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rocks Igneous Igneous Rocks Solidified molten rock 1,100 C to 650 C Depends on composition Earth is mostly igneous rock Magma Subsurface melt Lava Melt at the surface Magma erupts via volcanoes Igneous rocks Rock texture size, shape, orientation of minerals Composition minerals that compose the rock Igneous rock textures Igneous rock textures Phaneritic Glassy Aphanitic Vesicular Porphyritic Pyroclastic Igneous rock textures Intrusive igneous rocks Extrusive igneous rocks Igneous rock composition Felsic (high silica) Intermediate (medium silica) Mafic (low silica) Art 05.03b1 Texture? A. Phaneritic B. Aphanitic C. Porphyritic D. Pyroclastic Igneous Activity Distribution Igneous activity tracks tectonic plate boundaries Igneous Activity Distribution Igneous activity tracks tectonic plate boundaries. Hot spots Mafic volcanic activity Convergent boundaries Felsic igneous activity Divergent boundaries Mafic igneous activity Which of the following is a source of heat inside the Earth? A. Gravitational energy from the Sun B. Energy from the Earths formation C. Heat energy from the Sun D. Forest fires Magma Formation Why does magma form? Earth is hot inside. Why? Planetesimal and meteorite accretion. Gravitational compression. Differentiation. Radioactive mineral decay. Where does most volcanic material come from? Magma Formation Partial melting in crust / upper mantle. Melting is from Pressure release Volatile addition Heat transfer How many km would rock 600 km deep have to rise before it partially melts? A. B. C. D. 200 km 300 km 400 km The rock would never partially melt Addition of Volatiles Volatiles cause rocks to melt at much lower T. Water Carbon dioxide Adding volatiles to hot, dry rocks initiates melting Important subduction process Influence of volatiles on melting How shallow (minimum depth) in a subduction zone can magma form? A. B. C. D. 20 km 60 km 80 km Magma will never form What Is Magma Made of? Magmas have 3 (solid, components liquid, and gas). Solid Solidified minerals are carried by the liquid. Liquid The melt itself is comprised of mobile ions. What Is Magma Made of? Gas Volatiles dissolved in the melt. Dry magma No volatiles. Wet magma To 15% volatiles. Water vapor (H2O) Carbon dioxide (CO2) Sulfur dioxide (SO2) 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. Type Density Temperature Viscosity Felsic Very low Very low (600 to 850 C) Very High: Explosive eruptions. Intermediate Low Low High: Explosive eruptions. Mafic High High Low: Thin, hot runny eruptions. Ultramafic Very high Very high (up to 1,300 C) Very low Magma Variation Why are there different magma compositions? Magmas vary chemically due to Initial source rock compositions Partial melting Assimilation Fractional crystallization Magma Variation Source rock dictates initial magma composition. Mantle source Ultramafic and mafic magmas. Crustal source Mafic, intermediate, and felsic magmas. Bowens Reaction Series Bowens Reaction Series What mineral would you expect to melt first in a rock? A. B. C. D. Olivine Na rich plagioclase Quartz biotite Partial Melting Upon heating, silica-rich minerals melt first Partial melting, then, yields a silica-rich magma Removing a partial melt from its source creates: Felsic magma Mafic residue Fractional Crystallization As magma cools, early crystals settle by gravity Melt composition changes as a result Fe, Mg, and Ca are removed as solids settle Si, Al, Na, and K remain in melt and increase Fractional Crystallization Explain how felsic magma can evolve from mafic magma as magma is migrating up through rocks towards the surface. Assimilation Magma melts the country rock it passes through. Assimilated materials change magma composition. ... View Full Document

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