Continental Drift

Prof. John J. Renton

Outline Lecture Notes** (JA)

  • Plate tectonics has revolutionised our understanding of geology since 1965.
  • Earth cooled into inner core (heaviest), mantle and crust (lightest).
  • Liquid rock called magma below surface, lava above.
  • Siiica content of magma (granitic = 75%, basaltic = 45%) determines its viscosity. Basaltic like olive oil, granitic like molasses.
  • Thinner basaltic magma rose faster and spread out over surface. Granite hit surface later and formed islands in basalt.
  • Water vapour from magma formed cloud cover until surface temps allowed rain to fall and settle.
  • Idea that continental outlines fitted together and were once joined not well received in WW1. No energy source or mechanism to split continents.
  • WW2 SONAR mapping showed network of oceanic ridges, rather than flat plain, also trenches parallel to coasts, with associated volcanic arcs
  • Geomagnetism studies revealed mirror-image pattern of magnetic reversal on either side of ridges - implying ocean floor spreading.
  • Crust creation at ridges requires consumption elsewhere to maintain earth's size - trenches are where crust is forced down under continents (subduction) and melts to cause volcanic arcs.
  • Seismic studies showed plastic layer (asthenosphere) 100-250 miles down. Convection currents in this layer move plates above, along with continents. Up-currents spread ocean floor, down-currents drive subduction, providing both energy and mechanism for continental drift.
  • Entire plates move, taking continents with them. When continents collide, mountains are formed.

Further information

Plate Tectonicsdescription [Wikipedia]
Alfred Wegenerlife and works [Wikipedia]
Continental Drift theorydescription [Wikipedia]
Structure of Earthdescription [Wikipedia]

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