Another form of evidence came from continental rocks. Old rocks had different magnetic fields recorded in them than the field today.
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A D V E R T I S E M E N T
More precisely, the inclination of the field was different. Figure two shows the magnetic field of the Earth. The dip (inclination) of the field lines change as you move north or south. This is recorded in igneous rocks the instant they cool and is set permanently. From this you can tell the latitude that the rock form in. So, if you have a rock with a horizontal inclination, then it formed at the equator.
Rocks from all over the world were found to have inclinations that did not agree with the continents present locations. Two things could have caused this - the magnetics poles move over time or the continents move. The only way to match the data from all the samples was to move the continents around. However, this movement is plotted as if the poles moved in a "Apparent Polar Wander Path". Don't be confused though, it is the continents that move, not the poles. These "Polar Wander Paths" were also used by Wegner by calculating pole position from climate belts.
How do we know what the plates do when they are subducted or how they are formed? Most of the evidence of this come from geophysical studies. The most modern way of looking at plate tectonics is using GPS. The results from these studies, lasting 5 or more years, the rates of plate movement have been calculated with great accuracy. However, these values agree near perfectly with results from sea-floor spreading and other methods of calculating spreading rates.
Seismic tomography is a method of using seismic waves from earthquakes (plus some other data) to create 3D images of the mantle. These studies pick out areas of fast or slow mantle, which correspond to areas of high and low temperature. From these studies you can actually see subducting slabs and upwelling at mid-ocean ridges. Figure three is a seismic tomographic map at 50km depth. You can see features such as the East-Pacific rise and the Indian ridge systems, the volcanoes around the Pacific rim and even the Hawaiian hotspot (just), all of which show up as magenta
More traditional evidence comes from gravity studies. The gravity anomaly over a subduction zone shows many features. Figure four shows a typical gravity anomaly. A high gravity anomaly shows area of high density, whereas a low anomaly shows areas of low density.