The Evolution of Continental Crust The high-standing continents owe their existence to the earth’s long history of plate-tectonic activity by S. Ross Taylor and Scott M. McLennan xcept perhaps for some remote of Venus compares with that of our own island dwellers, most people have world. Although centuries of telescopic Ea natural tendency to view conti- observations from the earth could give nents as fundamental, permanent and no insight, beginning in 1990 the Ma- even characteristic features of the earth. gellan space probeÕs orbiting radar pen- One easily forgets that the worldÕs con- etrated the thick clouds that enshroud tinental platforms amount only to scat- Venus and revealed its surface with tered and isolated masses on a planet stunning clarity. From the detailed im- that is largely covered by water. But ages of landforms, planetary scientists when viewed from space, the correct can surmise the type of rock that cov- picture of the earth becomes immedi- ers Venus. ately clear. It is a blue planet. From this perspective it seems quite extraordi- Comparison with the Neighbors nary that over its long history the earth could manage to hold a small fraction ur sister planet appears to be blan- of its surface always above the sea, en- O keted by rock of basaltic composi- abling, among other things, human evo- tionÑmuch like the dark, Þne-grained lution to proceed on dry land. rocks that line the ocean basins on the Is the persistence of high-standing earth. MagellanÕs mapping, however, continents just fortuitous? How did the failed to Þnd extensive areas analogous earthÕs complicated crust come into ex- to the terrestrial continental crust. Ele- istence? Has it been there all the time, vated regions named Aphrodite Terra like some primeval icing on a planetary and Ishtar Terra appear to be remnants cake, or has it evolved through the of crumpled basaltic lavas. Smaller, ages? Such questions had engendered dome-shaped mounds are found on debates that divided scientists for many Venus, and these forms might indicate decades, but the fascinating story of that a substantially diÝerent bedrock how the terrestrial surface came to take composition does exist in some places; its present form is now essentially re- solved. That understanding shows, re- markably enough, that the conditions required to form the continents of the earth may be unmatched in the rest of the solar system. The 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 EARTHÕS CRUST is composed primarily of the basaltic rocks that line the ocean basins. Granitic rocks constitute the high-standing continental platforms. Venus is nearly the same size as the earth, yet radar imagery indicates that it is encrusted al- most entirely by basalt. Only a tiny fraction of that planetÕs surface exhibits pan- cake-shaped plateaus (detail above) that might, like the earthÕs continents, be built of granitic material. The crust of the earthÕs moon is largely covered by white high- lands formed as that body Þrst cooled from a molten state; volcanic eruptions later created dark so-called seas of basalt. 76 SCIENTIFIC AMERICAN January 1996 Copyright 1995 Scientific American, Inc. it is also possible that these pancake- These Þndings from Venus and simi- mixtures of rock with water, methane like features may be composed merely lar surveys of other solid bodies in the and ammonia ices, may also have aris- of more basalt. solar system show that planetary crusts en from catastrophic melting during After analyzing the wealth of radar can be conveniently divided into three initial accretion. data provided by Magellan, scientists fundamental types. So-called primary In contrast to the product of such ); have concluded that plate tectonics crusts date back to the beginnings of sudden, large-scale episodes of melt- s (that is, the continual creation, motion the solar system. They emerged after ing, secondary crusts form after heat enu V ) and destruction of parts of the planetÕs large chunks of primordial material from the decay of radioactive elements n Moo surface) does not seem to operate on came crashing into a growing planet, gradually accumulates within a plane- ( Venus. There are no obvious equiva- releasing enough energy to cause the tary body. Such slow heating causes a ); JPL/NASA ( l ); NASA lents to the extensive mid-ocean ridges original protoplanet to melt. As the small fraction of the planetÕs rocky in- h detai Eart or to the great trench systems of the molten rock began to cool, crystals of terior to melt and usually results in the ( earth. Thus, it is unlikely that the crust some types of minerals solidiÞed rela- eruption of basaltic lavas. The surfaces of Venus regularly recycles back into tively early and could separate from the of Mars and Venus and the earthÕs ocean Y/NASA ( OR that