Earth Structure, Magnetism, Continental Drift and Plate Tectonics

Earth Structure
1. Changes with depth
  1. Pressure-weight of the overlying rock increases with depth
  2. Temperature-Heat is generated in Earth's interior by radioactivity
  3. Geothermal gradient: Rate of T change varies with depth, geothermal gradient varies at about High 50 celsius per Km to low 15 celsius determined by tectonic setting
    1. Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior.
2. Investigation into the Earth
  1. Seismology-use of earthquake waves to map interior of Earth
    1. Seismic wave velocities change with density allowing us to determine depth of seismic velocity changes indicates density changes)
      1. 2 Types
        P-waves-compressional-travel through sold and liquid
        S-waves-shear-travel through solid NOT liquid
        Liquids not rigid enough to transmit shear
  2. Seusmological evidence for liquid outer core as no s-waves. Shadow waves caused via refraction to give us a size of the core to outer core.
    1. Seismic waves measured due to 1950's Global seismographic network to monitor nuclear tests
    2. Liquid outer core drives Earth's Magnetic field
      1. Core radius: 3,471Km
        Outer core:
        Liquid iron-nickel-sulphur
        2,255km thick
        Density 10-12g/cm3
        Inner core
        Solid iron-nickel alloy
        Radius of 1,220Km
        Density 13g/cm3 like core of proto-planets (iron meteorites)
Magnetic Field
1. Molten iron in core circulates along a spiraling path in columns aligned in the N-S direction generating electrical currents to set up the dipolar magnetic field
  1. Known as Geodynamo
2. Like a dipole bar magnet: N pole is near Earth's geographic S pole due to opposites attracting
3. Magnetic ffield lines extend into space, weaken with distance.
  1. Form a shield known as the Earth's magnetosphere
    1. Solar wind distorts the magnetosphere (shaped like a tear drop)
      1. Deflects solar wind, protecting Earth
        Magnetic field concentrates charged particles of the solar wind in the Van Allen belts
        Lack of protection would lead to atmosphere being "hosed away"
        Van Allen belts concentrate deadly plasma from sun and expand when increased solar activity
        Inner Van Allen Belt: Protons and electrons. Outer is mostly electrons
4. Revealed by spectacular auroe where charged particles are channeled along magnetic lines where they ionise atmospheric gases in polar regions and glow
  1. Northern Lights: aurora borealis. Southern Lights: aurora australis
Earth's Crust
1. Crust: Continental, Oceanic
  1. Continental underlies continents:
    1. Average thickness 35-40km - thickest 70km under mountain belts
    2. Average density of 2.7g/cm3 (more buoyant)
    3. Composition is Felsic (granite) to intermediate
  2. Oceanic underlies ocean basins
    1. Average thickness 7-10km - thinnest under ridges
    2. Average density of 3.0g/cm3 less buoyant
  3. Composition is Mafic (basaltic and gabbroic) in composition
  4. 99.5% of crust is comprised of 8elements, Oxygen being the most abundant (reflects importance of silicate minerals), occupies 93% of crustal volume
2. Mantle: Upper, Transitional, Lower
  1. Lithosphere
    1. Outermost 100-150km
    2. Behaves rigidly - non-flowing material
    3. Makes up Tectonic plates
  2. Asthenosphere
    1. Upper mantel below lithosphere
    2. Shallow under oceanic lithosphere, deeper under continental
    3. Flows as a soft solid
3. Core: Outer-Liquid, Inner-solid
4. Layers sub-divided on basis of seismic waves
5. Alfred Wegener - Continental Drift
  1. German meteorologist wrote 'The Origins of Oceans and Continents in 1915'
    1. Radical ideas of Continental drift and formation of supercontinent of Pangea
      1. Before, oceans and continents thought as FIXED
      2. Led to Plate tectonics where in 1960's 'paradigm' shift, Harry Hess proposed sea floor spreading, 1968 complete model made linking continental drift to sea floor spreading and subduction. 20 plates!
