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?
