The earth's magnetism has been recognized for many hundreds of years. In addition to geographic north and south poles there are magnetic north and south poles. Compasses work because of this. The earth rotates an its axis, which can be thought of as a straight line between the poles. The magnetic poles are thought to be moving slowly around the geographic poles. But, the magnetic poles do not match the geographic poles. In fact, they are currently about 11.5 degrees apart. It's usually assumed that they've never been much further apart than this.

It's now known that the magnetic poles can reverse. Now the flow of magnetic force exits the globe at the South pole and re-enters at the North pole. That's assumed to be normal. When the opposite happens, that's considered a magnetic reversal. If you were on earth during a time of reversal, your compass would always point south instead of north, as it does now.

A device called a magnetometer can scan the earth from the air, or over oceans, to reveal the various patterns and strengths of magnetism anywhere on earth. There are spots where magnetic strength is much higher or much lower than the surrounding area. Scientist try to explain causes of such differences or anomalies. Some causes include differences in rock types and differences in circulation patterns of the earth's liquid outer core.

Paleomagnetic studies, the study of ancient magnetic fields, are based on one key fact. As certain rocks are formed, they store a record of the direction of earth's magnetism at that time--and its strength. Magnetic crystals in lava and iron compounds in sedimentary rocks such as red sandstone are key types studied.

By comparing the magnetism of different rock samples, widely separated in time or location, it's possible to create a picture of magnetic changes over time. The magnetic field of studied rocks either points north, or south. The catch is that this is a good record only if the rocks have not been super-heated after the initial magnetic record has been stored. That would erase the original magnetic information and replace it with a more current one.

Studies of magnetism in North American lava flows indicate that during the Permian Period of the Late Paleozoic Era, the North magnetic pole was in eastern Asia.

In the Late Permian, mass extinctions of shallow water invertebrates mark the end of the Paleozoic Era, roughly 225 million years ago. Some think that shallow-water shelf environments were destroyed by the 'fusing' of the edges of masses that made up the super-continent Pangaea.

Other lava flows point to positions not in eastern Asia. In fact, many believe all this is evidence for that the idea that the continents carrying the lava moved while the magnetic poles themselves did not move.

The dip of magnetized crystals indicates the distance of the lava from the magnetic pole at the time. That's because the magnetic lines of force going through the earth dip more steeply as you get closer to the dominant magnetic pole. Currently that's the North pole. So, if you had a compass in your hand, you would notice the needle pulled further and further down as you came closer to the North pole. Standing right on the magnetic pole you would expect the needle to try to point straight down. During a reversal, that same would happen as you came closer to the South pole.

But again, there's a catch. Movement is relative. What moved? The poles themselves or the rocks?

Based on similarity of rock sequences across different continents, Alfred Wegener believed that in the early Mesozoic Era the so-called supercontinent or masses called Pangaea started to move apart.

All this raises at least two questions:

Assuming Pangaea existed in some form, exactly what mechanism is most likely to account for the convergences and divergences of these gigantic masses?

What are the best evidences and assumptions for determining the rate of change between the 'stages' of convergence and the 'stages' of divergence among Pangaea's masses?

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