Locations And Ages

As the photographs of other bodies taken from space began to be returned to the earth, the full extent of cratering in the solar system began to become apparent, at least to the more attentive, who concluded that there must be many undiscovered craters on earth, and set out to find them. They had their work cut out for them. A recent impact, like that at Meteor Crater some 50,000 years ago, leaves an obvious crater. But the earth is 4,500 million years old; most craters will have been so eroded that they no longer have any surface manifestation at all and may be detectable only through geological and geophysical methods. These techniques work because the shock of impact distorts the rocks at ground zero, raising central peaks and causing terraces to slump, as at Tycho. But these surface features, which on the earth are obscured by time and erosion, are underlain by structural ones—rock beds bent and twisted into concentric rings. Imagine, for example, that the moon had wind and water and that Tycho had been eroded for millions of years. Then the central peak, the terraces, indeed the crater itself would be gone, leaving no surficial hint that a crater had once been present at that spot. Buried in the rocks below the lunar soil, however, would be the bull's-eye imprint of the now vanished impact crater, detectable by geophysical

FIGURE 7 Tycho, a complex lunar crater 85 km in diameter. Note the central peak, the collapsed and terraced rim, and the hummocky ejecta deposits outside the crater. [Lunar Orbiter V-M125. Photo courtesy of the National Space Science Data Center, principal investigator L. J. Kosofsky.]

techniques. The magnetic, seismic, and gravitational properties of these rocks would reveal the bull's-eye pattern and show that it was once a crater.

By the time the Alvarez theory appeared, scientists had discovered some 100 terrestrial craters; today we recognize approximately 160, and the number increases by 3 or 4 each year. One-third are invisible at the surface, detectable only by geophysical properties. The first thing one notices about the distribution of terrestrial craters (Figure 8] is that although oceans cover 70 percent of the earth's surface, only a handful of impact sites have been found there (off Nova Scotia and eastern Russia). Since incoming meteorites would strike randomly, we assume that many more must have formed in the ocean basins, but that they have been hidden by younger oceanic sediments or carried down a descending tectonic plate. Most craters are located in the interiors of continents, as in North America, Europe, and Australia, which are geologically old, stable, and well studied (the older the surface, the more likely that it has been hit, but also the more likely that erosion will have removed the evidence). Approximately 20 percent of the craters are in Canada, a country that occupies only 1 percent of the total land surface of the earth. Oh, have the gods frowned on fair Canada? Emphatically not. Rather, the Geological Survey of Canada has mounted an intense search for impact craters in a country that contains a higher percentage of geologically old terrain than most. Few craters are known from South America and Central Africa, where rain forests make the search more difficult. About 60 percent of the craters discovered so far are younger than 200 million years, a period that represents only about 4 percent of

Was this article helpful?

0 0

Post a comment