Earth Sciences 1022a Lecture Brief 22 Sep 09 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 of tetrahedra]), and continuous (same structure but different proportions of sodium and calcium in plagioclase feldspar)- repeated removal of crystals ( crystal settling ) from 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 the heavy elements with them, then light crystals form later from the remaining light elements, eventually to
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