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ISAAC ASIMOV

Discovery of Ancient Rocks Casts New Light

February 18, 1990|ISAAC ASIMOV

Geologists, for many years, have tried to find really old rocks on Earth's surface, from which they might deduce Earth's earliest history. It's not easy, because you must find rocks that solidified billions of years ago and have rested in the ground ever since without being seriously disturbed.

Until recently, the oldest rocks known were in western Greenland, but now Samuel A. Bowring, of Washington University in St. Louis, and his colleagues have discovered still older rocks in northwestern Canada and reported on them in November, 1989. These rocks seem to be 3.96 billion years old. They formed, apparently, when the Earth was only about 600,000,000 years old, or only one-eighth its present age.

How is it possible to know the rocks are that old? The answer seems to lie in tiny crystals of zircon that exist within the rocks. Zircon is "zirconium silicate," a rocky substance that contains atoms of the not-very-rare metal zirconium, together with atoms of silicon and oxygen.

When the zircon crystals form, they set up regular lattices of atoms of zirconium, silicon and oxygen. There are, of course, other types of metallic atoms in the vicinity. Some of these metallic atoms can fit into the lattice and can replace an occasional zirconium atom. Other metallic atoms cannot fit and therefore stay out of the tiny crystal.

The fortunate thing about the zircon crystal is that it can accommodate uranium atoms, but not lead. The result is that the zircon crystals contain tiny amounts of uranium, but no lead.

At least, they have no lead at the start, but they develop it. You see, uranium atoms are radioactive. One of them occasionally breaks down to still another type, and so on. Eventually, the breakdowns end in the formation of a lead atom. The lead atom is stable and remains.

The breakdown of uranium is not very rapid. It proceeds so slowly, in fact, that it takes 4.5 billion years for half the uranium in a zircon crystal to turn into lead.

On the other hand, the breakdown is very regular and follows simple rules that have been worked out accurately in the laboratory. If a zircon crystal is analyzed and found to contain so much uranium and so much lead, one can calculate how long it must have taken the uranium to break down and produce that lead and that, in turn, tells you how old the rock is.

Of course, all this isn't quite as simple as it sounds. Making the actual measurements and interpreting them properly is not necessarily easy.

The logical thing to do might be to take the entire zircon crystal and analyze it for uranium and lead content. Unfortunately, nothing is perfect. The zircon crystal may have tiny hairline fractures in it, through which lead may leak out.

What one must do is analyze different parts of the tiny crystal to find those parts in which the lead content is at a maximum and where the crystal has consequently suffered the least loss of lead.

In order to do this, Bowring took his rocks to Australia, where there is a machine that is just right for this sort of measurement. It fired a beam of charged particles at the zircon crystal, and the energy of impact vaporized about two-billionths of a gram of material. This tiny bit of zircon vapor was then analyzed by means of something called a "mass spectrometer," which counts the lead, practically atom by atom. And thus the rocks were found to be 3.96 billion years old.

But even older zircon crystals have been found. Tiny crystals of zircon found in Australian rocks have been measured at 4.3 billion years old. However, other measurements show the rocks to be relatively young. They must once have been part of ultra-old rocks, but the forces of erosion broke down those rocks, and the crystals were then incorporated into newer rocks. The mere existence of these ultra-old zircon crystals doesn't tell us anything about the early Earth. We need to find such ultra-old crystals in their original rock, and there is no way of knowing whether geologists will ever succeed in doing so.

Meanwhile, the rocks of northwestern Canada are interesting. They are granitic in nature--the kind of rocks that make up the continents of the Earth. This would indicate that nearly 4 billion years ago, continents already existed on Earth.

What's more, these granitic rocks are not what we would expect of primeval rocks. Everything geologists have learned makes it clear that such granitic rocks have evolved from simpler predecessors. This means that 4 billion years ago, Earth had already undergone complex changes since its formation. Someday other rocks may give us additional clues to the details of those changes.

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