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Return to Igneous Rocks: Why bother?

Return to Igneous Rocks: Why bother?. Classifying igneous rocks based on chemistry/mineral content. Types of magma are defined by the relative proportion of silica (SiO 2 ) to the sum of Magnesium (Mg) and Iron (Fe) oxides When amount of SiO 2 increases, Fe + Mg decreases. Magma with high

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Return to Igneous Rocks: Why bother?

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  1. Return to Igneous Rocks: Why bother?

  2. Classifying igneous rocks based on chemistry/mineral content • Types of magma are defined by the relative proportion of silica (SiO2) to the sum of Magnesium (Mg) and Iron (Fe) oxides • When amount of SiO2 increases, Fe + Mg decreases

  3. Magma with high MAGNESIUM + FERROUS (iron) Is called MAFic magma (silica ~50%) Also called BASALTIC magma after the most common mafic rock Magma with ~ 40% silica is called ULTRAMAFIC

  4. Rocks with high amounts of iron-magnesium minerals (Olivine and pyroxene) are called MAFIC or ULTRAMAFIC rocks • Example: Gabbro or basalt (mafic) • Dunite and peridotite (ultramafic) • Rocks with high amounts of FELDSPAR and SILICA (quartz) are called FELSIC rocks (Silica >65%, also called SILICIC) • Example: Granite or rhyolite

  5. Also see pages 429-432 in your text

  6. At divergent boundaries and hot spots, mafic magma forms by PARTIAL MELTING (not complete melting) of the asthenosphere • Minerals with higher melting points stay in the asthenosphere. Minerals with relatively lower melting points melt to form mafic magma

  7. At divergent boundaries and hot spots, rocks melt because PRESSURE IS REDUCED (temperature is not a major factor) • Melting temperature of a solid goes UP under high pressure (becomes difficult to melt) • Melting temperature of a solid goes DOWN under low pressure (becomes easier to melt)

  8. Rocks start to melt (produces basaltic magma) Pressure is reduced At divergent boundaries and hot spots, hot rocks from asthenosphere move upward (convection)

  9. Sometimes mafic magma is produced below continental crust (e.g. East African Rift, Yellowstone, subduction zones etc.) • When it rises through continental crust, the continental crust melts • The rocks in continental crust are silica rich and SILICIC magma is formed (silica >65%)

  10. Common rocks produced from silicic magma are GRANITE and RHYOLITE • (Silicic rocks = Felsic rocks) • Mixing of silicic and mafic magma, as well as melting of ocean sediments produces INTERMEDIATE magma (ANDESITE, silica ~55%) in subduction zones

  11. In subduction zones, rocks melt because WATER UNDER PRESSURE gets added to the already hot mantle (pressure is not reduced, temperature is not raised) This lowers the melting temperature of the rocks in the mantle (wet rocks are easier to melt than dry rocks)

  12. When magma becomes solid, IGNEOUS ROCKS are formed (igneous = born of fire) Above the surface (EXTRUSIVE igneous rocks, pages 116-120) Igneous rocks can form Below the surface (INTRUSIVE igneous rocks, pages 110-115)

  13. We know whether an igneous rock formed above or below ground by looking at its TEXTURE (pages 110-111) • Texture of an igneous rock refers to • Size • Shape, and • Arrangement of mineral grains in the rock

  14. Texture depends on the rate of cooling and viscosity of magma when it solidifies Extrusive igneous rocks form above ground Cools fast Mineral grains do not have enough time to grow big (FINE GRAINED TEXTURE, meaning you cannot identify individual minerals without a microscope)

  15. When very viscous lava becomes solid, it forms volcanic glass with extremely fine grains (OBSIDIAN) Intrusive igneous rocks show COARSE GRAINED textures (individual mineral grains can be seen without a microscope) Implies slow cooling or cooling below surface

  16. Sometimes magma cools in stages Parts of it may solidify below the surface and form larger crystals (PHENOCRYSTS) Rest of the magma may be extruded and cool quickly (MATRIX) The overall texture of the rock is called PORPHYRITIC texture

  17. When gases bubble out of the lava as it becomes solid, the rock obtains a VESICULAR texture (vesicles=holes in the rock) • Special cases: • Scoria • Pumice • Tuff

  18. Economic deposits associated with igneous rocks Examples: • Metallic sulfide ores (ores of lead, zinc, silver, copper etc.) • Oxide ores (magnetite, chromite etc.) • Native metals (primary deposits of gold, platinum etc.) • Gemstones (diamond, tourmaline, peridot etc.)

  19. Most of the metallic minerals mined in the world, such as copper, gold, silver, lead, and zinc, are associated with extinct volcanoes at subduction zones. Rising magma does not always reach the surface to erupt; instead it may form batholiths beneath the volcano As the magma body cools, the metallic elements become concentrated in the residual fluids called HYDROTHERMAL SOLUTIONS

  20. The heat from the magma bodies moves and circulates such ore-bearing fluids by convection. The metals become concentrated by circulating hot fluids These can be deposited under favorable temperature and pressure conditions, to form rich mineral veins. • Movie clip: Nikko Sulfur pit eruption, 2006, Nikko Seamount, Mariana Arc. http://oceanexplorer.noaa.gov/explorations/06fire/logs/may12/media/movies/nikko2_video.html

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