Why is there plate tectonics on Earth? The recipe for plate tectonics seems simple enough at first glance. You need a planet differentiated into a thin, solid crust sitting atop an underlying region that is hot, fluid, and mobile. You need this underlying region to be undergoing convection, and for that you need heat emanating from even deeper in the planet. And you are likely to need water—oceans of water: Much new research suggests that without water you cannot have plate tectonics (though perhaps it is simply that without water you cannot get continents).
As in so much else in planetary geology, there is still a great deal we don't know about why our planet (and, more important, any planet) develops and then maintains plate tectonics. Because ours is still the only planet we know that has plate tectonics, we have nothing with which to compare it.
Much of the data pertaining to plate tectonics lies so deep that we are unlikely ever to sample it directly.
As an illustration of the degree of uncertainty about what we might call planetary plate tectonics, which we can define as the theoretical (as opposed to the Earth-based actual) study of plate tectonics, we cannot be certain whether plate tectonics would operate if Earth were 20% larger or smaller, or if it had a crust with more iron and nickel than it does, or if its surface had only 10% of the present-day volume of water. The best current work on these types of questions is being done by planetary geologists V. Solomatov and L. Moresi, who are using computational models to study how convection (the driving force of plate tectonics) works. Yet the abstract of their 1997 paper on the subject concluded, "The nature of the mobility of lithosphere plates on Earth has yet to be explained." We know the plates move, and we know convection moves them. The physics behind the convection is well understood, but its application to subduction is still an enigma.
When we asked about the physical condition necessary to produce plate tectonics on a planet, Solomatov responded, "It is a very interesting problem and we've just started exploring the physical conditions required for plate tectonics to occur on a planet. So far, we have been moving to the conclusion that water might be the factor which is crucial for plate tectonics: no water, no plate tectonics." Without water, the lithosphere (which is the plate of plate tectonics, the rigid surface region composed of the crust and uppermost part of the mantle) is strong and cannot break and descend back into the mantle— the process known as subduction that occurs along the linear subduction zones described earlier in this chapter. According to Solomatov, subduction is a major requirement for plate tectonics. Apparently, subduction zones operate only when the crust is "weak," or able to bend and break, which allows it to descend into the regions where the mantle convection cells sink downward. All of this work is being done with mathematical modeling. Solomatov and his colleagues are using computers to arrive at these generalizations—not trips to the center of the Earth with Jules Verne's heroes.
Even in the absence of water, plumes of hot magma may rise to a planet's surface. But this new material must ultimately go somewhere, and if subduction is not operating, the plates will not move, for the new crustal material must ultimately duck down into the mantle, along the linear subduction zones. Without subduction zones, there is no plate tectonics, even if mantle convection cells are operating inside a planet.
Venus and Mars both lack subduction zones and thus lack plate tectonics. Although both might have the internal mantle convection necessary to move surface plates, the surface itself is composed of "strong" rock (Soloma-tov's term) that cannot move. Because of its thickness and strength, the crust on these planets is now immobile. The lack of water on both of these plates may be the reason why this is so. Because both of these planets may in the past have had liquid water and crustal composition similar enough to that of Earth, we may find that Venus and Mars once did have plate tectonics—and perhaps lost it when they lost their liquid water. Venus and Mars may be experiencing what Solomatov and Moresi describe as a "stagnant lid regime": The viscosity difference between the convecting mantle and the solid surface is so great that little or no movement of the crust can occur. Yet heat continues to flow upward, and in the case of Venus, this heat caused the entire surface of the planet to melt about a billion years ago (the planetary "resurfacing" we alluded to at the start of this chapter). On Earth this great viscosity difference does not occur. Earth has a "small viscosity contrast regime," according to the technical scientific papers describing all of this, and the result is the very actively moving crust so important for mountain formation, nutrient cycling, and life.
Yet perhaps we have this story reversed. Perhaps Mars and Venus had water but lost it because they had no plate tectonics—and thus no planetary thermostat.
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