Rock Classification Essay

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October 11, 1999 Classifying Rocks Rocks are classified to make it easier on people to identify them in the future. This can be done by a numerous amount of ways. Each rock type has their own specific ways, but there are two distinct characteristics that apply to all. These are texture and composition. These two, along with many others helps to classify igneous, sedimentary, and metamorphic rocks. Igneous rocks are classified first by texture. This is broken down mainly into grain size. First there are intrusive, or plutonic igneous rocks. These types of rocks cool within the crust and forms large, visible crystals. The opposite would be extrusive, or volcanic rocks. These cool at the surface rapidly, forming small…show more content…
First you have limestone, which can be either organic or inorganic. An example would be fossiliferous or chalk. Next is dolostone, and it is formed from dolomite. Chert is next; and can be organic or inorganic also. Flint and jasper are some examples of chert. Rock salt and gypsum are what are known as evaporites. These form from the evaporation of saline waters in an arid environment. Finally there is coal, which is organic and forms from buried plant remains and carbon. There is one more type of rock that is classified the same way as the previous two. Metamorphic rocks are pre-existing rocks that are changed by heat and pressure. The pre-existing rock is called the parent rock, or protolith. Metamorphism occurs in these rocks when the minerals become instable. There are two types of metamorphism, contact and regional. Contact is high temperature, low pressure. It is basically a massive rock that is baked. Regional is high pressure and low temperature. Strong fabric, or layering, develops from this kind of metamorphism. Along with metamorphism, you get texture changes. The first is recrystallization, which is where you get new minerals from old. Some of the new minerals that form are micas, feldspars, and garnet. You can also get foliation, which is a planar fabric in a metamorphic rock. Simply, this is the alignment of minerals within the rock that are platy. Types of foliation are slaty cleavage, schistosity, and gneissic layering.
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