The Evolution of Continental Crust [Preview]

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

Late in this stage of formation, a massive planetesimal, perhaps one the size of Mars, crashed into the nearly fully formed Earth. The rocky mantle of the impactor was ejected into orbit and became the moon while the metallic core of the body fell into Earth. As might be expected, this event proved catastrophic: it totally melted the newly formed planet. As Earth later cooled and solidified, an early basaltic crust probably formed.

It is likely that at this stage the surface of Earth resembled the current appearance of Venus; however, none of this primary crust has survived. Whether it sank into the mantle in a manner similar to that taking place on Earth or piled up in localized masses until it was thick enough to transform into a denser rock and sink remains uncertain. In any event, there is no evidence of substantial granitic crust at this early stage. Telltale evidence of such a crust should have survived in the form of scattered grains of the mineral zircon, which forms within granite and is very resistant to erosion. Although a few ancient zircons dating from near this time have been found (the oldest examples are from sedimentary rocks in Australia and are about 4.3 billion years old), these grains are exceedingly scarce.

More information about the early crust comes from the most ancient rocks to have survived intact. These rocks formed deep within the crust just less than four billion years ago and now outcrop at the surface in northwest Canada. This rock formation is called the Acasta Gneiss. Slightly younger examples of early crust have been documented at several locations throughout the world, although the best studied of these ancient formations is in western Greenland. The abundance of sedimentary rock there attests to the presence of running water and to the existence of what were probably true oceans during this remote epoch. But even these extraordinarily old rocks from Canada and Greenland date from some 400 million to 500 million years after the initial accretion of Earth, a gap in the geologic record caused, no doubt, by massive impacts that severely disrupted Earths earliest crust.

From the record preserved in sedimentary rocks, geologists know that the formation of continental crust has been an ongoing process throughout Earths long history. But the creation of crust has not always had the same character. For example, at the boundary between the Archean and Proterozoic eons, around 2.5 billion years ago, a distinct change in the rock record occurs. The composition of the upper crust before this break contained less evolved constituents, composed of a mixture of basalt and sodium-rich granites. These rocks make up the so-called tonalite-trondjemite-granodiorite, or TTG, suite. This composition differs considerably from the present upper crust, which is dominated by potassium-rich granites.

The profound change in crustal composition 2.5 billion years ago appears to be linked to changes in Earths tectonic regime. Before this time, higher levels of radioactive decay produced more heat in the planet. The consequence was that in the earlier Archean the oceanic crust was hotter, thicker and more buoyant and was not able to be subducted. Instead, under thicker sections of crust that may resemble modern Iceland, denser crust melted and produced the sodium-rich igneous rocks of the TTG suite.

Somewhat similar rocks now form in a few places such as southern Chile, where young oceanic crust subducts. But these modern rocks, forming now because of plate tectonics, are subtly different from their older Archean cousins, which formed from sinking slabs under thick crust. Modern-style plate tectonics did not begin operating until the late Archean (between 3.0 billion and 2.5 billion years ago), when the oceanic crust became cooler, lost its buoyancy and was thus able to sink back into the mantle.

The early tendency for magma to form with a TTG composition explains why crust grew as a mixture of basalt and tonalite during the Archean eon. Large amounts--at least 50 percent and perhaps as much as 70 percent of the continental crust--emerged at this time, with a major episode of growth between 3.0 billion and 2.5 billion years ago. Since that time, the relative height of ocean basins and continental platforms has remained comparatively stable. With the onset of the Proterozoic eon 2.5 billion years ago, the crust had already assumed much of its present makeup, and modern plate-tectonic cycling began.

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