AMERICAN GEOPHYSICAL UNION MEETING : Meteorite-Strike Theory Postulates a Single Ice Age
Armed with controversial evidence of a meteorite strike 250 million years ago, scientists on Wednesday proposed a radically new view of historic geology that challenges accepted theories about glacial erosion and could imply that Earth weathered only one Ice Age instead of many.
In separate presentations at a meeting of the American Geophysical Union in San Francisco, Michael R. Rampino of New York University and Verne R. Oberbeck of NASA Ames Research Center in Mountain View, Calif., also hypothesized that the collision could have initiated the gradual breakup of a supercontinent called Gondwanaland into South America, Africa and other modern continents and subcontinents.
The provocative postulation hit the busy scientific meeting with the impact of one of the tens of thousands of cataclysmic collisions the geologists have proposed. Scientists packed meeting rooms to listen to the two presentations, smiling incredulously but applauding warmly as they concluded.
Rampino’s and Oberbeck’s hypothesis could resolve stubborn paradoxes that dog traditional theories about repeated ice ages, such as how they apparently occurred during relatively warm periods. But their idea was swiftly dismissed by many of their colleagues.
“I seriously doubt this is the start of a new paradigm,” said Peter Barker of the British Antarctic Survey. “When something is entirely new, there is a quality to the work that you can sense . . . a new instrument, a new test, a new mathematical model. . . . This does not have that quality.”
Thomas J. Crowley, a paleoclimatologist with the Applied Research Corp. in Texas, said his reaction to the hypothesis was “considerable disbelief, bordering on incredulity.” Crowley circulated a written critique of Rampino’s and Oberbeck’s abstracts before they even presented their full arguments.
Others took a more cautious, almost bemused, wait-and-see approach.
One expert on extraterrestrial craters, Virgil L. (Buck) Sharpton of the Lunar and Planetary Institute in Texas, said the new hypothesis is plausible enough--at least in some cases--to merit serious testing and challenging before it is accepted or dismissed.
“Every (meteorite) impact structure that we know of today was initially misinterpreted,” either as a salt dome or a glacier, Sharpton said. “When the level of understanding is as incomplete as it is about impact cratering, I don’t think we should dismiss anything.”
Oberbeck said he has an article in the next issue of the Journal of Geology that will fully lay out his argument that peculiar rock formations traditionally attributed to slow glacial erosion may actually have been created instantly by material thrown into the air by an asteroid or comet colliding with Earth.
At issue are diamictite and tillite, two types of rock made up of a chaotic mixture of boulders, cobbles, pebbles, sand and mud. Tillite is a special type of diamictite with unusual signs of erosion--polishing, faceting and striation, or grooving--generally attributed to the inexorable movement of glaciers.
Oberbeck and Rampino challenged the belief that tillite can be created only by glaciers. Oberbeck argued that some deposits of both diamictite and tillite that have been accepted as evidence of glaciers are really signs of meteorite strikes.
Rampino went even further. “Until somebody can prove different to me, I’m going on the assumption that all tillites are debris flow (from meteorite impact craters),” he said.
Oberbeck and Rampino base their hypothesis on the “ejecta blankets” that surround a number of known meteorite craters. Ejecta is the terrestrial material that is ejected, or displaced, when an asteroid or comet collides with Earth. When it falls to the ground, it creates a distinctive layer, or blanket, around the crater.
Since this violent process can both mix up different kinds of material and simultaneously sort it according to size and weight--with objects of similar mass tending to fall back to Earth together, regardless of their origin--the result can superficially resemble tillite.
Tillite also appears sorted because boulders and pebbles swept up along the path of a moving glacier fall out in similar fashion when the ice sheet flows into, say, a fiord and begins to melt.
Millions of years later, after the material is compressed into solid rock, it takes very close examination to differentiate tillite from diamictite and other outwardly similar rock, scientists said.
However, Oberbeck and Rampino contended that some tillite classifications have not been careful enough and that, in any case, the violent collision of a meteorite could produce characteristics indistinguishable from those of glacial material.
Because of this, they asserted that geologists have incorrectly concluded that a series of ice ages was responsible for some historic geological events that were probably the result of meteorite strikes.
Oberbeck said mathematical formula based on the number of craters found on the moon and other planets in the solar system indicates that there were plenty of opportunities for meteorites to have struck Earth over the last 2 billion years--far more meteorites than have been documented by craters.
According to this formula, he estimated that Earth was hit by an astounding 24,283 meteorites at least three miles wide. Three of them were likely to have been at least 400 miles wide, and 110 wider than 60 miles.
Rampino and Oberbeck argued that the most significant geologic event other scientists have traditionally misattributed to glaciers was a mass extinction that occurred 250 million years ago, at the boundary of the Permian and Triassic geologic periods.
That was the largest of many mass extinctions recorded by scientists; about 96% of all living species were exterminated within a few million years.
Based on his own studies of the Cape Fold Belt, an unusual ridge formation in South Africa, Rampino has concluded that this mass extinction actually was caused by the impact of a meteorite roughly 12 miles wide in the middle of the Southern Hemisphere supercontinent called Gondwanaland.
The extinctions, he said, could have been caused when a huge volume of fine ejecta from this collision blocked enough sunlight to kill off plants on land and in the sea, touching off a collapse of the entire food chain.
At the same time, he added, the collision probably also started the process that caused Gondwanaland to break up and drift apart over the next 100 million years.
Oberbeck agreed that the extinction and breakup of Gondwanaland were probably the result of some extraterrestrial force, but he hedged on the idea of a single massive meteorite. He said a number of smaller meteorite strikes might have done the job, throwing up similar volumes of ejecta while perforating the land like a sheet of stamps and thus letting normal tectonic forces pull Gondwanaland to pieces.
Scientists generally accept the existence of Gondwanaland, which eventually broke up to form the current continents of South America, Africa, Antarctica and Australia, as well as the Indian subcontinent and the Arabian peninsula. Most also accept the idea that tectonics alone caused the supercontinent to break up.
Goodby Gondwanaland?
Geologists have theorized that a massive meteorite broke into two pieces and struck Earth 250 million years ago with enough force to initiate the gradual breakup of a Southern Hemisphere supercontinent called Gondwanaland. Scientists generally accept the existence of Gondwanaland, remnants of which they believe drifted apart to form many of today’s continents. But many are skeptical of the idea that an asteroid or comet helped break it up.
Source: Michael R. Rampino, New York University
