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This 5 page Class Notes was uploaded by Kimberlee Fischer on Tuesday March 22, 2016. The Class Notes belongs to GEO207 at Illinois State University taught by Dr. Tranel in Spring 2016. Since its upload, it has received 4 views. For similar materials see Natural Disasters in Geology at Illinois State University.
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Date Created: 03/22/16
Geology 207: Natural Disasters and Hazards; Dr. Tranel Notes for 3/15/16 and 3/17/16 Internal Structure of Earth & Plate Tectonics Layers of the Earth: Compositional Properties Vs. Physical Properties -3 Layers: Compositional Properties -Crust: – Thin, rocky – 5-70 km thick – Composition: granite, basalt, & others – Density = 2.7-3 g/cm3 -Core – 3486 km radius – Iron-nickel alloy – Density = 11 g/cm3 -Mantle – Solid, rocky – 2900 km thick – Peridotite – Density = 3.3 g/cm3 -5 Layers: Physical Properties *change with depth; increase temperature, increase pressure -Crust -Upper Mantle (transition zone) -Lower Mantle -Liquid Outer Core -Solid Inner Core Lithosphere: (sphere of rock) Rigid, Cool & Strong, ~150 km thick Asthenosphere: (weak sphere) So7 & Warmer, Detached, Flows 10-15 cm/yr Mesosphere: Lower Mantle Higher temps, increased pressure, deeper & stronger Outer Core Liquid: iron alloy, very hot, flowing Inner Core Solid: iron alloy, highest temps & pressure How we know the interior Earth? -Seismic Waves -Vibrations from earthquakes, nuclear blasts -Detected across earth Seismic Waves -Pressure (P) waves – Through solid, liquid, & gas -Shear (S) waves – Only through solid Rock properties affect how seismic waves move. -Denser rocks = faster waves -Liquid rocks = slower waves -Different types of solid materials Brittle Materials – Permanent deformation – Rapid stress breaks rock – Earth surface Ductile Materials – Permanent deformation – Stress for a long time – High temps Elastic Materials – Recovers from deformation Waves refracted at boundary between different materials -A stick in water appears to bend -Light crossing the air-water interface -Seismic waves behave the same when crossing crust-mantle or mantle-core interface Seismic waves refracted by different layers -Differentiated planet - Earth – Waves travel in curved lines -Homogeneous planet – Waves travel in straight lines Geology 207: Natural Disasters and Hazards; Dr. Tranel Notes for 3/15/16 and 3/17/16 Travel times depend on wave path and materials S-Wave travel through Earth No S-waves through liquid + S-wave shadow after earthquake = Liquid core P-Wave travel through the Earth Refraction bends waves + Different materials = P-wave shadow Flowing iron alloy in the outer core creates the Magnetic Field Magnetic field surrounds Earth North & south magnetic poles -Magnetic minerals align north-south when magma cools -Important for plate tectonics The magnetic field also protects the Earth from solar wind and radiation. Behaviors of Earth's Layers Continental vs. Oceanic crust -Continental Crust • 30-50 km • Density: 2.7 g/cm3 -Oceanic Crust • 7 km • Density: 3.0 g/cm3 Crustal blocks “float” on a denser upper mantle -Buoyancy: Upward force acting on a less dense object floating in denser material -Isostacy: buoyancy force equals gravitation force *The weight of crustal materials affects the isostatic equilibrium. Crust rises or sinks to regain isostatic balance Glacier Example -weight added: crust bends downward -weight removed: crust rises upward. Crustal rebound observed in northern Canada & northern Europe where land was covered by ice ~10,000 years ago. Earth’s Heat Flow -Crust: heat flows by conduction -Mantle: heat flows by convection Mantle Convection: cold material sinks & hot material rises. Ridge Push: mid-ocean ridge is a topographic high, so gravity pushes plates away to either side Slab pull: old cold oceanic lithosphere is dense and sinks into the mantle, pulling the rest of the plate down behind it Plate Tectonics Earth’s lithosphere is broken into plates • 7 major plates + many smaller plates • Rigid plates move Continental Drift: Continents move relative to each other -Evidence 1: Continental Fit Geology 207: Natural Disasters and Hazards; Dr. Tranel Notes for 3/15/16 and 3/17/16 -Shorelines ft together -Pangaea: one large continent -Wegener, 1915: The Origins of Oceans and Continents -Evidence 2: Glacial deposits -Evidence 3: Paleoclimate -Permian time -Pangaea at South Pole -Wegner predicted locations of: Tropical coals, Tropical reefs, Subtropical deserts, Subtropical evaporates **A.k.a. if you have different temperatures, you have different rocks and features formed -Evidence 4: Fossils -Land animals could not have crossed the oceans -Evidence 5: Geologic