At best, radiometric techniques can indicate whether these impacts took place within a million years of each other. Dennis V. Kent of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., has tried to check the hypothesis by looking at magnetic studies of rocks from Manicouagan and Rochechouart conducted in the 1960s and early 1970s. As rocks cool from a molten state, they record a snapshot of Earth's magnetic field, which occasionally turns over, switching north and south magnetic poles. If the craters had the exact same age, reasoned Kent, magnetic particles in their rocks should have the same orientation. According to the available evidence, however, rocks from the two sites have opposite orientations, indicating that the direction of Earth's magnetic field reversed in the interval between impacts.
Geomagnetic researchers think that it takes thousands of years to reverse the field.
Kent says his simple test argues against the likelihood that these two craters formed hours apart, leaving unexplained the straight line with the Saint Martin crater. Spray contends that the magnetic test results may not be as clear-cut as they initially seem. Because the Manicouagan crater is so much larger than Rochechouart, the molten rocks would have cooled much more slowly at the Canadian site, taking thousands or hundreds of thousands of years longer to lock in a record of Earth's magnetic field. I f Spray and others can strengthen their claim of an impact chain, it would raise some problems. Standard theories say there is almost no chance of finding such a string of craters on Earth.