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IGNEOUS ROCKS are “born of fire”. In other words, they were once molten and upon cooling, the magma (molten rock) crystallized into solid rock. Igneous rocks may form deep inside the Earth or at the Earth’s surface when a volcano erupts.
Slow cooling deep beneath the Earth’s surface allows crystals to grow to large size (1/8” or more). These crystals are easily visible and distinguish this group of igneous rocks as INTRUSIVE.
Rapid cooling near or at the Earth’s surface, produces many small crystals that are not readily seen by the unaided eye. This group of igneous rocks is called EXTRUSIVE and are typically volcanic in origin. Cooling may be so rapid that crystals do not have a chance to form and instead a glass is produced.
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Granite - intrusive
GRANITE is a coarse to medium-grained rock that forms from the cooling of magma deep within the Earth (intrusive). It is made up mainly of varying amounts of the minerals: quartz, orthoclase, muscovite, biotite and hornblende. The name is from the Latin granum, for “grains”.
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