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Issue date:
June 26-28, 1998



Ancient Armageddon

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Learn more about comets, collisions, continents and craters.

The real story of what hit Earth 200 million years ago is more spectacular than any Hollywood fantasy.

By James Trefil

It's turning out to be a tough summer for poor old Earth. First it got slammed by a giant comet in Deep Impact, and this week it's threatened by a Texas-sized asteroid in Armageddon.

   
WHAT HAPPENED?

The continents were part of a prehistoric land mass when -- perhaps in a single day 214 million years ago -- at least five huge objects from space struck Earth as it rotated, leaving craters.


TODAY

Over time, the land mass split apart; the craters ended up on different continents. Only by dating rocks at the sites did scientists conclude that the craters were made by a string of comet fragments.


Source: David Rowley, University of Chicago

 

 
Though these collisions are purely cinematic, we know of about 150 craters around the world created when large objects fell from the sky. Recently, scientists teamed up to explore whether, millions of years ago, there were astronomical fireworks even more explosive than this summer's big-screen extravaganzas.

The story began last year in west-central France, where a team led by scientists from Canada's University of New Brunswick was trying to establish a date for a crater near the town of Rochechouart. This involved finding pieces of rock that were melted during the impact, then measuring radioactive atoms to determine how long ago that melting occurred.

When the age of the rocks was determined to be 214 million years, the Canadian scientists were intrigued. The age was remarkable because it was identical to the age assigned to rocks found in Quebec's Manicouagan crater. With further research, they found that three more craters -- one in Canada, one in North Dakota, one in Ukraine -- had approximately the same dates.

How could five objects hit Earth simultaneously? A few years ago, we had an example of this happening on Jupiter, when Comet Shoemaker-Levy broke up into a string of projectiles that hit Jupiter in rapid succession as the planet turned beneath them. Had a similar event happened on Earth, scientists would expect to see a row of craters of the same age lined up across Earth's surface, following a single line of latitude.

On the face of it, this was a wonderful explanation for the scientists' finding. One catch: The 214 million-year-old craters weren't in a straight line.

But one thing we know about Earth is that the continents move around, and in 214 million years they can move great distances. Had these craters been in a straight line on that day millions of years ago, but shifted in the intervening time?

To find out, the New Brunswick scientists consulted with David Rowley, who has spent years studying the positioning of continents over the past 500 million years. Rowley, of the University of Chicago, entered the positioning information into a huge computer database.

"I get about one call a month from people who want to know where a particular spot on Earth was millions of years ago,-- Rowley says. When he heard from the crater team, "I sat at my computer and watched it draw a map showing the craters falling on a line of constant latitude." In other words, Rowley's map of Earth's surface 214 million years ago showed the five craters roughly lining up.

So, assuming further research confirms that the craters' ages match up, we are left with this scenario:

On a fateful day 214 million years ago, a string of comet fragments fell to Earth, landing thousands of miles apart as Earth turned between impacts. The smaller pieces dug craters miles across, creating shock waves and a rain of fiery rock over hundreds of square miles. The largest piece, which made the Manicouagan crater, might well have wrought continent-wide, even worldwide, destruction: massive clouds of dust and toxic acid rain.

So while you're munching your popcorn and enjoying summer's astro-disaster movies, bear this in mind: The characters on the screen have to worry about only one collision at a time. Small potatoes, compared with 214 million years ago.


James Trefil, USA WEEKEND's contributing editor for science, is an author and George Mason University professor.

Illustration Credit: CHUCK CARTER FOR USA WEEKEND


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