F4 - Earth Structure and Global Tectonics

• Palaeomagnetism - ancient magnetism preserved in rocks.
• 'Self-exciting Dynamo':

  • temperature different creates convection currents in outer core (convecting mass of molten iron generate electricity)
  • generation of electricity induces magnetism and creates a dynamo effect + balance between generation/destruction allows earth's weak magnetic field
  • circulation of convection currents affected by earth's rotation (affects position of magnetic poles) + convection not possible without solid inner core
• Earth's Magnetic Field; inner core solid with temperature of 5700$^oC$ + outer core liquid with temperature at mantle-core boundary of 3500$^oC$(temperature above Curie Point where magnetic materials lose magnetism, so earth's magnetism constantly generated).

  
• Variation in field-magnetic reversals; 'self-exciting dynamo' runs down as convection in core changes so magnetic field gradually fades away over several thousand years, then increases again with poles in reverse.

  • evidence found in rocks as remanent magnetism; recorded in rocks as alignment of magnetic materials according to earth's magmetic field at time of formation
• Palaeomagnetism:

  • iron-rich magnetic minerals in lava align with earth's magnetic field retaining this magnetism permanently; act as 'frozen compasses' showing direction to poles at time of formation
  • minerals in lavas erupted in normal polarity aligned north-south/in lavas erupted in reverse polarity aligned south-north + age of the lavas (+date of magnetic reversal) known from radiometric dating/from fossils in interbedded sediment
  • sensitive magnometers show normal field strength over normal polarity zone/weak field strength over reversed polarity zone
• Magnetic Inclination - the angle of dip of the lines of a magnetic field (dip angle made with horizontal/earth's magnetic field lines).

  • palaeolatitude calculated from measurement of magnetised mineral's angle from horizontal
• Magnetic Anomalies

  • earth's magnetic field reverses up to 4 times in a million years; as new magma is erupted at mid-ocean ridges, iron particles line up parallel to earth's existing magnetic field
  • as rocks cool, particles remain magnetised; results in striped magnetic anomaly pattern on sea floor shown as normal/reversed polarity (pattern parallel to MOR but vary in width due to length of time magnetic field stayed in one direction/amount of lava)
• Symmetry is evidence that crust on two sides of ridge moving apart; basalt erupts at ridge to form new oceanic crust (spreads away equally on both sides of ridge). Radiometric dating of basalt gives absolute dates from which rate of spreading can be calculated (proof of seafloor spreading).
• Apparent Polar Wandering Curve:

  • iron-bearing minerals in some rocks hold a record of earth's magnetic field at time of their formation; rocks are collected/dated with direction of palaeomagnetism measured (data plotted as apparent polar wandering curve)
  • curves for South America/Africa suggest before 160Ma north pole is 2 positions (assumed pole is fixed so it was continents that moved) + if 2 continents re-positioned next to each other, the 2 curves match up with no position for pole + curves diverge only after continents started to drift apart (167Ma)
• Apparent Polar Wandering Curve - a line on a map which joins up the apparent positions of the magnetic north pole over time.

  
• Plate Movement:

  • loss of heat from lithosphere causes it to become cold/dense enough to have negative buoyancy allowing to to sink into warmer/ductile asthenosphere; drags surface plates by slab pull (main force driving plate movement)
  • this creates tension forces elsewhere on plate thinning it and causing passive convectional upwelling of hotter mantle rock in reponse to plate movement (induced upwelling)
  • forms ridge that slides away laterally as it cools, pushing plates apart by ridge push contributing to slab pull
• • Mantle Dynamics; subducting slabs sink down to mantle transition zone at bottom of asthenosphere, before descending towards core-mantle boundary + movement of these slabs is integral part of convection that moves plates.
  • Upwelling at divergent margins appears to be passive process of the upper asthenosphere induced by thinning of the plate above by tension forces.

  
• Plate Tectonics:

  A) Arabia

  B) Scotia

  C) Nazca

  D) Caribbean

  E) Cocos

  F) Juan de Fuca

  G) Filipino

  
• Conservative/Transform Plate Margins:

  • plates slide past each other in horizontal movement (no subduction, volcanoes or creation/destruction of crust)
  • eg. San Andreas Fault; plate movement 5cm/yr + shallow focus earthquakes (active and locked sections of fault)
  • opposing movement of plates caused shear stresses, eg. Ventura Anticline, Los Angeles has 500,000yr old sediments/small thrusts
• Continental Rifting (Divergent Margin):

  • rift valley is linear strip of crust that has slipped down along normal faults that dip towards valley; normal faults caused by crustal extension (pulled apart by rising magma)
  • strip of crust subsides between fractures due to gravity to form valley + shallow focus earthquakes common along fault lines
  • eg. East African Rift Valley; divergent boundary where new basaltic ocean crust is formed (split started 40Ma)
• 170Ma, South America seperated from Africa along major rift system in centre of Gondwanaland; extrusions of plateau basalts accompanied rift. Rift valley widened/subsided below sea level + continental crust become thinner leaving wide continental slopes no each side of plate.
• Oceanic-Oceanic Convergent Boundary:

  • slower, older, denser, and colder oceanic plate subducted at trench into area of high temperature (partial meltining of basaltic crust starts); melted minerals seperate/rise as intermediate magma erupted at surface (andesitic/basaltic magma)
  • resulting island arc reflects curving shape of convergent plate boundary beneath (rocks on arc are metamorphosed by increase in temperature/pressure)
  • eg. Pacific-Eurasian Plate Margin, Japan
• Oceanic-Continental Convergent Boundary:

  • magma rising from subducting oceanic plate causes partial melting of continental crust + magma reaching surface form rhyolitic volcanoes (some mix with mafic magma to form intermediate lava), eg. Andes Strato Volcanoes
  • compression of continental crust sediment forms fold mountain chains (+large batholiths intruded deep in core of fold mountains); rocks are regionally metamorphosed (gneiss/migmatite in core grade outwards through schist/slate)
• Continental-Continental Convergent Boundary:

  • two continental plates of similiar composition/density meet forming fold mountains, eg. collision of India with Eurasia formed Himalayas
• Mid-Ocean Ridges (Divergent Boundary):

  • MORs made of mafic/ultramafic igneous material from upper mantle + magma accumulates in shallow magma chambers and moves up through feeder sheeted dolerite dykes to erupt on seafloor as fissure lava flows (plates melt from decompression melting) + shallow focus earthquakes occur along MORs
• Rheid - a non-molten solid that deforms by viscous/plastic flow, in response to an applied force.

  • Rheid Asthenosphere; upper mantle at high temperatures so further increase causes low-melting point minerals to melt + only 1-5% of molten material needed for asthenosphere to act as rheid + molten fraction found between solid crystals act as lubricant reducing rigidity/seismic wave velocities.