planetÕs mantle. Nor would there body of magma. This process, for exam- ßoors are covered by secondary crusts T seem to be much need to make room ple, probably created the white high- created in this way. The lunar maria (the for new crust: the amount of lava cur- lands of the moon after low-density ÒseasÓ of the ancient astronomers) also The Geosphere Project rently erupting on Venus is roughly grains of the mineral feldspar ßoated to formed from basaltic lavas that origi- equivalent to the output of one Hawaii- the top of an early lunar ÒoceanÓ of mol- nated deep in the moonÕs interior. Heat an volcano, KilaueaÑa mere dribble for ten basalt. The crusts of many satellites from radioactivityÑor perhaps from AN SANT OM V the planet as a whole. of the giant outer planets, composed of the ßexing induced by tidal forcesÑon JET PROPULSION LABORA T MOON EARTH VENUS Copyright 1995 Scientific American, Inc. SCIENTIFIC AMERICAN January 1996 77 some icy moons of the outer solar sys- en several billion years to produce its 6 tem may, too, have generated second- tertiary crustÑthe continents. Yet these 4 ary crusts. features amount to just about one half Unlike these comparatively common of 1 percent of the mass of the planet. CONTINENTS 4 2 types, so-called tertiary crust may form if surface layers are returned back into Floating Continents 2 the mantle of a geologically active plan- 0 et. Like a form of continuous distillation, any elements that are otherwise volcanism can then lead to the produc- Mrarely found on the earth are en- VENUS 0 tion of highly diÝerentiated magma of riched in granitic rocks, and this phe- –2 a composition that is distinct from ba- nomenon gives the continental crust saltÑcloser to that of the light-colored an importance out of proportion to its –2 –4 igneous rock granite. Because the recy- tiny mass. But geologists have not been EARTH ELEVATION (KILOMETERS) cling necessary to generate granitic mag- able to estimate the overall composi- VENUS ELEVATION (KILOMETERS) –4 mas can occur only on a planet where tion of crustÑa necessary starting point –6 plate tectonics operates, such a compo- for any investigation of its origin and OCEAN BASINS sition is rare in the solar system. The evolutionÑby direct observation. One formation of continental crust on the conceivable method might be to com- 0 0.5 1.0 RELATIVE AREA earth may be its sole demonstration. pile existing descriptions of rocks that JANA BRENNING Despite the small number of exam- outcrop at the surface. But even this SURFACE ELEVATIONS are distributed ples within each category, one general- large body of information might well quite diÝerently on the earth (blue) and ization about the genesis of planetary prove insuÛcient. A large-scale explora- on Venus (gold ). Most places on the surfaces seems easy to make: there are tion program that could reach deeply earth stand near one of two prevailing clear diÝerences in the rates at which enough into the crust for a meaningful levels. In contrast, a single height char- primary, secondary and tertiary crusts sample would press the limits of mod- acterizes most of the surface of Venus. form. The moon, for instance, generated ern drilling technology and would, in (Elevation on Venus is given with re- its white, feldspar-rich primary crustÑ any event, be prohibitively expensive. spect to the planetÕs mean radius.) about 12 percent of lunar volumeÑin Fortunately, a simpler solution is at only a few million years. Secondary hand. Nature has already accomplished crusts evolve much more slowly. The a widespread sampling through the ero- ly useful in deciphering crustal compo- moonÕs basalt maria (secondary crust) sion and deposition of sediments. Low- sition because their atoms do not Þt are just a few hundred meters thick and ly muds, now turned into solid rock, neatly into the crystal structure of most make up a mere one tenth of 1 percent give a surprisingly good average com- common minerals. They tend instead to of the moonÕs volume, and yet these position for the exposed continental be concentrated in the late-forming gra- so-called seas required more than a bil- crust. These samples are, however, miss- nitic products of a cooling magma that lion years to form. Another example of ing those elements that are soluble in make up most of the continental crust. secondary crust, the basaltic oceanic water, such as sodium and calcium. Because the REE patterns found in a basins of our planet (which constitute Among the insoluble elements that are variety of sediments are so similar, geo- about one tenth of 1 percent of the transferred from the crust into sedi- chemists surmise that weathering, ero- earthÕs mass) formed over a period of ments without distortion in their rela- sion and sedimentation must mix dif- about 200 million years.