In contrast to the product of such sudden, large-scale episodes of melting, secondary crusts form after heat from the decay of radioactive elements gradually accumulates within a planetary body. Such slow heating causes a small fraction of the planets rocky mantle to melt and usually results in the eruption of basaltic lavas. The surfaces of Mars and Venus and Earth's ocean floors are covered by secondary crusts created in this way. The lunar maria (the "seas" of the ancient astronomers) also formed from basaltic lavas that originated deep in the moons interior. Heat from radioactivity--or perhaps from the flexing induced by tidal forces--on some icy moon's of the outer solar system may, too, have generated secondary crusts.
Unlike these comparatively common types, so-called tertiary crust may form if surface layers are returned back into the mantle of a geologically active planet. Like a form of continuous distillation, volcanism can then lead to the production of highly differentiated magma of a composition that is distinct from basalt--closer to that of the light-colored igneous rock granite. Because the recycling necessary to generate granitic magmas can occur only on a planet where plate tectonics operates, such a composition is rare in the solar system. The formation of continental crust on Earth may be its sole location.
Despite the small number of examples within each category, one generalization about the genesis of planetary surfaces seems easy to make: there are clear differences in the rates at which primary, secondary and tertiary crusts form. The moon, for instance, generated its white, feldspar-rich primary crust--about 9 percent of lunar volume--in only a few million years. Secondary crusts evolve much more slowly. The moons basalt maria (secondary crust) are just a few hundred meters thick and make up a mere one tenth of 1 percent of the moons volume, and yet these so-called seas required more than a billion years to form. Another example of secondary crust, the basaltic oceanic basins of our planet (which constitute about one tenth of 1 percent of Earths mass), formed over a period of about 200 million years. Slow as these rates are, the creation of tertiary crust is even less efficient. Earth has taken several billion years to produce its tertiary crust--the continents. These features amount to just about one half of 1 percent of the mass of the planet.
MANY ELEMENTS that are otherwise rarely found on Earth are enriched in granitic rocks, and this phenomenon gives the continental crust an importance out of proportion to its tiny mass. But geologists have not been able to estimate the overall composition of crust--a necessary starting point for any investigation of its origin and evolution--by direct observation. One conceivable method might be to compile existing descriptions of rocks that outcrop at the surface. Even this large body of information might well prove insufficient. A large-scale exploration program that could reach deeply enough into the crust for a meaningful sample would press the limits of modern drilling technology and would, in any event, be prohibitively expensive.
Fortunately, a simpler solution is at hand. Nature has already accomplished a widespread sampling through the erosion and deposition of sediments. Lowly muds, now turned into solid sedimentary rock, give a surprisingly good average composition for the exposed continental crust. These samples are, however, missing those elements that are soluble in water, such as sodium and calcium. Among the insoluble materials that are transferred from the crust into sediments without distortion in their relative abundances are the 14 rare-earth elements, known to geochemists as REEs. These elemental tags are uniquely useful in deciphering crustal composition because their atoms do not fit neatly into the crystal structure of most common minerals. They tend instead to be concentrated in the late-forming granitic products of a cooling magma that make up most of the continental crust.