structures & rocks Ocean Bathymetry & Magnetic Rocks -Topographic maps of ocean floors -Sonar in 1950’s -Deepest areas near land -Mountain range runs through every ocean -Volcanoes create lines across ocean floors -Magnetic minerals in magma Magnetic minerals randomly oriented in magma -Minerals align with magnetic poles Alignment is preserved when magma solidifies -New crust moves away from mid-ocean ridges -Magnetic Rocks Positive and negative anomalies in magnetic orientation can be observed across sea floor Seafloor Spreading Theory Volcanic activity mid-ocean -Upwelling mantle at ridges -New crust forms & moves away from ridge Subduction at ocean edges -Oceanic crust sank -Trenches formed Hotspots Isolated volcanoes are born and die as plate moves over a stationary magma chamber. Examples in the United States: Hawaii & Yellowstone -Hotspots leave a track of how the plate has moved over time -Magma plumes rise to surface -Volcanic rock layers build up -Exposed above water surface -> creates islands -Island moves away from magma chamber -Plate moves -New island forms -Seamount or guyot -Forms if plate moves off magma chamber before volcano reaches sea level Geology 207: Natural Disasters and Hazards; Dr. Tranel Notes for 3/15/16 and 3/17/16 Types of Plate Boundaries in Plate Tectonics Continental Margins Passive Margin: Continental border that is not a plate boundary Active Margin: Continental border that is a plate boundary Plate Boundaries- 3 types -Transform: Moving aside -Convergent: Pressure together -Divergent: Splitting apart Divergent Boundary Plates split apart -Lithosphere stretched out, thinned & broken to where the mantle pushes up -Fast spreading > 10 cm/yr (In geologic time, this is very fast; you would see this is you were in the pacific) -Magmatism dominates -Gentle slopes on ridges -Slow spreading (~1-5 cm/yr) (You would see this in the Atlantic) -Faulting dominates -Steep slopes on ridges & rift valley (1500-3000 m) Rifting: Breaking apart continental or oceanic lithosphere -Cross-section of the East Africa Rift -Extension removed lithospheric mantle below the Red Sea -Cross-section of the Basin and Range -Blocks of rocks are broken & tilted -Thinning causes basins and mountain ranges -Mid-Continent Rift: -Buried under layers of rock -Observed with gravity anomalies Convergent Plate Boundaries -Ocean-ocean -Continent-ocean -Continent-continent Convergent Boundary Plates are pushed together causing lithosphere to bend, fold, & sink (subduct). Subduction -Fault forms -Trench forms -Plates move together -Crust melts at depth -Volcanic arc forms along boundary Ocean-Ocean Convergence -Fore-arc: area in front of volcanic arc, above subducting slab -Back-arc: area behind volcanic arc, above overriding plate Ocean-Continent Convergence -Oceanic lithosphere sinks because denser Geology 207: Natural Disasters and Hazards; Dr. Tranel Notes for 3/15/16 and 3/17/16 -Japan is an example of an island arc Earthquakes get deeper as the plate moves farther beneath the overriding plate. -Sinking lithosphere is observed by earthquakes in the Wadati-Benioff zone. -Down to 670 km below surface -Asthenosphere is not rigid enough to cause earthquakes, so earthquakes at this depth have to be from the subducted lithosphere -Deep focus earthquakes occur along convergent boundaries with subducting crust. -Continents get closer as subduction continues. Collision: Continent-Continent Convergence -No more oceanic crust to be sub ducted -Rocks deformed & create mountains -Plates are sutured together (organic belts). -The Himalayas formed when Indian plate collided with Eurasian plate. Crust thickened and deformation was distributed across a wide zone. Transform Boundary -No crust is gained or lost -Plates move past each other -Transform faults are found near mid ocean ridges to accommodate spreading. -Transform faults are found near mid ocean ridges to accommodate spreading. -San Andreas Fault: Transform boundaries form major plate boundaries. -Parts of the Juan de Fuca Plate were completely sub ducted. -The convergent boundary changed to a transform boundary. Review: Based on compositional properties, the crust is what plates are made from. Based on physical properties, the plates are in the lithosphere How do we know the Earth has different layers? Seismic Waves, as they travel through the Earth What evidence supports the theory that plates move? Pangaea, glacial deposits, paleoclimate, fossils, geologic structures and rock deposits, mountain features, paleoclimate **(if you have different temperatures, you have different rocks and features formed). **Alignment of magnetic materials and hotspots are pushed away from oceanic ridges.
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