The fossil record suggests that there are more species of animals and plants alive on Earth today than at any time in the past; estimates vary between about 3 and 30 million species. This great diversity has come about through many physical and evolutionary factors. We contend that the effects of plate tectonics are among the most important. But once created, does high biodiversity require the continued presence of plate tectonics? We can examine this question with a thought experiment.
Imagine that all volcanism on Earth's surface suddenly ceases. This will stop the many dozens of volcanic eruptions that occur on the continents each year (usually causing great media fanfare and little damage). But the cessation of volcan-ism will have a far more profound effect. If all volcanism stops, so does sea floor spreading—and thus plate tectonics as well. And if plate tectonics stops, Earth eventually (through erosion) loses most or all of the continents where most terrestrial life exists. In addition, CO2 is removed from the atmosphere via weathering, causing our planet to freeze. Of all of the attributes that make Earth rare, plate tectonics may be one of the most profound and—in terms of the evolution and maintenance of animal life—one of the most important.
Only cessation of the flow of heat up from Earth's interior or thickening of the crust would stop volcanism. It is this heat that causes convective motion of the interior, the subterranean motor of plate tectonics. To stop plate tectonics would require eliminating these great lithic boiling pots, and that cannot be done unless all heat emanating from the Earth's interior is stopped (which would require that all the radioactive minerals locked away there decay to stable daughter products) or the composition of Earth's crust or upper mantle changes such that movement can no longer occur. This could happen if the crust became too thick or the mantle too viscous to allow movement. None of these conditions is likely to occur on Earth in the foreseeable future, but there is speculation that just such events occurred on both Venus and Mars in the past.
If Earth's tectonic plates did suddenly stop moving, subduction would no longer occur at the contacts between colliding plates. Mountains—and mountain chains—would cease to rise. Erosion would begin to eat away at their height. Eventually, the world's mountains would be reduced to sea level. How long would it take? The problem is a bit more complicated than simply measuring average erosion rates and calculating the number of years required for the mountains to disappear. This is because of the principle of isostacy. Mountains (and continents) are a bit like icebergs: If you cut off the top, the bottom rises up relative to sea level, causing the entire iceberg (or mountain)
to rise. Eventually, however, even this isostatic rebound effect would be overcome by the extent of the erosion.
What would sea level be in a world without further plate tectonics? All of the sediment produced by the simultaneous erosion of the world's mountains would have to go somewhere—and that somewhere is the ocean. The eroding continental mass carried into the oceans by river and wind transport would displace seawater and cause the level of the sea to rise. Calculations by David Montgomery, a geomorphologist at the University of Washington, suggest that the entire Earth might become covered by a global ocean— much shallower than the oceans of today, of course, but global in extent nevertheless. Our planet would have returned to its state of 4 billion years ago: a globe covered completely (or nearly so) by ocean. And with the continents awash, Earth would witness a mass extinction more catastrophic than any in the past. All land life would die off under the lapping waves. Paradoxically, the increase of ocean area would probably also be accompanied by extinctions in the sea. Ocean life depends on nutrients, and most nutrients come from the land as runoff from rivers and streams. With the disappearance of land, the total amount of nutrients (though initially higher as so much new sediment entered the ocean system) would eventually lessen, and with fewer resources, there would be fewer marine animals and plants.
How long before such a water world would be achieved? Tens of millions of years would be required for the mountains and continents to erode to sea level. Yet mass extinction would ensue long before that. Planetary calamity for complex life would occur shortly after the cessation of plate movement, for plate tectonics is not only the reason we have mountains; it turns out to control our planet's climatic thermostat as well.
The temperature of Earth must remain in a range suitable for the existence of liquid water if animal life is to be maintained. The range of temperature that Earth experiences is the result of many factors. One is the existence of the at mosphere. The average temperature of the Moon is — 18°C, for example, well below the freezing point of water, simply because it has no appreciable atmosphere. If Earth did not have its cloaking atmosphere, including such insulating gases as water vapor and carbon dioxide (producing the much-discussed Greenhouse Effect), its temperature would be about the same as that of the Moon. Yet the Earth, thanks to the greenhouse gases, has an average global temperature of 15°C (33°C warmer than the Moon). Greenhouse gases are keys to the presence of fresh water on this planet and thus are keys to the presence of animal life—and many scientist now believe that the balance of greenhouse gases in Earth's atmosphere is directly related to existence of plate tectonics.
Greenhouse gases are those with three or more atoms, such as water vapor (H2O; three atoms), ozone (O3), carbon dioxide (CO2; three atoms), and methane (CH4; five atoms). All can capture outgoing infrared energy from Earth's surface and, in so doing, warm the planet. Their role in keeping Earth's temperature within the critical levels necessary not only for allowing the presence of liquid water (0°C to 100°C) but also for maintaining animals (about 2°C to about 45°C) has been beautifully summarized by Columbia University geologist Wally Broecker in How to Build a Habitable Planet. Broecker describes the following scenario. Imagine that the sun's energy was diminished for a period of time brief by geological standards but long enough for the oceans to freeze. If the sun then resumed its normal output of today, Earth would remain frozen. Once frozen, water reflects much of the light that hits it, and even the current volume of greenhouse gas would be insufficient to reheat the planet to a temperature at which the water would thaw. This condition is called a Global Icehouse, and it is one way a planet can lose its animal life. They freeze to death.
Now, say we reversed this situation and allowed the sun's energy to increase for a geologically short period of time, but long enough so that all of Earth's oceans boiled away, filling the atmosphere with steam. If we then reduced the sun's energy to its present-day levels, the oceans might not recondense, and the planet would stay hot. Once in the atmosphere, the steam would keep the planet hot through its properties as a greenhouse gas, even when solar radiation hitting the planet had decreased. This situation is called a Runaway Greenhouse.
The Earth's greenhouse gases are rare compounds of our planet's atmosphere. It turns out that the major constituents of our atmosphere, nitrogen and oxygen, play little role in the greenhouse warming, because they do not absorb infrared radiation. Carbon dioxide and water vapor, on the other hand, do, even though they make up only a tiny fraction of the gas volume of the atmosphere (carbon dioxide constitutes only 0.035% of the atmosphere). Plate tectonics plays an important part—perhaps the most important part— in maintaining levels of greenhouse gases, and these in turn maintain the temperatures necessary for animal life.
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