Unformatted text preview: surface. The pockets of trapped magma cool very slowly. Slowly cooling magma results in large mineral crystals rocks known as phaneritic. Plutonic rocks are usually light in color because of the high silica content; these rocks are also known as felsic rocks. Intrusive rocks can occur in different shapes in sizes. Volcanic rocks, sometimes called extrusive rocks, cool rather quickly. Extrusive rocks are usually dark in color because of the low silica content. Sometimes these low silica high iron rocks are known as mafic rocks. When mineral crystals are present in volcanic rocks, they are usually microscopic and known as aphanitic. Sometimes crystals are absent from volcanic rock because of how rapidly these rocks cool. References National Park Service U.S.Department of the Interior. (2007). Igneous rocks. Retrieved May 27, 2011 from: http://www.nps.gov/brca/forteachers/randmact3.htm...
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- Spring '11
- rocks, National Park Service, Basalt