Answered by Thomas Ahrens, professor of geophysics, Caltech.
The replenishment of lava—the molten rock which flows out on the surface from the rocky silicate mantle of the Earth, back into the Earth—is a key process that appears to be unique to our planet (relative to the other silicate mantle planets with iron cores: Mars, Venus, and Mercury).
Basalt is the most prevalent lava type found on Earth. This is true also for the other above-mentioned planets. Basalt rock, which has a specific gravity 2.7 to 3.1 times denser than that of water, consists of two key mineral groups: plagioclase, which contains mostly calcium, sodium, potassium, aluminum, silicon, and oxygen; and pyroxene, which contains mostly calcium, magnesium, iron, silicon, and oxygen. These essential minerals react at pressures in the range of 10 to 15 thousand atmospheres to form a denser garnet-bearing rock, called eclogite. Eclogite has a density in the range of 3.4 to 3.5.
Thus, as a large pile of basalt accumulates on the Earth's surface, a process called subduction, or sinking, occurs when the basalt-ecologite transition begins. This causes sections of the crust of Earth containing basalt, or rock of similar composition with thicknesses of 30 km or greater, to transform to eclogite. Because their density is greater than the underlying mantle (3.3), the materials sink, or subduct, into Earth. The basalt-eclogite reaction requires elevated temperatures and some moisture.
If the temperature becomes too high, however, greater than 1,300 degrees centigrade, the dense mineral garnet, the major dense constituent of eclogite, will not be stable and subduction does not occur. This seems to be the case for Venus, which both has a hotter interior and lacks even a fraction of the percentage of water required to assist the basalt-eclogite reaction. This is also true for the sun's nearest planet neighbor, Mercury.
Interestingly, Mars' interior appears to be too cold for eclogite formation. The subduction process is a key element in the process of recycling rock back into the Earth. Earth scientists began to understand this process in the early 1960s, and it is now recognized as a major feature of the theory of plate tectonics. Moreover, exploration of the properties of the other silicate mantle planets with iron cores suggests that only the Earth has active plate tectonics.
