What we want is a story that starts with an earthquake and builds to a climax.
Our planet has been destructive in recent years. Earthquakes in Turkey and India have caused huge loss of life; smaller quakes in America and Japan have caused inconvenience; and as I write, Mount Etna is spewing forth lava that threatens the livelihood of several hundred villagers.208 It therefore seems strange that some geologists consider the existence of plate tectonics — the process that gives rise to earthquakes and volcanic eruptions — to be necessary for the existence of complex life. But there is a serious reason for believing that three phenomena — life, water oceans, and plate tectonics — are linked. And this linkage may be unique to Earth.
The various planets of the Solar System have different methods of disposing of their internal heat. In Earth's case, the heat generated by radioactive decay in the interior is transported by the convective method known as plate tectonics. Consider what happens near a mid-ocean ridge. Hot material from the deep mantle region of Earth is brought to the surface in a convection cell, and at the surface it spreads out and solidifies into ocean crust — it becomes part of the lithosphere. Over geological timescales, the new material floats on the hot mantle underneath it and moves away from where it was born. During this process it cools and collects masses of igneous rocks. The material becomes heavier, and after many tens of millions of years it sinks back, under its own weight, deep into the mantle at places called subduction zones. Eventually, the cycle repeats. On geological timescales, the outer regions of our planet resemble one of those kitsch lava lamps.209
Some scientists think plate tectonics may be the most important requirement for the development of animal life. There are several reasons why plate tectonics might be vital. Let us look at just three of them.
First, the mechanism of plate tectonics seems important in the creation of Earth's magnetic field. The theory of planetary magnetism is formidably complicated, but, in essence, planets generate a magnetic field by means of an internal dynamo. Such a dynamo requires three things: the planet must rotate, it must contain a region with an electrically conducting fluid, and it must maintain convection within the conducting fluid region. It is difficult to be sure, but in Earth's case it seems likely that without plate tectonics the convective cells would cease to export heat to the surface, the dynamo would not function, and Earth's magnetic field would be a tiny fraction of its present value. The relevance of all this is simple: Earth's magnetic field helps prevent high-energy particles in the solar wind from scattering atmospheric particles into space; over time, such sputtering could cause the Earth's atmosphere to dissipate. In short, without Earth's magnetic field surface life might not have evolved.
Second, plate tectonics, or continental drift, created Earth's continents — and continues to refresh them. Continents are important. A world with a mixture of oceans, islands and large continents is more likely to offer evolutionary challenges than is a world dominated solely by water or land. Furthermore, plate tectonics causes environmental conditions to alter, and thus helps promote speciation. (For example, suppose the splitting of a piece of land from a continental land-mass results in a particular species of bird living on both the new island and the original continent. Over time, the environment on the island will differ from the continental environment; the birds will face different challenges and will evolve in different ways. Over time, there will be two species where before there was one.) Plate tectonics thus promotes biodiversity, which, as we have seen, is important during times of mass extinction. The larger the number of species, the greater the chance of some of them surviving the extinction event.
Third, and most important, for a billion years or more plate tectonics has played a key role in regulating Earth's surface temperature. The climate on our planet has long been balanced on a razor's edge. If the temperature drops too much, and the icecaps begin to increase in size, then a runaway icehouse effect may occur: Earth freezes. If the temperature increases too much, and the oceans start to simmer, then the extra water vapor in the atmosphere causes a runaway greenhouse effect: Earth boils. Certain prokaryotes might survive these temperature extremes, but complex life-forms can flourish only over a much narrower range of temperatures. Plate tectonics, some scientists argue, has a fine-tuning mechanism that keeps the planetary thermostat set "just right" for animal life.
The way plate tectonics controls temperature is rather complicated, and more than one mechanism is involved.210 The key role it plays, however, is in its regulation of atmospheric carbon dioxide. CO2 is an effective greenhouse gas: if the atmosphere contains too much CO2, then global temperatures can rise (as mankind seems hell-bent on demonstrating experimentally). On the other hand, if there is too little atmospheric CO2, then Earth fails to benefit from the greenhouse effect, and the planet cools.
Now, CO2 does not remain in the atmosphere indefinitely. Carbon dioxide reacts with water to form carbonic acid; rainfall thus "washes" it out of the atmosphere. This carbonic acid weathers the rocks on Earth's surface, and the chemical products of this weathering get transported by rivers to the ocean. The products end up as calcium carbonate (CaCO3) and quartz (SiO2) on the seafloor, both through the formation of rocks and through the formation of the shells of living organisms. Eventually, the plate tectonics mechanism causes this CaCO3 and SiO2 to be subducted down into the depths of the Earth. Thus, atmospheric CO2 is removed. But that is not the end of the story! The high temperatures and pressures deep within Earth convert the calcium carbonate back into CO2 and CaO. Plate tectonics then recycles the CO2 — and lots of other useful materials — by creating volcanoes (which vent tremendous amounts of CO2; as we saw earlier, this mechanism allowed an escape from Snowball Earth episodes.)
If the atmospheric CO2 were not replaced, Earth would undergo a global cooling. But what if too much CO2 is put back into the atmosphere? Do we not run the risk of a runaway greenhouse effect? It turns out that, as the planet warms, the chemical weathering of rocks increases — which causes more CO2 to be removed from the atmosphere, which causes the planet to cool (thus slowing the rate at which CO2 is removed from the system, which causes the planet to warm . . . and so on, in a classical feedback mechanism). This CO2-silicate cycle is rather complicated, and the details are not fully understood, but the cycle seems to be crucially important for the long-term stabilization of global temperature.
One can argue that the development of animal life here on Earth required plate tectonics — to promote biodiversity, to generate a magnetic field, to stabilize global temperature, and so on. And yet there is nothing inevitable about plate tectonics. Only Earth, as far as we know, uses this mechanism to dispose of its internal heat. Perhaps the process is rare, and other planets lack animal life because they lack plate tectonics.
We do not know how frequently plate tectonics will occur because we lack a good general theory of the process. The type of questions one might ask — How does the existence of plate tectonics depend on a planet's mass? How does it depend on the chemical composition of the mantle? — cannot be answered with present models, so it is impossible to provide a good estimate of how many planets might develop, and maintain, plate tectonics. In the absence of hard facts, from either experiment or theory, one can argue either way. Some scientists believe the titanic collision that formed the Moon laid the seeds from which plate tectonics developed; in this case, plate tectonics may be rare. On the other hand, the basic conditions for plate tectonics seem relatively simple: a planet should have a thin crust floating on top of a hot, fluid region undergoing convection due to rising heat from the core. Perhaps water oceans are also necessary to "soften" the crust and allow subduction. Such conditions are probably not rare. Scarce, perhaps, but not rare. In other words, we simply do not know whether or not plate tectonics is common.
Even if plate tectonics is rare, does it necessarily follow that animal life is rare? Although plate tectonics seems to have played (and continues to play) a beneficial role for the development of life on Earth, is it the only mechanism that can provide these benefits? Plate tectonics is an extremely complicated and poorly understood process; the very existence of the CO2-silicate cycle has only been known about for two decades. In cases like these, it often happens that there is more than one way to skin a cat. Maybe right now the scientists of a planet orbiting some anonymous M-class star are marveling at the cooling mechanism of their world and how it almost miraculously stabilizes their global environment.
My guess is that — like so many factors we have discussed — the possible rarity of plate tectonics is by itself insufficient to provide an answer to the Fermi paradox. But it may be yet another factor making it less probable that ETCs will develop on other planets.
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