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Igneous Rock Identification

Igneous Rock Identification

Rock - Igneous - GraniteIntroduction: Igneous rocks form when molten earth material cools and solidifies. This can happen rapidly when lava is ejected from a volcano and quickly solidifies when it comes in contact with air or water. It can happen over extremely long periods of time as magma slowly cools, allowing different minerals to crystallize at various temperatures. Geologists identify igneous rocks based mainly on their composition and texture. These properties provide clues to conditions under which the rock formed.

Composition depends on the chemical makeup of the magma or lava the minerals crystallize from while the rock forms. Identifying the chemicals in a rock is difficult and requires laboratory equipment. Instead, igneous rocks are often classified into two broad categories based upon their colors: 
  • Mafic rocks are dark in color and rich in iron and magnesium minerals like olivine and pyroxene.
  • Felsic rocks are light in color and rich in silicon and aluminum minerals like quartz and feldspar.

Igneous textures refer the size of the rock's mineral crystals. In general, the larger the size of the crystals, the longer time the molten material that formed the rock had to cool. 
  • Intrusive rocks, also called plutonic rocks, cool slowly from magma deep within the earth. These rocks tend to have phaneritic textures with large crystals easily seen by eye. 
  • Extrusive rocks, also called volcanic rocks, cool rapidly from lava eruptions on the earth's surface. These rocks tend to have aphanitic textures with crystals too small to be seen by eye. 

Using an igneous rock chart like the one shown can help with the process of rock identification.
Chart - Igneous Rocks

 

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