        EVIDENCE FOR ALFRED WEGENER
        Present shorelines fit: Bullard 1965 continental shelf edges make a better fit
        Glacial evidence of Permain glaciers found on 4 continents. Glacial deposits converge in Pangea model
        Fossil evidence of identical fossils on widely spearted landmasses:
        Mesosaurus: freshwater reptile
        Glossopteris: subpolar fern with heavy seeds
        Cynognathus and Lystrosaurus-mammal like reptiles
        Geological units
        Distinctive rock assemblages across Atlantic of structures, rock types and ages: Proterozoic 2,500-542Ma and Archean 4-2.5 Ga
  2. Criticism though due to No Mechanism thus hypothesis died
    1. Revived in WW2 via Polar wandering, Palaeomagnetism Bathymetry of the oceans via echo sounding, global seismology, sea-floor spreading detected by magnetometers
      1. Palaemogagnetism: Geographic and magnetic poles not parallel where the difference (declination) between geographic North and Magnetic North is constantly varying: 11.5 degrees declination atm)
        Curved magnetic fields causes magnetic needle to tilt this is called inclination influenced via latitude, normal or reverse polarity
        Magnetic signals archived by iron minerals: Hot magma no magnetization due to kinectic energy of atoms very high, magnetic dipoles randomly oriented
        Cools through 'curie temperature) 540 celsius for basalts, dipoles align with mag field, magnetic dipoles 'frozen' into rock
      2. Polar Wandering
        Layered basalts record magnetic changes overtime
        Inclination and declination indicate change in position
        Known to be an artifact as it is not the signature of a wandering pole on a fixed continent,
        Signature of a fixed pole on a wandering continent
        Apparent polar wandering is strong evidence for drift!
        Each contient has a separate polar wandering path, paths align when continents joined as Pangea
        The slow erratic movement of the earth's poles relative to the continents throughout geological time, due largely to continental drift.
      3. Sea-Floor Bathymetry
        Until 1940-lead weight used to measure depth
        Echo-sounding (sonar) created Bathymetric maps revealing:
        Mid-ocean mountain range (every ocean), Deep Ocean trenches occur near volcanic island chains, submarine volcanoes ocean floor, huge fracture zones segment the mid ocean ridge, oceanic transform faults.
        Belts of subsea earthquakes found limited to mid ocean ridge axes, parts of oceanic fracture zones, deep ocean trenches
        1920's seismology identified zones of earthquakes parallel to oceanic trenches inclined 40-60 horizontal and extending several 100km into Earth - subduction zones
      4. Sea-Floor Spreading
        1960, Harry Hess published 'Essay in Geopoetry', theory 'Sea-floor spreading'
        Upwelling mantel erupts at mid ocean ridges
        New crust moves away from ridges gathering sediment
        At trenches, sea-floor subducts back into mantle
        Provides mechanism for continental drift
        Contient move apart as sea-floor spreading occurs, continents move together as sea-floor is subducted
        Layered lava flows reveal reversals in magnetic polarity which are geologically rapid - time markers
        Magnetism in sea floor varies across MORs, stripes of positive (stronger) and negative (weaker) magnetic intensity (symmetrical about MOR)
        Sea floor spreading explains stripes as magnetic polarity are imprinted in sea-floor as sea floor continues to spread.
        Determine rate of sea floor spreading: Mid Atlantic Ridge 2cm/yr, East Pacific Rise 10cm/yr
        Age-Dating rock gives timing of polarity reversals which occur in uneven intervals
        Longer intervals (500 to 700+ Ka) called chrons
        Shorter intervals (200Ka) are subchrons
        Magnetic reversals time scale extended to 170 Ma
Plate Tectonics
1. 1968, evidence overwhelming
  1. Mechanism explains: Origin of continents and ocean basins, distribution of earthquakes and volcanoes, distribution of igneous, sedimentary and metamorphic rocks, distribution of fossil platts and animals and the genesis and destruction of mountain chains "orogeny"
2. Tectonics in Lithosphere
  1. Principles of Buoyancy and "isostacy"
    1. Archimedes 2000 years ago of floating solids displace water equal to their mass. e.g. iceberg sinks until mass of water it displaces is equal to the total mass of iceberg, applies to lithosphere plates
    2. Maintaining "Isostatic equilibrium"
      1. Glaciation:
        1) Isostatic equilibrium destroyed by loading of ice sheet, mantle moves to restore
        2) Ice sheet melts destroys equilibrium, mantle moves to restore equilibrium e.g. Post-glacial rebound (glacial-isostatic adjetment)
      2. via erosion and redeposition of sediment:
        Flow in asthenosphere acts to form isostatic equilibrium
        Erosion of mountains, transport and redeposition destroys equilibrium, mantle flows to compensate
        Movement of asthenosphere mantle restores isostatic equilibrium
3. 20 Tectonic plates, average 15cm/yr, identified via active margins, subduction zones, volcanoes, plate interiors almost equarthquake free.
