Rocks fall from the sky. This is fact. It was not always so — two centuries ago the Academie Française pooh-poohed farmers, who claimed as much, as being ignorant and superstitious peasants. The peasants, however, were right. Rocks, on occasion, fall from the sky.
Usually they are small rocks. Usually they don't make it to the ground. If you go out some night when one of the really spectacular meteor showers is scheduled, you will see brilliant fireworks formed by grains of sand, perhaps the tiniest of pebbles, hitting our atmosphere at escape velocity or greater.
Sometimes they get a little larger and more spectacular. I've had the supreme good luck to see — quite accidentally! — three such marvels in my lifetime. One was the largest such meteor ever sighted over Great Britain. I was driving up Hughenden Valley, north of High Wycombe, on my way home one evening when I saw what I at first thought was the full moon. But the full moon would not have been falling in the approximate direction of the North Pole at a noticeable speed. The newspapers the next day were full of this heavenly phenomenon (of which a piece apparently impacted on a barn in Northern Ireland).
The second, which I saw late one night in May, 1975, streaked across the sky out of the west over Sacramento, California. It was green, of all things — not a forest green, nor the green we Esperanto speakers like to associate with hope (the word from which the language took its name), but an iridescent, poisonous green. It disappeared behind a line of trees and buildings to the east and then, apparently, exploded in a flash of green light that turned the sky bright as day for a moment. The Griffith Park Observatory, which may or may not have actually observed it, explained to the world that it was a falling rock (meteorite); having seen what I saw, I tended to discount the airline pilot's observation that, as the object rose out of the west, it was flashing red, white and blue ...
The third came down, also from west to east, one morning when I was driving to work in Milpitas, and crossed the sky directly ahead of me — whether a mile away or a hundred miles away, I never knew. I actually saw a piece break off and go plummeting down at a somewhat steeper angle. This one also got a lot of news on KCBS, the local news radio station. Wherever it was, though, it must have been pretty large; you rarely see breakoffs from a grain of sand.
Really big falling rocks are, fortunately, rare. I'm talking now about the ones that are yards across, or kilometers across. These can cause damage. Almost a hundred years ago such a rock came out of the sky over eastern Russia and apparently detonated while still above the ground. The explosion levelled hundreds of square miles of forest — you can see a similar, if smaller, phenomenon on the north slopes of Mt. Saint Helens in Washington, the result of its 1980 volcanic eruption. A little further back in time — say, sixty-five million years — an even bigger rock impacted the earth in the region of what is today Yucatan, with the apparent result that entire species — almost all the dinosaurs, apparently — went extinct. If a rock as large were to land on earth, with a similar speed, today, it's likely that all the larger mammals — say, any species with an average body weight over a few pounds — would also disappear. As you can guess, this includes humanity.
Of course, such large impacts are fairly rare. Barringer Crater in Arizona, the largest known remnant of such a fall in the United States, caused by a rock which would have annihilated a major city if it fell on it but would not have affected the survival of any species, major or minor, is around 50,000 years old. So large impacts don't happen often ... do they?
The Holocene Impact Working Group seems to believe that they do. They've been working with chevrons, great — and deep — swathes of sediment left behind when megatsunamis — great waves hundreds, not dozens, of feet high — raised by such impacts sweep over areas of land. They found four such chevrons in southern Madagascar, all pointed towards an area of the Indian Ocean where, in the depths, there seems to be an impact crater some eighteen miles across. And the interesting thing is that the chevrons seem to date not from the geologically distant past but from around 4800 years ago, a date not inconsistent with several "flood" myths that they've also been studying.
The HIWG (also known as the "Band of Misfits", a name probably first applied by their detractors, but one that they've picked up and run with) needs more study before they can really say something for sure — for instance, a deep-sea study of the newly-discovered Indian Ocean crater. But the evidence looks interesting. And proof that civilization-destroying, if not species-destroying, events occur every few thousand years rather than every few million is something that we really need — so that we can be ready if it happens again, as, sometime or another, it surely will.
Posted by Don Harlow at November 25, 2006 09:14 AM