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Fractional crystallisation 

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When magma cools it begins to crystallise as the crystals seperate from the magma the composition of the melt changes.The melt becomes increasingly rich in silica as it is the silica rich minerals that crystallise last. Through this process the melt becomes less dense and rises further within the crust. A basalt melt can change to andesite then dacite. If rising magma assimilates continental crust on its ascent then large amounts of Si rich melt can be generated leading to emplacment of granite. Within a granitic pluton the top can be highly rich in the lighter Si rich minerals of Quartz and Feldspar whereas the base of the intrusion can dark and rich in Fe and Mg minerals. Schematic diagrams showing the principles behind fractional crystallisation in a magma. While cooling, the magma evolves in composition because different minerals crystallize from the melt. 1: olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene and plagioclase crystallize; 4: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rock formsFractioanl crystallisation of magama ccurs when the melt cools. More dense minerals crsytallise first and sink to the bottom of the magma chamber