Week 1: Plate tectonics and forces

• 2 main classifications of plates

  Continental - silica rich, buoyant, typically younger in age than oceanic, 10-70km thick

  Oceanic - basaltic rich, dense and heavy plates, oldest material is found in area furtherest from spreading zones, 5-10km thick
• 3 main type of plate boundaries and examples

  o   Convergent – Himalayas

  o   Divergent – Mid Atlantic Ridge

  o   Transform – San Andreas Fault
• Main drivers of plate motion

  o   Slab pull – when a subducting plate experiences a gravitational force, the subducted slab will pull the rest of the tectonic plate towards it. E.g., The Northern boundary of the Pacific plate

  o   Ridge push – typically a mid-ocean ridge where the spreading zone of the ridge creates new crust, the creation of this crust displaces and pushes the plates apart on both sides of the spreading zone. E.g., The Pacific-Antarctic Ridge (Southern boundary of the Pacific plate)
• How fast is the Pacific plate moving

  7-11cm NW each year, it is one of the fastest moving plates
• Why is the Pacific plate one of the fastest moving plates

  It is one of the fastest moving plates because at these boundaries is where the main drivers of plate motion occur. These are slab pull and ridge push.

  • Slab pull to the North (Subduction under the North American Plate)
  • Ridge push to the South (Pacific-Antarctic Ridge)

  By experiencing constant driving forces on both sides of the plate, without orogenic zones- results in the Pacific Plate moving fast
• Subduction resistance

  • As the subducted plate is pulled beneath into the mantle, there exists frictional forces before rocks melt down
  • This force slows the rate of subduction and therefore the motion of the rest of the plate (slab pull)
• Viscous drag

  A resisting force exerted by the asthenosphere onto the underside of lithospheric plate which opposes plate motion
• Collisional resistance

  • At an orogenic zone where two plates are colliding exist frictional forces
  • The amount of frictional force varies based on density of rock and pre-existing structures
  • Over time the rate of two plates crashing into each other will slow
  • E.g., the Indian plate colliding with the Asian plate is slowing due to the Himalaya Orogeny
• Tectonic setting of the North American Plate

  • North American Plate:
  • Western boundary-subduction zone directly next to a major transverse fault
  • Eastern boundary- mid-Atlantic ridge
  • Southern boundary- dominated by strike slip boundaries and other divergent boundaries
• Tectonic setting of African Plate


  -The eastern side of the African continent (east African rift) is currently experiencing active rifting; this rifting is contributing to the formation of a new mid-ocean ridge as overtime an ocean will open up in this area as a result of spreading.

  -The western boundary is the mid-Atlantic ridge while to the north the plate Is converging and subducting under the Eurasian plate.
• Himalayan Mountain Range

  • Active orogenic zone between continent-continent plates
  • Thick, wide orogen that extends into the interior of the Asian plate
  • The convergence of tectonic plates resulted in this mountain building, when orogens are created the plates experience crustal thickening and surface uplift
  • Surface uplift and isostasy in this region, due to buoyancy of the crust lifts the mountains higher.
• Why is the Tibetan Plateau only so high

  -Isostatic uplift is important when it comes to the formation of the Himalaya/Tibetan orogen

  -When the critical thickness of the rocks is reached and they begin to flow outward, the stress and density on the portion of mountain has decreased, as a response to this they will equilibrate by rebounding higher, this restarts the process.

  -As a result, the Tibetan plateau can only get so high, and this is why the mountain range is consistently spreading into Asia
• What is the lithosphere

  -       The lithosphere is composed of the outmost layer of the Earth- the crust and the top, solid layer of the mantle

  -       Together they form the lithosphere

  -       The crust ranges between 10-70km thick for continental and 5-10km thick for oceanic

  -       The upper mantle is between 10-200km thick

  -       The lithosphere is broken into approximately 7 major plates, with 6-8 minor plates. These plates experience motion both relative and independently from each other