Currently oceanic crust forms by the eruption of basaltic lava along a globe-encircling network of mid-ocean ridges. More than 18 cubic kilometers of rock are produced every year by this process. The slab of newly formed crust rides on top of an outer layer of the mantle, which together make up the rigid lithosphere. The oceanic lithosphere sinks back into the mantle at so-called subduction zones, which leave conspicuous scars on the ocean floor in the form of deep trenches. At these sites the descending slab of lithosphere carries wet marine sediments as well as basalt plunging into the mantle.
At a depth of about 80 kilometers, heat drives water and other volatile components from the subducted sediments into the overlying mantle. These substances then act as a flux does at a foundry, inducing melting in the surrounding material at reduced temperatures. The magma fractionates, producing andesites, while the more basic substratum probably sinks back into the mantle in a process called delamination. The andesite magma produced in this fashion eventually reaches the surface, where it causes spectacular, explosive eruptions. The 1980 eruption of Mount St. Helens is an example of such a geologic cataclysm. Great chains of volcanoes--such as the Andes--powered by boiling volatiles add on average about two cubic kilometers of lava and ash to the continents every year. This andesite provides the bulk material of the continents.
But the more silica-rich granitic rock, which we see at the surface of the continents, comes from within the crust. The accumulation of heat deep within the continental crust itself can cause melting, and the resultant magma will ultimately migrate to the surface. Although some of this necessary heat might come from the decay of radioactive elements, a more likely source is basaltic magma that rises from deeper in the mantle and becomes trapped under the granitic lid; the molten rock then acts like a burner under a frying pan.
Crustal Growth Spurts
ALTHOUGH THE MOST DRAMATIC SHIFT in the generation of continental crust happened at the end of the Archean eon, 2.5 billion years ago, the continents appear to have experienced episodic changes throughout all of geologic time. For example, sizable, later additions to the continental crust occurred from 2.0 to 1.7, from 1.3 to 1.1 and from 0.5 to 0.3 billion years ago. That Earth's continents experienced such a punctuated evolution might appear at first to be counterintuitive. Why, after all, should crust form in spurts if the generation of internal heat--and its liberation through crustal recycling--is a continuous process?
A more detailed understanding of plate tectonics helps to solve this puzzle. During the Permian period (about 250 million years ago), the major continents of Earth converged to create one enormous landmass called Pangaea [see "Earth before Pangaea," on page 14]. This configuration was not unique. The formation of such "supercontinents" appears to recur at intervals of about 600 million years. Major tectonic cycles driving the continents apart and together have been documented as far back as the Early Proterozoic, and there are even suggestions that the first supercontinent may have formed earlier, during the Archean.
Such large-scale tectonic cycles serve to modulate the tempo of crustal growth. When a supercontinent breaks itself apart, oceanic crust is at its oldest and hence most likely to form new continental crust after it subducts. As the individual continents reconverge, volcanic arcs (curved chains of volcanoes created near subduction zones) collide with continental platforms. Such episodes preserve new crust as the arc rocks are added to the margins of the continents.
For more than four billion years, the peripatetic continents have assembled themselves in fits and starts from many disparate terranes. Buried in the resulting amalgam is the last remaining testament available for the bulk of Earths history. That story, assembled from rocks that are like so many jumbled pieces of a puzzle, has taken some time to sort out. But the understanding of crustal origin and evolution is now sufficient to show that of all the planets Earth appears truly exceptional. By a fortunate accident of nature--the ability to maintain plate-tectonic activity--one planet alone has been able to generate the sizable patches of stable continental crust that we find so convenient to live on.