TIPS IGNEOUS ROCKS FOR FRESHERS AS WELL 1. Magma is molten rock that originates from the partial melting of the lower crust and the upper mantle, usually at depths between 10 and 200 km below the surface. 2. The texture of a rock provides important insight into the cooling history of the magma.The major textures of igneous rocks are (a) glassy, (b) aphanitic, (c) phaneritic, (d) porphyritic, and (e) pyroclastic. 3. Most magmas are part of a continuum that ranges from mafic magma to silicic magma. 4. Silicic magmas produce rocks of the granite-rhyolite family, which are composed of quartz,K-feldspar, Na-plagioclase, and minor amounts of biotite or amphibole. 5. Basaltic magmas produce rocks of the gabbro-basalt family, which are composed of Ca-plagioclase and pyroxene with lesser amounts of olivine and little or no quartz. 6. Magmas with composition intermediate between mafic and silicic compositions produce rocks of the diorite-andesite family. 7. Basalt, the most abundant type of extrusive rock, typically either erupts from fissures to produce relatively thin lava flows that cover broad areas or erupts from central vents to produce shield volcanoes and cinder cones. Volcanic features developed by intermediate to silicic magmas include viscous lava flows, ash-flow tuff, composite volcanoes, and collapse calderas. The abundance of water in silicic magma is critical to its development and eruption. 8. Masses of igneous rock formed by the cooling of magma beneath the surface are called intrusions or plutons.The most important types of intrusions are batholiths, stocks, dikes, sills, and laccoliths. 9. The wide variety of magma compositions is caused by variations in (a) the composition of the source rocks, (b) partial melting, (c) fractional crystallization, (d) mixing, and (e) assimilation of solid rock into the molten magma. 10. Most basaltic magma is generated by partial melting of the mantle at divergent plate boundaries and in rising mantle plumes. Most intermediate to silicic magma is produced at convergent plate boundaries. Partial melting of continental crust at rifts and above plumes can also produce silicic magma.
Posted on: Wed, 18 Sep 2013 11:02:43 +0000
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