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8/30/10 1:25 PM Igneous Rock ? Texture Texture ? size, shape and arragenment of minerals Igneous rocks > Interlocking crystals Size of the crystals help us make the first division Slow cooling ? Intrusive (plutonic) Large, visible interlocking crystals Phaneros = ?to see? PHANERTIC texture Rapid cooling ? Extrusive (volcanic) Small, interlocking crystals Crystals generally too small to see without a hand lens APHANITIC textures The Nature of Igneous Rocks Form from Magma Hot, partially molten mixture of solid liquid and gas Mineral crystals form in the magma making a crystal ?slush? Gases ? H2O, CO2, etc. ? are dissolved in the magma Magma is less dense than solid rock Igneous Rocks ? Composition Composition = the mineralogy of the magma and hence, rocks While texture allows us to separate Plutonic from Volcanic rocks, composition allows us to identify and name the rocks in each category Name change between Mineral composition and Magma/Rock composition Magma that is high in Ferromag minerals is called MAFIC Magma that is high in Sialic minerals is called FELSIC Magma that is intermediate in composition is called INTERMEDIATE Magma Differentiation (1) Fractional crystallization ? high T mafic minerals crystallize first and settle out of magma. Magma becomes more Felsic over time Magma Differentiation (2) Partial Melt ? Low T minerals in surrounding country rock melt into magma, making it more felsic over time Xenoliths Fragments of country rock (sometimes ultra mafic mantle) Magma Differentiation (3) Magma Mixing Minerals, Magma, and Rocks 8/30/10 1:25 PM 8/30/10 1:25 PM