the rates at which organic compounds and sulfur-containing compounds and minerals (such as pyrite) are buried, or not, and the rate at which they are weathered or not. Many things control burial and weathering rates. One of these is continental position, and it is this that can best be predicted for the future.

PLATE TECTONICS AND THE FATE OF GEOGRAPHY

We have been in an ice age now for more than 2.5 million years, and there is a real possibility, based on past experience, that our planet awaits at least that much time again in the seemingly endless cycles of ice and warming for another 2.5 million, or 5 million, or even 10 million years. But eventually the ice ages will end, and indeed with humans producing greenhouse gases at such a prodigious rate it may well be that the time of ice is over, perhaps on timescales of geological periods (tens of millions of years). While global temperature is a function of many factors (including the presence of eroding high mountains, such as the Himalayas, which serve to increase the rate at which the global thermostat removes carbon dioxide from the atmosphere), geography also plays a huge role. Our planet is heading for greater warmth caused both by the increased energy of the sun and by a new global geography. Continental drift is moving enough of the northern landmasses out of high latitudes and moving Antarctica out of the extreme southern latitudes, to end the tyranny of ice. We have a good idea of how and even when that will come about, based on powerful new computer simulations of plate movements.

Plate movements for the past 600 million years are fairly well established (although ambiguity increases for older time). Certainly the positions of continents for the past 200 million years are very well known. Five hundred million years ago, at the time of the major animal diversification known as the Cambrian Explosion, the continents were widely dispersed along the equator. For the next 200 million years, large-scale drift and continental collision resulted in the formation of ever-larger land bodies and major mountain chains, including the Appalachians of the eastern United States. By about 300 million years ago the major continents had coalesced into a single united block, a super-continent named Pangea. By about 200 million years ago, the huge



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