Plate Tectonics

Prof. J. Renton

Outline lecture notes** (JA)

  • Recap on important words/terms
  • Earth has inner iron core, peridotite mantle, brittle, 40mi thick outer crust
  • Lighter things always float on denser ones
  • Basaltic oceanic crust, granitic continental crust
  • Plastic asthenosphere 100-150mi down can flow, above it is lithosphere (crust + upper mantle)
  • Convection cells in asthenosphere have alternating rising/falling streams, creating tension/compression in crust
  • Tensional forces break brittle lithosphere into plates. Under continental crust they create cracks up to surface -> rift zone
  • Rising asthenosphere liquifies to low-viscosity granitic magma, rises up thru cracks, meets groundwater -> hot springs, fumaroles. Eventually magma surfaces -> small lava flows, fire fountains. Descending lava cools -> cinders, which collect -> cinder cone.
  • Rift zones in Albuquerque (aborted) and East African Rift (central part collapsed -> rift valley)
  • When one end of rift reaches ocean, seawater spills in (Red Sea) -> linear ocean
  • When other end breaks, forms an island and starts new ocean (with oceanic ridge)
  • Pangea broke up this way. Alfred Wegener matched up rock types, fossils, glacial deposits/striations between S America and Africa.
  • If oceans growing, crust must be consumed. At deep sea trenches, compressional forces force denser oceanic lithosphere down under lighter continental lithosphere -> zones of subduction.
  • Oceanic lithosphere melts -> viscous granitic and runny andesetic lavas.
  • Andesitic lavas rise to surface -> arc volcanoes, continental arc if trench near coast, island arc if further offshore
  • Granitic magma rarely reaches surface, big blobs attach to edge of continent, adding granitic rocks to continent.
  • Always (denser) oceanic lithosphere subducts below (lighter) continental, never other way round.
  • Opening oceans create new ocean bottom, so oldest oceanic crust ~200m years old and nearest to continents.
  • Pacific Ocean closing on 3 sides, eaten up faster than new ocean floor created.
  • Crust formed 3.5-4bn years ago, some continental rocks dated to 4.1bn years old. They don't subduct so are never recycled, plus extra added by rising granitic magma.
  • J. Tuzo Wilson proposed 500m year cycle of supercontinent formation and breakup. Continents drift apart for 250m years and then come together to form new, different supercontinent.
  • Assuming plate tectonics started 3bn years ago, now in 6th cycle. Atlantic Ocean still opening, Indian as big as it will get, Arctic "resting", Pacific closing.
  • Americas will only travel another 8-900mi West, not even as far as Hawaiian Islands
  • Called the Wilson Cycle

Further Information

Plate Tectonics Infodescription [Wikipedia]
Plate Tectonics Revision notesfirst 3 pages [BBC BiteSize]
GeolSoc Plate Tectonicsdescription [British Geological Society]
Wilson Cycledescription [Wikipedia]
Geology Page Wilson Cycledescription [Geology Page]

** more detailed lecture notes available to group members on request.