The Evolution of Continental Crust [Preview]

The high-standing continents owe their existence to Earth's long history of plate-tectonic activity

Because the REE patterns found in a variety of sediments are so similar, geochemists surmise that weathering, erosion and sedimentation must mix different igneous source rocks efficiently enough to create an overall sample of the continental crust. All the members of the REE group establish a signature of upper crustal composition and preserve, in the shapes of the elemental abundance patterns, a record of the igneous events that may have influenced the makeup of the crust.

Using these geochemical tracers, geologists have, for example, determined that the composition of the upper part of the continental crust approximates that of granodiorite, an ordinary igneous rock that consists largely of light-colored quartz and feldspar, along with a peppering of various dark minerals. Deep within the continental crust, below about 10 to 15 kilometers, rock of a more basaltic composition is probably common. The exact nature of this material remains controversial, and geologists are currently testing their ideas using measurements of the heat produced within the crust by the important radioactive elements uranium, thorium and 40K, the radioactive isotope of potassium. But it seems reasonable that at least parts of this inaccessible and enigmatic region may consist of basalt trapped and underplated beneath the lower-density continents.

It is this physical property of granitic rock--low density--that explains why most of the continents are not submerged. Continental crust rises on average 125 meters above sea level, and some 15 percent of the continental area extends over two kilometers in elevation. These great heights contrast markedly with the depths of ocean floors, which average about four kilometers below sea level--a direct consequence of their being lined by dense oceanic crust composed mostly of basalt and a thin veneer of sediment.

At the base of the crust lies the so-called Mohorovicic discontinuity (a tongue-twisting name geologists invariably shorten to "Moho"). This deep surface marks a radical change in composition to an extremely dense rock rich in the mineral olivine that everywhere underlies both oceans and continents. Geophysical studies using seismic waves have traced the Moho worldwide. Such research has also indicated that the mantle below the continents may be permanently attached at the top. These relatively cool subcrustal "keels" can be as much as 400 kilometers thick and appear to ride with the continents during their plate-tectonic wanderings. Support for this notion comes from the analysis of tiny mineral inclusions found within diamonds, which are thought to originate deep in this subcrustal region. Measurements show that diamonds can be up to three billion years old and thus demonstrate the antiquity of the deep continental roots.

It is curious to reflect that less than 50 years ago, there was no evidence that the rocks lining ocean basins differed in any fundamental way from those found on land. The oceans were simply thought to be floored with foundered or sunken continents. This perception grew naturally enough from the concept that the continental crust was a world-encircling feature that had arisen as a kind of scum on an initially molten planet. Although it now appears certain that Earth did in fact melt very early, it seems that a primary granitic crust, of the type presumed decades ago, never actually existed.

The Evolution of Geodiversity
HOW WAS IT that two such distinct kinds of crust, continental and oceanic, managed to arise on Earth? To answer this question, one needs to consider the earliest history of the solar system. In the region of the primordial solar nebula occupied by Earths orbit, gas was mostly swept away, and only rocky debris large enough to survive intense early solar activity accumulated. These objects themselves must have grown by accretion, before finally falling together to form our planet, a process that required about 50 million to 100 million years.

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