EESA06H3 Chapter Notes - Chapter 2: Subduction, Convergent Boundary, Asthenosphere

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12 Aug 2016
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Geophysical research:
Polar wandering: wegener's work dealt with the wandering of Earth's geographic poles
of rotation
The magnetic poles are located close to the geographic poles.
Historical measurements show that the position of the magnetic poles moves from year
to year but the magnetic poles stay close to the geographic poles as they move.
We are referring to an apparent motion of the magnetic poles.
The magnetism of old rocks can be measured to determine the direction and strength of
the magnetic field in the past.
The study of ancient magnetic fields is called paleomagnetism.
Old rocks reveal very different magnetic poles positions to those at present and it was
once thought that this indicated movement of the poles "polar wandering" but it is not
recognized that the rocks (and their paleomagnetic records) have moved as part of
migrating tectonic plates.
Apparent polar wandering paths are now used to reconstruct continental movement
over time.
Every continent shows a different position for the Permian pole. A different magnetic
pole for each continent seems highly unlikely.
A better explanation is that a single pole stood stil while continents split apart and
rotated as they moved.
Recent evidence for continental drift:
If the continents are fitted together, like the pieces of a jigsaw puzzle , geological
features should match from piece to piece
Many of the boulders in South American glacial deposits have been traced to a source
that is now in Africa.
Rock matches show when continents were together; once the continents split, the new
rocks formed are dissimilar.
Pangea split up 200 million years ago to form our present continents.
Pangea was formed by the collision of many small continents long before it split up
What is sea floor spreading ?
Harry Hess proposed that the sea floor might be moving too
Contrasted with Wegener's idea because he said that the ocean floor remained
stationary as the continents ploughed through it
Hess's 1962 proposal is named sea floor spreading for it suggests that the sea flood
moves away from the mid-oceanic ridge as a result of mantle convection.
The initial concept of sea floor spreading said;
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