S11_GEOL_1001_Lecture_8-1

S11_GEOL_1001_Lecture_8-1 - Magma and Igneous Rocks Earth:...

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Unformatted text preview: Magma and Igneous Rocks Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Magma Formation Magma Partial melting in Partial crust / upper mantle. crust Geothermal gradient – Earth is hot inside. Crustal temperature (T) increases with depth. At the base of the lithosphere T ~ 1,280 °C. Melting is from… Pressure release. Volatile addition. Volatile Heat transfer. Heat Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Magma Formation Pressure release Pressure (decompression melting). melting). Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Magma Formation Heat transfer. Rising magma Rising carries mantle heat. carries This raises T in This crustal rock. crustal Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Addition of Volatiles of Volatiles cause rocks to melt at much lower T. Water. Carbon dioxide. Adding volatiles to hot, dry rocks initiates melting. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Magma Compositions 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 Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Magma Compositions Composition controls density, T, 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. Density Very low Low High Very high Type Felsic Intermediate Mafic Ultramafic Temperature Very low (600 to 850°C) Low High Very high (up to 1,300°C) Viscosity Very High: Explosive eruptions. High: Explosive eruptions. Low: Thin, hot runny eruptions. Very low Chapter 6: Up from the Inferno: Magma and Igneous Rocks Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Bowen’s Reaction Series Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Magma Variation Why are there different magma compositions? Magmas vary chemically due to… Magmas Initial source rock compositions. Partial melting. Assimilation. Fractional crystallization. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Partial Melting Upon heating, silica-rich minerals melt first. Partial melting, then, yields a silica-rich magma. Creates: Felsic magma. Mafic residue. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Assimilation Magma melts the country Magma rock it passes through. rock Assimilated materials Assimilated change magma composition. composition. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Magma Mixing 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 Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Fractional Crystallization (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. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks ...
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