Geology 1001 Tiffany Roberts 03 08 2014 Chapters 4 7 03 08 2014 Chapter 4 Magma Igneous Rocks and Intrusive Activity Magma Completely or partially molten rock Components Melt is liquid Crystals is solid Volatiles is gas Buoyant material At the surface it is called lava Cools into igneous rock Crystallization occurs while the magma is cooling Intrusive igneous rocks are plutonic Lava extrusive igneous rocks are called volcanic Origin from the outer core Geothermal Gradient pressure increases with depth Magma Generation Decompression Melting decrease the pressure o Ex at Divergent boundaries and mantle plumes hot spots spread apart that decreases the pressure and allows for the magma to rise Flux Melting Add water volatiles o Ex At subduction zones Adding water steam to the melt When the oceanic crust subducts water comes down with it allowing melts to occur o Water reduces the melting temperature Heat Transfer Melting Increase the temperature o Ex Once the hot magma goes up it melts the crust which allows it to continue to travel towards the surface o It is independent of plate Can occur anywhere Q The rate of temperature change with depth inside the earth is called Geothermal gradient Q Heat transfer will cause melting to produce magma Magma Compositions Silicates most common minerals on earth Ultramafic o Has 38 45 SiO2 Often found in mantle Mafic dark silicates are rich with magnesium and ferrum iron o Lower in silica o Darker color more dense and part of oceanic crust o Has 42 52 SiO2 Intermediate Composition o Has 52 66 SiO2 o Part of continental crust Felsic light silicates comes from feldspar and silica o Higher in silica o Lighter color less dense and part of continental crust o Has 66 76 SiO2 Bowen s Reaction Series Felsic will start to melt at lower temperatures than Intermediate Mafic Ultramafic etc Changes to Composition 1 Magmatic differentiation changes to composition over time If a magma pool underground does not immediately erupt minerals like Pyroexene and Olivine will solidify This will then erupt with silica rich magma more feltic than mafic than had it erupted immediately 2 Assimilation When hot mafic magma rises towards continental crust it will melt the crust and add silica to the magma 3 Magma Mixing two magma body s mix 4 Partial Melting similar to magmatic differentiation When unmelted mafic residue is still solid Q A magma comprised of a high proportion of silica would be called felsic Q Magma composition can be changed through the process of partial melting assimilation and mixing Viscosity resistance to flow Igneous Rock Composition rock will be felsic Same as Magma composition If the Magmas composition was felsic the Texture mineral crystal size Depends on time and temperature Very small form when magma cools extremely cooling Larger crystals form when magma cools underground slowly Porphyritic mixed sizes cooling history Vesicular gas bubbles Glassy very rapid cooling Pyroclastic fragmental angular Pegmatitic late crystallization in fluid rich environments Nomenclature composition and texture Felsic Granitic o Granit Phaneritic coarse o Rhyolite Aphanitic fine Intermediate Andesitic o Diorite Phaneritic coarse o Andesite Aphanitic fine Mafic Basaltic o Gabbro Phaneritic coarse o Basalt Aphanitic fine Q Igneous rocks with a composition of high silica are called Felsic Q Fine grained mafic igneous rocks are called Basalt Nature of Igneous Rocks Intrusive interior below the surface Intrusions Plutons Extrusive Volcanic Intrusive Igneous Bodies Tabular o Dikes cuts perp through layers o Sills are horizontal along layers Massive o Batholiths a big pool of magma o Xenoliths a fragment of rock that is different in origin from the igneous rock in which it occurs o Laccoliths mushroom shape Intrusive Igneous Bodies Coarser grain size More resistant to erosion Inverted topography Ch 5 Volcanoes and Volcanic Hazards Ring of volcanoes located along subduction zones Volcano an igneous mountain that is a conduit to release pressure and Formation Volcanoes form where magmas is rising under the Ring of Fire magma surface Nature of Eruptions Viscosity more viscous more resistance to flow o Temperature The cooler the materials the more resistant to flow of magma o Composition o dissolved gases Silica content affects viscosity Mafic releases gases easily felsic does not Felsic is normally more viscous than mafic Ex Hawaiian islands are mafic and low viscosity Materials From Eruptions Lava Flows o Mafic basaltic Aa sharp jagged lava Pahoehoe ropey lava Pillow Lava found in ocean at Mid Ocean Ridge o Felsic rhyolitic Block Lava similar to Aa mafic lava Gases o H2O CO2 SO2 Pyroclastic Tephra materials plaster fragments o Ash tiny shards of glass o Welded Tuff bunch of ash o Lapilii tiny cinder balls that fall from air o Blocks Bombs large materials o Scoria o Pumice General Anatomy of a Volcano Magma Chamber Conduit Vent Crater Caldera large boulder on top of volcano that is made my sinking materials Volcano Morphology Shield Volcanoes large laterally wide at base o Not viscous so lava flows out more easily and layers upon itself Composite strato taller skinnier normal volcano o Alternate layers of rock o Felsic composition Cinder cones even smaller o Short lived volcanoes that release cinder Q Which volcanic type is built of many successive lava flows of low viscosity mafic lava Shield volcano Q label the volcano Calderas Landforms Yellowstone type Large Igneous Provinces LIP o Fissures o Flood Bassalts Lava Domes a cap at a vent Velsic settings Hazards superheated air Pyroclastic Flows huge wall of ash and materials combined with Lahars viscous mudwater It can move bury bridges Lava Flows much lower than pyroclastic flows but can be avoided Ash Falls not good to breathe Landslides can take down forests Blasts Tsunamis linked to underwater earthquakes and volcanoes Gases can be deadly Hazard Mitigation Inflation deflation of volcano as the magma chamber fills the volcano will actually bulge Gas emissions an increase in sulfur dioxide precedes eruptions Seismicity Plate tectonics Check slides Feb 25th Chapter 6 Weather And Soil Weathering the physical disintegration and chemical decomposition of rock at or near Earth s surface Mechanical Breakdown without mineralogical change Increases Surface Area It causes a volume to break into smaller pieces Frost Wedging Salt Crystal Growth apart Sheeting Biological Activity o Ex tree roots o Water gets into cracks then freezes and