Lecture 6 and 6' Notes
Lecture 6 and 6' Notes Geo 002
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This 5 page Class Notes was uploaded by Linda Tan on Wednesday February 17, 2016. The Class Notes belongs to Geo 002 at University of California Riverside taught by Prof. Tim Lyons in Winter 2016. Since its upload, it has received 37 views. For similar materials see Earth's Climate Through Time in Geology at University of California Riverside.
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Date Created: 02/17/16
GEO 2 Lecture 6 Notes Plate Tectonics Alfred Wegener: Father of Continental Drift Argument for Drift: Similar fossils found on multiple continents Continental fit (South America and Africa) Similar rock types and structures on either side of Atlantic Ocean Paleoclimatic evidence (pole shift) Conventional explanation was large land bridges, but Wegener proposed a super continent called Pangea and said that the continents were moving over time, but did not propose mechanism for movement. Said that ~Carboniferous period all continents were one (Pangea) Arthur Holmes: Proposed mechanism for continental drift Convection cells in mantle are moving-transfer of heat energy by the circulating motions of a liquid that is heated from below. Paleomagnetism: Iron bearing minerals, when cooled below a certain point (Curie point) act as fossil compasses that point toward the existing magnetic poles. True North = North pole (axis) vs. Magnetic Pole from magnetic field created from core Curie Point: Point where magnetic material, as it cools, lines up with the magnetic field present at that location. Different for every metal. Ex 770 C for Iron Declination: The direction of magnetic north at the time of cooling. Ɵ (off set of magnetic north) vs. True north Inclination: Provides the distance from the pole because the angle of tilt correlates to the magnetic fields of earth The shift of magnetic north through time shown by minerals in separate continents not consistently pointing in one direction means… 1. The Poles are moving = True Polar Wander Path or 2. The continents are moving = Apparent wander path Magnetic Dynamo Convection in outer core creates magnetic field that is horizontal at the equator and vertical at the poles. The magnetic field protects earth by deflecting solar winds and keeps atmosphere in place. Polar migration curves of magnetic field can be mapped over time by iron bearing minerals in the earth’s crust. Earth’s layers 1. Crust: The solid moving outer layer. 2. Mantle/Asthenosphere: Layer of convection cells. Like liquid plastic. 3. Liquid Core: Also convection layer and reacts with solid core to create magnetic field 4. Solid Core Magnetic Dynamo: mechanism that transforms energy from fluid motions (e.g., convection of liquid currents that create a magnetic field (converts mechanical energy to electricity/magnetism) Image of the magnetic field produced by Earth’s core. Magnetic north (changing) Can be reversed Evidence from iron minerals in bands of changing directions on the ocean floor. Harry Hess: Proved seafloor spreading from the convection of the mantle Through use of sonar, Hess discovered that ocean bridges are upwelling zones and trenches are subduction. Mid ocean bridges: Mountain ranges in the middle of ocean basins Chains of volcanoes, spewing and cooling ocean crust due to upwell of convection cell in mantle This means that more crust is being made all the time and the earth is not getting bigger 2 plates have to move away from each other to accommodate new ocean basin And other earth is being pushed back into the mantle Relationship to CO 2nd sea level Polarity reversals: Evidence of convection mantle Record of polarity reversals by the study of the magnetic properties of the seafloor (Vine and Matthews) Normal polarity: Magnetic north and true north are the same Reversed polarity: Magnetic north and true north are opposite Mantic stipes (polarity) are symmetric along mid ocean ridge. Alternating stripes of normal and reversed polarity roughly parallel the ridge crust. New basalt added to ocean floor at the ridge becomes magnetized according to existing magnetic field. Age of ocean floor is older the farther you move away from the ridge Theory of Plate Tectonics (1968): United seafloor spreading and continental drift Lithosphere: ~12 individual rigid plates that contain ocean floor, continent, or some combination of both. Upper most rigid portion of mantle Crust: ocean crust (mafic) is about ~5km thick and continental crust (felsic) 35 km thick Interaction between plates = Earthquakes and volcanoes (above surface) 3 types of plate boundaries Divergent-constructive, ocean ridges. Convergent-destructive, subduction zones. Equal density = no subduction = mountain building Transform-(off set ocean ridge segments) or may cut through continental crust, conservative, strike-slip motion Ex. San Andreas fault Driving forces = mantle drag, ridge push Convergent boundary types: 1. Ocean-continent: Subduction zone, deep-ocean trench, accretionary prism, volcanic (magmatic) arc 2. Ocean-ocean: Subduction zone, deep-ocean trench, accretionary prism, island arc 3. Continent-continent: Mountain building (continental crust not effectively subducted) Active (above boundaries) versus passive (transition between oceanic and continental lithosphere which is not an active plate margin) continental margins • Modes and role of convection • Mantle drag (lithosphere frictionally “piggybacking” on the convecting mantle) • Ridge push (gravitational in response to topographic differences—i.e., ridge high) • Slab pull • More complicated than Hess model for seafloor spreading Hot spots-relatively stationary mantle plumes while continental plate is moving over it creating volcanoes and islands Ex Hawaii and Yellowstone Super Continents: are formed and broken in cycles (Wilson cycles) Rodinia ~1000-750 Ma Pangea (Permian period) In relation to climate: Break up of super continents increase CO 2 o Chemical weathering of land masses Formation of super continents decrease CO 2 o Less ridge activity and organic matter burial Ridges increase sea level (break up of super continents increase sea level)
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