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Rock Types and the Rock Cycles
Sedimentary rocks are those that are formed from smaller amounts of other rocks, broken down through processes such as erosion and weathering to form sediment. Layers of sediment build up, compressing the layers beneath to form new rocks. Metamorphic rocks are formed when sedimentary rocks, or a third type of rock formed from cooled magma, called igneous rocks, are exposed to intense heat, pressure or both. The process by which sedimentary rock is transformed into metamorphic rock is called metamorphism. The rock cycle is the cycle that describes this continual breaking down and reforming of the Earth's rocks.
Temperature
One of the conditions that cause sedimentary rock to become metamorphic rock is through chemical changes caused by extreme heat, at temperatures of more than around 400 degrees Fahrenheit. One source of this level of heat is magma, which is forced upwards through the earth towards the surface, warming the rocks surrounding it. If sedimentary rock is heated to high temperatures through this method it can become metamorphic rock. Sedimentary rock can also be exposed to these high temperatures by being buried deeply within the earth, where temperatures become consistently higher depending on proximity to the Earth's core, through plate movement or through layers of sediment building up on top of it.
Pressure
Another condition that can cause sedimentary rock to go through a process of metamorphism is a high level of pressure. This may be an independent cause or it may be combined with heat. This pressure may be caused by the movement of tectonic plates. The top layer of these tectonic plates is composed of sedimentary rock. When these plates are forced together, for example, this sedimentary rock is exposed to extreme pressure and heat, causing the state to change and become metamorphic rock. Another cause of the pressure that can alter the state of sedimentary rock is the weight of large amounts, or layers, of rock on top of it.
Changes in Composition
When sedimentary rocks are subject to a great deal of heat and pressure, the minerals within that rock often line up and form layers in the direction in which these forces were exerted upon it. Sedimentary rocks that have been exposed to the greatest levels of heat and pressure may contain minerals that they did not have before they became metamorphic rocks, due to the chemical changes caused by these forces. Other sedimentary rocks are composed of one mineral only. In this case, the sedimentary rock can only form a metamorphic rock that is composed of the same one mineral. No new minerals can be formed despite the pressure and heat that it is put under.