Plate Tectonics And Global Warming

Plate tectonics is one of the three major types of climate forcing in the natural world (the other two being changes in the Earth's orbit and changes in the strength of the Sun). Climate forcing is when a mechanism "forces" the climate to change. These are both natural and human-caused climate forcing mechanisms. Unlike some forces that have a direct effect on climate, such as the atmosphere, the land's surface, and vegetation, this one works very slowly, over millions of years.

Because scientists have been able to reconstruct the past positions of continents and the shapes of ocean basins, it has allowed them to identify periods of global cooling (icehouse) intervals, when massive ice sheets covered much of the Earth, and greenhouse intervals, when no ice existed. In addition, when scientists can accurately measure plate tectonic processes as they have changed the Earth's surface and then compare these changes to the climate that occurred at that point in geologic time, they can determine possible cause-and-effect relationships between changes in the Earth's tectonic system and its climate.

One of the most helpful characteristics that has allowed scientists to look back through the window of time and reconstruct the ancient positions of the continents is the Earth's natural magnetic field. Some of the Earth's molten rocks, such as basalts, are rich in highly magnetic iron. When they cool, the iron-rich elements orient themselves to the magnetic north, turning them into fossil compasses. When scientists use the magnetic orientations of the rocks to determine how the rock was positioned relative to north at the time the formation was created, they can recon-

struct the past positions of continents and ocean basins. This discipline is called paleomagnetism.

Paleomagnetism has been successfully used to reconstruct movements of plates and rates of seafloor spreading. The drawback to this technique is that it can only date rocks back 175 million years, because that is the age of the oldest rock; anything older has already been subducted beneath a continental plate and remelted in the plate tectonic process. In order to go back farther in time, scientists must look at rock records on continents.

The technique is also limited to continental shifts in latitude, not longitude. This is because when continents are at the equator, their compass setting will be horizontal to north and at the North Pole the compass will be nearly vertical. This is because the equator is the farthest point away from the poles, giving the largest declination. Longitudinally, it does not matter because the longitude reading does not affect the north/south reading on the compass (because it is a constant north/south direction).

Throughout the Earth's history, the Earth's magnetic field has repeatedly reversed direction. For example, compasses that point to magnetic north today would have pointed to magnetic south (a position near the Earth's South Pole) during those intervals when the field was in a reversed orientation. These reversals are irregular. Sometimes they do not happen for several million years, other times they may happen after a few thousand years.

When scientists drill core samples out of basaltic rock and find evidence of these magnetic reversals, they can use the information to determine past continent location and climate change. Other methods of determining past locations of the Earth's continents come from analysis of landforms, similar to fitting the pieces of a jigsaw puzzle together. Overall, scientists can reconstruct the positions of continents on the Earth's surface with good accuracy back to about 300 million years ago, and less accurately back to 500 million years ago.

Concerning plate tectonics, climate, and global warming, three theories have been proposed: (1) the polar position hypothesis, (2) control of CO2 by seafloor spreading, and (3) control of CO2 by uplift and weathering.

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