the main fluid present within rocks of the crust, and the only one considered here. The presence of water is important for two main reasons. First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. This speeds the process up so metamorphism might occur more rapidly, or metamorphic processes that might not otherwise have had time to be completed are completed. Secondly, water-especially hot water-can have elevated concentrations of dissolved substances, making it an important medium for moving ions from one place to another within the crust. Processes facilitated by hot water are called hydrothermal processes (hydro refers to water, and thermal refers to heat). 5. **The amount of time available for metamorphism.** Most metamorphic reactions occur very slowly. Estimates of the growth rates of new minerals within a rock during metamorphism suggest that new material is added to the outside of mineral crystals at a rate of approximately 1 mm per million years. Very slow reaction rates make it difficult to study metamorphic processes in a lab.