|MadSci Network: Earth Sciences
The crust of the earth is comprised of 20 or so tectonic "plates." The plates are rigid, and move about. Some plates are comprised only of continents, while others are part continent and part ocean floor. As the plates move, the continents move with them. The theory of "plate tectonics" deals with the movements of the plates, and the concept of "continental drift" is a part of plate tectonic theory.
New ocean floor is created at certain ridges in the oceans. At these "midocean" (they aren't always at the center of the ocean) ridges, the crust is pulled apart and molten rock rises and cools into new crust. This is called "seafloor spreading." Old crust is destroyed in ocean trenches and along some continental edges. At these "subduction zones," the oceanic crust plunges deep into the interior of the earth where it probably melts (although there is some debate about exactly what happens down there).
Geologists and geophysicists use paleomagnetism to work out the movements of the plates, and consequently, of the continents. Every million years or so the magnetic poles of the earth flip. The earth keeps spinning on its axis as it does now, but the directions compass needles point is reversed. Periods when compass needles would have pointed the same direction as now are known as periods of "normal polarity," and periods when they would have pointed south are known as periods of "reversed polarity."
The coupling of seafloor spreading with magnetic reversals produces a neat result! Rocks of the ocean crust acquire a very weak magnetic field when they cool from the molten state at the midocean ridge. The ocean crust, then, acts like a tape recorder, recording the direction of magnetic polarization at the time each portion of the oceanic crust was created. Using very sensitive magnetometers, it is possible to measure the magnetic fields of oceanic rocks. When these are mapped, they make stripes of alternating normal and reversed polarity, centered on the midocean ridge.
To work out the past motions of the plates, all you have to do is to pretend that time is moving in reverse. You can remove the first stripe of normal polarity from the midocean ridges, and move the plates to fill up the gaps, and that tells you where the plates and continents were when the most recent pole reversal occurred. By radiometric dating of the rocks formed near the time of the magnetic flip, you can tell when the plates were in that position. By doing this stripe-by-stripe, you can roll history backwards to the age of the oldest existing oceanic crust.
Since oceanic crust is being destroyed in the subduction zones, our record of plate motions is incomplete. So, reconstructions of plate motion become less reliable the further back you go because we have progressively less ocean crust to work with. The oldest surviving crust is younger than 250 million years old, which means that reconstructions are pretty shaky when you get back that far.
Paleomagnetic studies can be made of continental rocks too, and that helps reconstruct plate motions before 250 Ma. By determining the ages of continental rocks and the orientations of their paleomagnetic fields, geologists can work out how the continents moved in earlier times. The geology of continents is more complex than the geology of ocean basins, so "apparent polar wandering curves" constructed from continental data are less precise than reconstructions based on oceanic crust. That means that the reconstructions before 250 Ma are very uncertain. The word "fantasy" comes to mind!
The Archean Era ended 2,500 million years ago, about one-half of the way back to the origin of the earth. Plate reconstructions from that time are complicated by the fact that less continental crust existed then (the continents grow through time), and the geology of exposed Archean crust is very complicated because of its great age. Furthermore, plate tectonics during the Archean was different than today, probably because there was more heat being generated by the decay of radioactive isotopes. If you find Archean plate reconstructions they might be good for wrapping fish.
We can't use seafloor stripes or continental polar wandering curves to predict future movements of the plates. For that we would have to use theories of what causes the plates to move. We know that plate motion is closely related to the convection of hot material in contact with the bottoms of the plates. The plates tend to ride on this hot material as though they were on a conveyor belt. However, there may be other things, like bumping into other plates, influencing plate motions. At this time, we don't know enough about the processes involved to make accurate predictions of how the plates will move in the distant future, maybe beyond 25 million years.
In the near term, less than, say 25 million years, we can predict plate motions by assuming they won't change much from their present motions.
Interestingly enough, an inexpesive CD which shows past and future plate motions has just been released. You can download a free demo from: http://www.cdvision.com/
The sun, earth, and the rest of the solar system are about 5 billion years old. The sun has another 5 to 7 billion years to go in its present state. Then it will become a red giant and toast anything on the earth.
Here are a couple of questions for you: Are predictions of future plate motions science? If you say "yes," how will these predictions be experimentally verified? If you say "no," what is plate prognostication if it is not science?Additional Info submitted by Steve Collins:
"Like any historical endeavour, the farther you go back in time, the less information you have to work with.
"I have been making maps of the past for over 25 years now, based on
my experience I would say that the accuracy of the maps varies roughly
0 - 200 Ma 100-90% accurate
200 - 350 Ma ~80% accurate
350 - 500 Ma ~60% acurrate
500 - 750 Ma ~50% accurate
750 - 1,100 ~30% accurate
"By accuracy, I mean if you could travel back in time, how much of a match would there be between the observed positions of the continents and the "mapped" positions.
"As your reply states, magnetic anomalies are the best way to reconstruct the continents (back to 200 Ma). Further back in time we have to rely on paleomagnetism, paleocliamtic evidence, biogeographic evidence and tectonic history. Though less precise, these lines of evidence do constrain the past positions of the continents.
"I agree with you, however, and doubt that we will ever be able to map the Archaen (earlier than 2,500 Ma).
"I would make a minor point. Most geodynamicists would not agree with your statement that the plates are passively dragged along by mantle convection cells. The plates actively drive themselves. The weight of the slab going down the subduction zone pulls the slab, and the pressure of molten rock upwelling at the midocean ridge pushes from the other edge. It's convection, but of a different sort."
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