Rock Classification

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 grains. A combination of the two would be porphyritic, large grains in an aphanitic, or extrusive matrix. Secondly, composition is used to classify igneous rocks. There are four types, ultramafic, mafic, intermediate, and felsic. Ultramafic rocks are very dark and contain and extreme amount of iron and magnesium. Mafic rocks are also dark in color; they too contain high iron and magnesium amounts. An example would be olivine, or pyroxene. Intermediate igneous rocks are made from silica and plagioclase. They tend to be grays and browns in color. Finally, felsic socks are light in color and contains high amounts of silica. Quartz and potassium feldspar are examples of felsic igneous rocks. Other types of rocks are classified similarly.

Sedimentary rocks form from the weathering of pre-existing rocks. The broken down particles are then compacted and cemented together after the sorting process is complete. Depending on what the sedimentary rock is formed by, determines whether is known as clastic or chemical. Clastic rocks are composed of particles from weathering. They are then sorted by grain size, gravel being the largest and clay being the smallest. A few examples would be sandstones and shales. Chemical sedimentary rocks are biochemical, and contain ions in the solution from weathering. These are also further classified. 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.