Rocks that were formed in the last 2,800 million years can be found in every continent, but rocks more than 3,500 million years old are extremely rare. Some rocks in the American Minnesota River valley and others in western Greenland have been dated at about 3,800 million years. The world’s oldest rocks, however, are probably in Australia, where, in 1983, scientists claimed they had found crystals of the mineral zircon that were 4,200 million years old.
The reason why extremely old rocks are so rare is probably because the surface of the Earth was molten for many millions of years after its formation. Whenever the surface cooled and hardened, it was broken up and remelted. It now seems certain that the Earth had a solid crust by about 3,800 million years ago and that, by 2,500 million years ago, about half of the present continents had been formed.
The oceans are, by contrast, recent features. About 275 million years ago, all the land areas were fused in one super-continent, surrounded by one vast ocean. This continent broke up about 180 million years ago and the pieces drifted slowly to their present positions. So most oceanic rocks were formed in the last 200 million years.