GEO 104 Chapter 2 Notes
GEO 104 Chapter 2 Notes Geo 104-001
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This 6 page Class Notes was uploaded by Jennifer Gintovt on Tuesday September 6, 2016. The Class Notes belongs to Geo 104-001 at University of Alabama - Tuscaloosa taught by Rona J. Donahoe in Fall 2016. Since its upload, it has received 8 views. For similar materials see Hazardous Earth in Geology at University of Alabama - Tuscaloosa.
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Date Created: 09/06/16
GEO 104-001 Chapter 2: Internal Structure of Earth and Plate Tectonics Earths Structure: • Layers are different o By composition/density § Mafic • Low in silica, high iron and magnesium, dark in color, high density (ferromagnesian) § Silicic (felsic) • High silica, low iron and magnesium, lighter in color, low density (non-ferromagnesian) o By physical properties (solid/liquid) o Components § Inner core • Solid • High temperature • About 90% iron • High density § Outer core • Liquid • Composition similar to inner core • Less dense than inner core § Mantle • Solid • Largest volume of any layer • Composed of iron/ultramafic rocks § Crust • Outer rock layer of earth • Low density • Moho discontinuity o Separates lighter crustal rocks from more dense mantle § Lithosphere • Strong and rigid • Compromised of the crust and the outer mantle § Asthenosphere • Underlies the lithosphere • Weak and plastic • Capable of slow flow Continents v. Ocean basins • Continental crust o Thick o Old o Light density o Granitic basement rocks • Oceanic crust o Thin o Young o Heavy density How do we know so much about earth’s interior? • Seismology o The study of earthquakes o Earthquakes cause seismic energy to move through earth § Some waves move through solids but not liquids § Waves can be reflected § Waves can be refracted • Change in path § Info on wave movement gives a picture of the Earth’s interior § Travel times/paths of different seismic waves through Earth allow us to infer its • Structure, physical properties, composition • Earth’s internal structure is complicated • Magma is created in asthenosphere • Slabs of lithosphere that have sunk deep into the mantle exist • Lithosphere thickness is extremely variable- reflects age and history Plate Tectonics: • Continental Drift- Alfred Wegener (1913) o Africa and South America “fit together” o Restricted plant & animal fossils found on widely separated land masses o Ancient glacial deposits on southern continents § Hypothesis abandoned for lack of plausible driving mechanism for movement of the continents Plate Tectonics II: • Exploration of sea floor (1950s) o Glomar Challenger- sonar used to map topography of sea floor (bathymetry) § Discovered ocean ridges and deep sea trenches • Theory or Seafloor Spreading- Harry Hess (1962) o New ocean crust created at oceanic ridges o Crust moves laterally from ridge to trench o Ocean crust is destroyed at oceanic trenches § Brought about a new interest in continental drift o Validity of seafloor spreading established by § Identification and mapping of oceanic ridges § Dating of volcanic rocks on the ocean floor • Youngest rocks found at ridges • Oldest rocks found at edges of ocean basins • Ocean sediment thickness increases away from ridge § Mapping of earthquake focal depths near deep ocean trenches (Benioff Zones) § Understanding and mapping of paleomagnetic history of ocean basins Paleomagnetism: • Study of rock remnant magnetism • Earth’s magnetic field can be represented by a dipole o Force lines extend from North Pole to South Pole o Caused by convection in outer core and Earth’s rotation • Igneous rocks that have minerals containing iron preserve the orientation of the Earth’s magnetic field when cooled below the Curie Point o Time at which an iron-rich mineral “locks-in” the Earth’s magnetic field • Magnetic reversals are cyclic but random o Occur on average every few 100,000 years o Change in polarity takes place over a few 1000 years • Magnetic reversals help to date Earth’s rocks History of Plate Tectonics III: • Magnetic mapping of ocean floor (early 19600s) o Magnetometers- instrumentals that measure magnetic properties of rocks • Magnetic anomalies on seafloor discovered- Vine & Matthews Theory (1963) o Noticed that the ocean floor had stripes when mapped § Areas of “normal” and “reversed” magnetic fields o Stripes were parallel to oceanic ridges Seafloor Age: • Geologists can infer ages for the ocean rocks using magnetic anomalies • Seafloor is no older than 200 million years o But there has been ocean crust found in the Mediterranean that may be as old as 340 million-years old • Spreading at the mid-ocean ridges can explain stripe patterns • Rising magma at ridge is extruded o Cooling rocks are normally magnetized o Field is reversed with new rocks that push old rocks away Plate Tectonics: • Large scale geological processes that deform Earth’s lithosphere • Produce landforms (ocean basins, continents, mountains, etc.) • Processes are driven by forces that occur within the Earth (internal energy) • Earth’s lithosphere is broken into plates o 7 major plates § N. American § S. American § Pacific § Eurasian § African § Indo-Australian § Antarctic o ~ a dozen minor plates • lithosphere is created at ridges and destroyed at subduction zones • Triple plate boundary- where three plates come together- higher rates of strain Locations of earthquakes/volcanoes define plate boundaries: • Boundaries between lithospheric plates are geologically active areas • Plate boundaries are defined by areas of seismic activity • Earthquakes/volcanoes are associated with plate boundaries Seafloor spreading and plate tectonics: • At oceanic ridges new crust is added to edges of divergent lithospheric plates o Continents are carried along with moving plates • Lithosphere is destroyed along convergent plate boundaries (subduction zones) • Earth remains constant in size Subduction plates generate e-quakes and magma: • Subducting ocean plates are cold+ brittle o Carry wet sediment down with them • E-quakes occur along the path of the descending plate • Plates and sediment come in contact with hot asthenosphere • Wet sediments melt first to generate magma; eventually the basaltic plate will begin to melt • Magma rises to produce either plutons or volcanic activity Convection: • Earth’s internal heat causes mantle to heat up and become less dense • Less dense mantle rises • Mantle moves laterally, cools and falls back downward • Like convection of water boiling in pan Convergent boundaries: • Ocean-continental o Ocean plate subducts under continental plate • Continental-continental o Two continents approaching each other o Because both plates are buoyant, neither one wants to subduct under the other o Continents essentially merge and create massive earthquakes • ocean-ocean o convergent boundary forms between plates of oceanic lithosphere o older, thicker, and denser plate subducts Divergent boundaries: • two plates separating from each other • generally oceanic, but can be continental Transform boundaries: • ocean-ocean or continent-continent • this is where two plates slide past one another • earthquakes are common along transform boundaries Rates of Plate motion: • plate motion = fast in terms of geology o plates move a few cm’s per year • movement may not be smooth/steady, especially at rough edges of plates • can be displaced by several meters during a significant earthquake Hot Spots: • volcanic centers resulting from hot materials from deep in the mantle • hot material moves up through mantle and overlying plates (believed to come up from core-mantle boundary) o found under both oceanic and continental crust o ex. Yellowstone National Park • plates move over hot spots, creating a chain of island volcanoes o seamounts = submarine volcanoes o ex. Hawaiian-Emperor Chain Plate tectonics, continental shape + Mountain ranges: • movement of tectonic plates is responsible for the present shapes/locations of continents • most recent episode of plate tectonics began 180 mil years ago- break up of Pangea (Alfred Wegner) o supercontinent- extended from pole to pole + halfway around earth o break-up of Pangea produced: § Laurasia (Northern hemisphere) § Gondwana (Southern hemisphere) Plate tectonics helps solve geologic problems: • Reconstruction of Pangea and recent continental drift clears up o Fossil data difficult to explain with separated continents o Evidence of glaciation on several continents o Geologic “coincidences” on either side of the Atlantic Ocean § Rock types and ages § shortened mountain belts that span two continents Driving mechanism: • 2 possible driving mechanisms to explain plate tectonics o ridge push § gravitational push away from crest of mid-ocean ridges o slab pull § occurs when cool, dense ocean plates sink into the hotter, less dense asthenosphere • weight of descending slab pulls the plate along § evidence suggests that slab pull is the more important process of the two Tectonics and hazards • divergent boundaries (Mid-Atlantic Ridge) o earthquakes and volcanic eruptions • Transform fault boundaries (San Andreas Fault) o Earthquake hazards • Convergent boundaries where there are active subduction zones o Explosive volcanoes and earthquakes • Convergent plate boundaries where continents collide (Himalayas) o High topography and earthquakes
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