Except perhaps for some remote island dwellers, most people have a natural tendency to view continents as fundamental, permanent and even characteristic features of Earth. One easily forgets that the worlds continental platforms amount only to scattered and isolated masses on a planet that is largely covered by water. But when viewed from space, the correct picture of Earth becomes immediately clear. It is a blue planet. From this perspective it seems quite extraordinary that over its long history Earth could manage to hold a small fraction of its surface always above the sea--enabling, among other things, human evolution to proceed on dry land.
Is the persistence of high-standing continents just fortuitous? How did Earths complicated crust come into existence? Has it been there all the time, like some primeval icing on a planetary cake, or has it evolved through the ages? Such questions had engendered debates that divided scientists for many decades, but the fascinating story of how the terrestrial surface came to take its present form is now essentially resolved. That understanding shows, remarkably enough, that the conditions required to form the continents of Earth may be unmatched in the rest of the solar system.
Earth and Venus, being roughly the same size and distance from the sun, are often regarded as twin planets. So it is natural to wonder how the crust of Venus compares with that of our own world. Although centuries of telescopic observations from Earth could give no insight, beginning in 1990 the Magellan space probes orbiting radar penetrated the thick clouds that enshroud Venus and revealed its surface with stunning clarity. From the detailed images of landforms, planetary scientists can surmise the type of rock that covers Venus.
Our sister planet appears to be blanketed by rock of basaltic composition--much like the dark, fine-grained rocks that line the ocean basins on Earth. Magellans mapping, however, failed to find extensive areas analogous to Earths continental crust. Elevated regions named Aphrodite Terra and Ishtar Terra appear to be remnants of crumpled basaltic lavas. Smaller, dome-shaped mounds are found on Venus, and these forms might indicate that volcanic rocks with the composition of granite do exist in some places, but radar reflections show that these pancakelike features may be composed merely of more basalt.
After analyzing the wealth of radar data provided by Magellan, scientists have concluded that plate tectonics (that is, the continual creation, motion and destruction of parts of the planet's surface) does not seem to operate on Venus. There are no obvious equivalents to the extensive mid-ocean ridges or to the great trench systems of Earth. Thus, it is unlikely that the crust of Venus regularly recycles back into that planet's mantle. Nor would there seem to be much need to make room for new crust: the amount of lava currently erupting on Venus is roughly equivalent to the output of one Hawaiian volcano, Kilauea--a mere dribble for the planet as a whole. These findings from Venus and similar surveys of other solid bodies in the solar system show that planetary crusts can be conveniently divided into three fundamental types.
So-called primary crusts date back to the beginnings of the solar system. They emerged after large chunks of primordial material came crashing into a growing planet, releasing enough energy to cause the original protoplanet to melt. As the molten rock began to cool, crystals of some types of minerals solidified relatively early and could separate from the body of magma. This process, for example, probably created the white highlands of the moon after low-density grains of the mineral feldspar floated to the top of an early lunar "ocean" of molten basalt. The crusts of many satellites of the giant outer planets, composed of mixtures of rock with water, methane and ammonia ices, may also have arisen from catastrophic melting during initial accretion.