Oceanography Chp 2

• Seismic waves


  • Low frequency pulses of energy
  • Generated by forces that cause earthquakes
• Seismic waves


  • Speed increases with depth and density
  • In cooler rigid areas seismic waves have a quicker velocity
  • In hotter liquid substances they have a slower velocity
  • Analysis of these waves and the time it takes for their passage proves that Earth is layered
• Scenarios for Earth's structure


  • Homogeneous Earth
  • Density increases evenly with depth
  • Earth is layered

  
• Lithosphere


  • Rigid outer layer
  • Uppermost mantle and crust
  • Includes continental and oceanic crust

  
• Continental crust


  • Low in density (2.7 g/cm3)
  • Essentially granite
  • High in silica, aluminium, potassium, calcium and sodium
  • Sial
• Oceanic crust


  • Denser than continental crust 3 g/cm3
  • Essentially basalt
  • High in silica, magnesium and iron
  • Sima
• Asthenosphere


  • Plastic layer
  • 10% molten
  • Extends to depth 350-650 km below lithosphere

  
• Earth's mantle


  • 80% of Earth's total volume
  • Oxygen, iron, magnesium and silicates
• Core


  • Outer core: dense, viscous
  • Inner core: solid
  • Both approximately 5500 degrees celsius
• Internal heat


  • Main source is from radioactive decay
  • Today most of radioactive heating takes place in the crust and upper mantle
  • Heat pathways: conduction, convection
• Convection in asthenosphere leads to isostatic equilibrium
• Ice retreated 97 km since 1794 in Glacier Bay, Alaska, showing the land beneath to rebound by 5.5m
• • Earthquakes do not occur at random places
  • Seismic activities appears concentrated in lines and arcs
  • Continents are like a jig saw puzzle
  • Fossil evidence
  • Glacial deposits
• Continental drift


  • Theory developed by Alfred Wegener
  • Suggested Earth's land had been a supercontinent named Pangaea and Panthalassa
  • Opposed by geologists' view of the makeup of the mantle at the time

  
• In 1935, Kiyoo Wadati speculated earthquakes and volcanoes near Japan to do with continental drift
• Seismic waves reflected and refracted through inner layers, with slower speeds in upper mantle, suggests the mantle is not solid
• Seafloor spreading to plate tectonics

  1. Mantle convection brings magma up

  2. Fractures crust

  3. Cools to form new seafloor

  4. Builds mid-ocean ridges

  5. Spreads seafloor laterally causing rifts
• Plate tectonics


  • Approximately a dozen or so major lithospheric plates
  • Move about 2.4 inches per year
  • Interact with plate boundaries

  
• Plate boundaries


  • Divergent
  • Convergent
  • Transform
• Plate movement


  • Powered by heat generated convection currents in the mantle
  • Plate movement powered by gravity
• Divergent boundaries


  • Spreading centers with upwelling of mantle
  • Zones of tension and constructional processes
  • E.g. Mid Atlantic Ridge and Great Rift Valley in East Africa
• Divergent plate boundaries


  • Separating continents
  • The Red Sea
  • Mid Atlantic Ocean Ridge
• Convergent boundaries


  • Collision zones
  • Zones of compression and crustal loss
  • Destructional process
  • Mountains, volcanoes and earthquakes

  
• Transform boundaries


  • Plates slide laterally past each other at right angles
  • Result in right angle fractions
  • Usually no volcanoes associated
  • Transform faults are parallel to the direction in which the plate is moving
  • Due to horizontal motion
  • No new crust formed
  • No old crust subducted

  
• Confirmation of plate tectonics: new ocean floor was hot at spreading centers, older oceanic crust shrank in volume and became more dense, ocean is deeper over the older oceanic crust farther away from spreading center

  
• Hot spots


  • Stationary sources of heat in mantle
  • Not always at plate boundaries
  • As lithosphere plates slide over hot spots they are weakened from below
• Terranes


  • Floating crustal pieces that attaches to a continent
  • Thickness and low density of terranes prevent their subduction
  • Differ from rock composition to their new continental homes

  
• Paleomagnetism


  • Tiny magnetic particles found in basaltic magma
  • As rock cools it fixes the orientation of Earth's magnetic field in the rock at that time
  • Use magnetometer to measure amount and direction of residual magnetism

  
• Earth's magnetism


  • Outer core generates 90% of Earth's magnetism
  • Magnetic field protects Earth from solar wind and cosmic radiation
• Wilson cycle


  • Plate tectonics shows an active cycling of Earth's surface
  • Ocean floors are formed and destroyed
  • Mountains erode
  • Sediments subduct
  • Continents rebuild
  • Seawater moves from basin to basin