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Rocks, Sediment and Soils Products of an Active Planet Earth’s structure leads to intense geologic activity Inner core: Solid iron Outer core: Liquid iron, convecting (magnetic field) Mantle (Asthenosphere) : Solid iron-magnesium silicate, plastic, convecting Crust (Lithosphere): Rigid, thin Crust: Rigid, Thin Mantle: Plastic, Convecting O, Si, Al, Fe, Ca, Na, K, Mg… 47%, 28, 8, 5, 4, 3, 3, 2 TDE pg.8-11 The Major Lithospheric Plates And directions of movement (TDE pg.5) Geologic Phenomena at Plate Boundaries Divergent Plate Boundaries: where plates move away from each other new oceanic crust is generated from melting mantle material that cools and forms Basaltic Rock (a.k.a. Mafic rock) Iron-rich Silicon-poor Lithosphere Dense Lithosphere Simplified Young Block Asthenosphere TDE pg.15-20, 32-36 Diagram Geologic Phenomena at Divergent Plate Boundaries Divergent Plate Boundary New Oceanic Crust Forming at Mid-Ocean Ridge Shallow Earthquakes Fissure Eruptions Crust Lithosphere Asthenosphere Magma Generation Locations of Divergent Plate Boundaries Mid-Ocean Ridges East Pacific Rise Mid Atlantic Ridge Mid Indian Ridge Mid Arctic Ridge (Mid-Arctic Ridge) Divergent Plate Boundaries and Hotspots Beneath Continents Flood Basalts on Uplift Shallow earthquake activity Volcanic activity Fissures eruptions Lava floods, and volcanoes Hot, non-explosive Basaltic rocks formed (i.e. iron-rich/silica poor) The Rocks (igneous) Basalt bedrock is usually formed in the shallow crust by recent (<~200mya) volcanic activity at divergent plate boundaries or hotspots (>1,000oC) E.g., volcanoes, feeder dikes, volcanic stocks, basalt floods Hawaii, Ship Rock NM, Devils Tower AZ, Yellowstone WY E.g., Red Sea and East African Rift Valleys Thinning crust, flood basalts, long lakes Rift Valley Shallow Earthquakes Linear sea, uplifted and faulted margins Oceanic Crust Rift Valley Passive continental shelf and rise Fig. 19.21 Fig. 19.22 Formation of Mafic Igneous Rocks Mafic Magmas Hot (>1000oC) Non-Viscous (runny, flows easily) “Dry” (no H2O or C02) Mafic Rocks Usually Extrusive, Fine-grained, Mafic (Basalt) rock forms oceanic crust, Shield Volcanoes and Flood Basalts If Intrusive, course-grained mafic rocks are formed Gabbro. If intrusive, Dikes and Sills more common. (Plutons don’t form) Sill Dike Dike Dike Geologic Phenomena at Plate Boundaries Convergent Plate Boundaries: where plates move toward each other, oceanic crust and the underlying lithosphere is subducted beneath the other plate (with either oceanic crust or continental crust) Oceanic Trench Lithosphere Lithosphere Subducted Plate Asthenosphere Simplified Block Diagram TDE 37-43 Formation of Magma How are rocks melted? 1. Heating ■ 2. Depressurization 3. Increase water content 4. Increased silica content Where do rocks melt? Subduction zones (Silicic and Intermediate) Mantle Plumes (“Hot Spots”) not only at Divergent Boundaries Hot and Low Pressure Mafic Hot and High Pressure Formation of Silicic Igneous Rocks Silicic (a.k.a, felsic) Magmas Cool (<700oC) Viscous (sticky, doesn’t flow easily) Gaseous (steam of H2O and C02) Composite Volcano Silicic Rocks Usually intrusive, course-grained, Silicic (Granite) to Intermediate (Diorite) rock forms plutons If extrusive, fine-grained rocks formed by explosive volcanoes Rhyolite or Andesite Volcanoes Also injects surrounding rocks with silica laden steam Batholith of Plutons Dikes: Intruded near a pluton Silica rich fluids are injected into cracks in all directions Discordant: cutting across layers Ores, rare elements and minerals, gems Geologic Phenomena at Convergent Plate Boundaries Volcanic Activity Explosive, Composite Volcanoes Granitic rocks formed (iron-poor/silica-rich) Shallow earthquakes near trench Shallow and Deep Earthquakes over subduction zone Physiographic Features at Convergent Plate Boundaries Island Arcs (chains of volcanic Islands), Chains of Volcanoes Oceanic Trenches The “Ring of Fire” A ring of convergent plate boundaries on the Pacific Rim New Zealand Tonga/Samoa Philippines Japanese Isls. Aleutian Island arc and Trench Cascade Range Sierra Madre Andes Mtns. Fujiyama Pinatubo Composite Volcanic Arcs (Granitic, Explosive) Basaltic Volcanism (Non-Explosive) Depth of Earthquakes at convergent plate boundaries Seismicity of the Pacific Rim 1975-1995 0 33 70 150 Shallow quakes at the oceanic trench (<33km) Deep quakes over the subduction zone (>70 km) 300 500 800 Depth (km) The 3 rock types form at convergent plate boundaries Sedimentary Rock: Sediments (e.g., ocean sand, silt, and clay) are compressed cemented (lithified) Metamorphic Rocks: rocks are compressed, heated and change minerals but do not melt Igneous Rocks: When rocks melt, Magma is formed, rises, cools and crystallizes. Below surface Intrusive. LavaExtrusive Transform Plate Boundaries Where plates slide parallel to each other: Transform faults are created Mid-ocean ridges are offset If occurring beneath a continent the continent is sheared and faulted Transform Plate Boundaries Examples Every mid-ocean ridge is offset along by transform faults San Andreas Fault Shallow earthquakes are generated Volcanic activity is rare See Figure 19.27 http://pubs.usgs.gov/publications /text/San_Andreas.html