9-9 - Magma and Igneous Rocks Magma Formation Magma Partial...

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Unformatted text preview: Magma and Igneous Rocks Magma Formation Magma Partial melting in crust / upper mantle, mostly at Partial divergent plate boundaries. divergent Melting is from… Pressure release. Volatile addition. Volatile Heat transfer. Heat Magma Formation Magma Pressure release Pressure (decompression melting). melting). Geothermal gradient – Earth is hot inside. Crustal temperature (T) increases with depth. Magma Formation Heat transfer. Rising magma Rising carries mantle heat. carries This raises T in This crustal rock. crustal Addition of Volatiles Addition of Volatiles cause rocks to melt at much lower T. Water. Carbon dioxide. Adding volatiles to hot, dry rocks initiates melting. Magma Compositions Magma 4 major magma types based on % silica (SiO2). Felsic (Feldspar and silica) Intermediate Mafic (Mg and Fe-rich) Ultramafic Ultramafic 66 to 76% silica. 52 to 66% silica. 45 to 52% silica. 38 to 45% silica. 38 Magma Compositions Magma Composition controls density and viscosity. Most important is the content of silica (SiO2). Silica-rich magmas are thick and viscous. Silica-poor magmas and thin and “runny.” These characteristics govern eruptive style. Type Density Viscosity Felsic Very low Very High: Explosive eruptions. Intermediate Low High: Explosive eruptions. Mafic High Low: Thin, hot runny eruptions. Ultramafic Very high Very low Magma Variation Magma Why are there different magma compositions? Magmas vary chemically due to… Magmas Initial source rock compositions. Partial melting. Assimilation. Fractional crystallization. Bowen’s Reaction Series Partial Melting Partial Upon heating, silica-rich minerals melt first. Partial melting, then, yields a silica-rich magma. Creates: Felsic magma. Mafic residue. Assimilation Magma melts the country Magma rock it passes through. rock Assimilated materials Assimilated change magma composition. composition. Magma Mixing Magma Different magmas may blend in a magma chamber. Different The result combines the characteristics of the two. The Often magma mixing is incomplete, resulting in Often blobs of one rock type suspended within the other. blobs Fractional Crystallization Fractional (magmatic differentiation) As magma cools, early crystals settle by gravity. Melt composition changes as a result. Fe, Mg, and Ca is removed in early settled solids. Si, Al, Na, and K remain in melt and increase. ...
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