From Scientific American:
In its infancy, beginning about 4.5 billion years ago, the earth glowed like a faint star. Incandescent yellow-orange oceans of magma roiled the surface following repeated collisions with immense boulders, some the size of small planets, orbiting the newly formed sun. Averaging 75 times the speed of sound, each impactor scorched the surface–shattering, melting and even vaporizing on contact. These fiery conditions had to subside before molten rock could harden into a crust, before continents could form, before the dense, steamy atmosphere could pool as liquid water, and before the earth’s first primitive life could evolve and survive. But just how quickly did the surface of the earth cool after its luminous birth? Most scientists have assumed that the hellish environment lasted for as long as 500 million years, an era thus named the Hadean. Major support for this view comes from the apparent absence of any intact rocks older than four billion years–and from the first fossilized signs of life, which are much younger still.
In the past five years, however, geologists–including my group at the University of Wisconsin-Madison–have discovered dozens of ancient crystals of the mineral zircon with chemical compositions that are changing our thinking about the earth’s beginnings.
More here.