  1. Volcanoes mark subduction zones and some spreading ridges and rifts
    1. Continental margin is where land meets ocean
      1. Margins near plate boundaries are 'active', far are 'passive'
      2. Passive-margin continental crust thins seaward: transition into oceanic crust, traps eroded sediment, develops continental shelf (the area of seabed around a large land mass where the sea is relatively shallow compared with the open ocean. ) (mainly Atlantic)
4. Plate Boundaries:
  1. Divergent Boundary-Plate moves apart, Lithosphere thickens away from ridge axis, spreading boundary, mid ocean ridge
    1. How plates break up
      1. Phase 1: Rifting, Lithosphere stretched, doming and faulting, crust thins, mantle upwells, pressure causes melting thus volcanism develops e.g. East African Rift Valley
        Phase 2: Drifting - Ocean crust formation, develops MOR, initiation of sea-floor spreading, development of new continental margins
        Fast spreading=smoother topography (EPR) 55mm/yr, slow=faulted central rift valley and more wide spread volcanism 12mm yr (MAR), Ultraslow spreading 7mm/yr (Southwest Indian Ridge)
  2. Convergent Boundary - Plates move together, subduction zone, trench, etc
    1. Subducting plate descends at an 45 degree angel revealed by Wadati-Benioff Earthquakes (lanar zone of seismicity corresponding with the down-going slab in a subduction zone.) marks frictional contact and mineral transformations, earthquakes deepen away from trench
      1. Quakes cease below 660km, plate descent may continue past earthquake limit, lower mantle may be plate graveyard
        Subduction features - Accretionary prisms (sediment scraped off), Back-arc basins, Volcanic arcs (Andes) (RESEARCH)
        Plate collision: Subduction, ocean closure, continental collision, continental crust too buoyant to subduct
        If continental and continental, mountains uplift e.g. Himmalayas
  3. Transform Boundary - Plates move sideways, Plate material neither created or destroyed, transform fault.
    1. Lithosphere slides past, not created or destroyed: many transforms offset spreading ridge segments and can cut through continental crust (Continental tranforms e.g. San Andreas Fault)
      1. Characterised by: Earthquakes, absence of volcanism
        Oceanic Transforms: MOR axis is offset by transform faults, a geometric necessity for a line spreading on a sphere
        Abundant earthquakes common when offsets are opposed, earthquakes vanish when either side of fault moves in same direction: Strong evidence of sea floor spreading
  4. Spreading ridges: Black smokers which are metal and sulphide rich plumes, unstable in seawater where precipitation of metal ores occur, energy can support life without light - chemosynthesis
    1. Oceanic lithosphere, hot asthenosphere at base of MOR, aging ocean crust moves away from heat, moves cools causing increased density and sinking, accumlating increasing thickness of sediment
  5. Subduction: Old oceanic lithosphere more dense than mantel, flat lying oceanic crust doesn't subduct easily, once bent downward leading edge sinks
  6. Driving mechanism: Old idea of convecting mantle but plate motions too complex for this, not prime mechanism. Model of a custard shape (see google images)
    1. Plate tectonics started as rigid crust - continuous 'stagnant lid regime' (like Mars). crust cracks and becomes a mobile regime, Venus is episodic regime (crust cracks and entirely subducts to reform
      1. Modern Thinking: Two froces
        Ridge Push-elevated MOR pushes lithosphere away
        Slab-pull-gravity pulls a subducting plate downward
        Convection in the asthenosphere adds or subtracts
  7. Triple Junctions: Where 3 plate boundaries intersect, multiple boundary combinations, these migrate and change across time
  8. Hot Spots: Plumes of deep mantle material independent of plates burn through plates and add lines of volcanoes to them. Hot spot seamounts age away from originating hot spot, line of seamount indicates direction of plate motion and age change marks rate of plate motion
    1. Plate Velocities mapped by Plotting plate motion relative to a fixed spot in the mantle or measuring ages/distance along a hot spot track
  9. Tectonics essential for life: Drives rock cycle by providing a global thermostat recycling chemicals crucial to keeping CO2 content relatively uniform. Enabled liquid Water, avoided runaway greenhouse gases (Venus), controls long term sea level chage, also important for CO2
    1. Fostered the development and maintenanceof complex life by creating and maintaining a changing variety of different environments ranging for mountain chains to deep ocean to allow and maintain global biodiversity, best defense against mass extinction. Overtime time, continents provide key to terrestrial environments for animal life to develop.

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	Creado por sebastian1862 hace alrededor de 10 años