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Western University
Speech 2001
Christine Sprengler

Earth Sciences 1022b Lecture Brief 17 Jan 13 IGNEOUS ROCKS Igneous rocks crystallize from molten silicate material called magma that forms at high temperatures and pressures deep in the Earth. Magma rises through the crust and either reaches the surface by volcanoes (extrusive) or cools below surface (intrusive). How Magmas Evolve - according to Bowen’s Reaction Series: discontinuous (different molecular structures of olivine [single tetrahedra], pyroxene [single chains], amphibole [double chains], biotite [sheets]), and continuous (same plagioclase structure) - repeated crystal settling in a magma chamber results in the melt crystallizing through a series of minerals according to their decreasing melting temperatures - remaining crystals react with the melt that is enriched in lighter elements (magmatic differentiation); dark crystals form early, settle, and take heavy elements with them, then light crystals form from the remaining light elements, eventually to quartz; thus granite can ultimately evolve from an original magma having a basaltic composition - magma composition can also change through assimilation (pieces of host rock fall into the magma and melt) and magma mixing where one body of magma invades another, producing a composition intermediate between the two Rock Composition: from dark (mafic, less silica) to light (felsic, more silica) mine
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