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October 31 Lecture

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University of Toronto St. George
Earth Sciences

GLG 105 – October 31, 2011 Last Class – Differences between 2 hypothesis – sea-fthor spreading accounts for the magnetic aspect Sea-floor spreading made it into the 20 century Lecture 13 - Plate Tectonics 1. Guyots Harry Hess – mid 1900’s – mapped deepest part of ocean – found “guyots” = flat-topped mountains (as if tops were eroded but 2km below the surface) – Paper History of Ocean Basins 1960-2 – explanation for guyots = new oceanic crust (& mantle) generated at MOR – guyots formed near MORs and sink as the move away because ocean depth increases (aside, Darwin - formation of coral reefs built up from submerged volcanoes, volcanoes had once broken above sea-level) - why would a mountain below the sea-level be eroded? - what used to be a mountain just at sea-level moves further away from the MOR 2. Magnetic Sea-Floor Anomalies Fred Vine, Drummond Matthews, Lawrence Morely – 1963 – explain magnetic bands along MORs - Vine Matthews hypothesis - very difficult to read, Vine-Matthews-Morely Hypothesis – bands formed together at MOR then separated as the MOR created new rock – that’s why you have the parallel lines Ex. Vancouver Island – ages are symmetric away from the MORs Magnetic anomalies can be explained by sea-floor spreading! Sea-floor anomalies allow for estimate of plate velocities in recent geologic past - feedback between how fast plates move ad magnetic timescale 3. Hot-Spot Tracks Tuzo Wilson – changed sides - in 1963 wrote articles in support of plate tectonics – “hot spot” area on the earth’s surface that originates in the mantle – volcanoes forming above hot mantle upwelling – Hawaiian Island Chain – each island used to be a volcanoe, but as the MOR pushes them away a new volcanoe is created due to upwelling of the hot mantle - hot spot tracks allow us to estimate plate velocities (distance over age) 1. Can hot spots not be explained by the expanding Earth hypothesis? – yes, but only if they are close enough to the MOR – hot spots – because you have the sea-floor moving over the hot spots, not the hot spots moving 2. What may have caused Wilson to change sides? – 3. What is the difference between continental drift and sea-floor spreading? – the continents moving vs. the sea-floor spreading (the whole plate that moves including the continents – adding that material com
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