|MadSci Network: Earth Sciences|
As with most things geological, this question has a complex answer. The oldest known rocks are about 3.8 billion years old, and rocks of about this age are found in several parts of the world, such as Greenland, northeastern Canada, and Australia. The period of time that saw formation of the oldest known rocks was called the Archaean.
Although the oldest known rocks are less than 4.0 billion years old, the oldest known sand grains are radiometrically dated at 4.1 billion years. These grains are very hard zircons that were redeposited in sandstones millions of years after they formed as part of a volcanic rock suite (which no longer exists).
The oldest rocks themselves are found in two suites or belts: greenstone belts and gneiss belts. Gneisses are metamorphosed granites. The gneiss belts are dominated by metamorphosed granite and similar rocks, but also contain volcanic and sedimentary rocks. Granite and its relatives are the rocks that make up most of the continental crust. Greenstone belts are dominated by metamorphosed dark (mafic) volcanic rocks, such as basalts. Basalt dominates the oceanic crust in the modern world.
It appears that the Archaean world was very different from that of today. Most of the earth was covered by something similar to modern oceanic crust, and continents were small. (However, there may have been little liquid water on top of the oceanic crust at first, because it may have been too hot.)
Because granitic rocks are lighter they tend to ride up on the basaltic rocks. This is why continents have gotten bigger over the past 4 billion years or so: their rocks tend not to be sucked down into subduction zones and recycled by plate tectonics, but instead are jammed together when they meet. This is also why the ancient gneiss belts still exist. The small pieces of greenstone belt that are very old somehow missed being recycled. Instead, they got mixed in with continental rocks, and that is exactly where we find them today. Most of the ancient basaltic terranes returned to the mantle, and some of these rocks may appear now in a new form on the floor of the modern ocean (which is no older than 200 million years, because of the constant recycling of plate tectonics).
I take this information from personal knowledge, but also from a good college textbook:
Dott, R. H., Jr., and Batten, R. L., 1988, Evolution of the Earth, 4th edition: New York, McGraw-Hill, 643 p.
I hope this helps,
David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 Web site www.gsa.state.al.us
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