Sunday, December 17, 2006
Santa Fe Sits on the Site of a Comet or Asteroid Impact, Geologists Say
By John Fleck
Journal Staff Writer
SANTA FE There is not much to say yet about the day a giant asteroid or maybe a comet hit what is now Santa Fe.
The impact was certainly enormous, but it is hard to say how big it was, or how long ago, or how massive a crater it made in what is now northern New Mexico.
Answers to those questions will come. For now, what is remarkable is that, thanks to the curiosity and persistence of a retired Santa Fe geologist Tim McElvain, scientists can say anything at all about the event.
McElvain was walking his terrier, Tintin, on a trail in the Santa Fe foothills two years ago when he spotted an odd bit of rock.
To the untrained eye, the rock's strange striations just look weird. But McElvain recognized it for what it was, and slowly began convincing others.
To the experts who have been making pilgrimages here in the two years since, the striations bear the unmistakable signature of geologic shock. On that day, maybe hundreds of millions of years ago, a giant rock from space, perhaps as large as a mile across, screamed down through Earth's atmosphere in a cataclysmic impact on the landscape that would later become Santa Fe.
On the surface above, life was likely wiped out over a large area, leaving a vast smoking scar of a crater on the Earth, likely miles across.
Something like that, but likely more massive, is what scientists believe caused the extinction of the dinosaurs.
The resulting crater is long gone. But below, the collision slammed a shock wave through the underlying rock, leaving a characteristic imprint for McElvain to find on a spring walk with energetic little Tintin.
Scientists beginning a detailed study of the site praise McElvain's sharp eye and persistent follow-up. Impact structures like it are rare just 174 have been confirmed worldwide, according to the University of New Brunswick's "Earth Impact Database." The nearest to New Mexico is Meteor Crater, near Flagstaff, Ariz.
As he scrambled across a hillside leading a team examining the rocks last week, University of New Mexico researcher Horton Newsom said, "We wouldn't all be here without Tim."
The discovery had its public unveiling at a scientific meeting in Philadelphia in late October. But scientists' current understanding of the event is more hole than doughnut.
"It's a very early stage here," said Wolf Elston, geology professor emeritus at the University of New Mexico.
Elston, who has traveled to South Africa to study the rocks formed by what may be one of Earth's great impact craters, was the first expert to give McElvain's discovery a serious look.
He was convinced as soon as he saw the rock.
At 78, Elston is one of the grand old men of New Mexico geology. He uses a cane, but that did not stop him on a recent field trip from scrambling through the snow and up the hillsides to look at the remarkable rocks McElvain found.
If this were the moon, figuring out what had happened, and when, would be relatively easy.
Throughout its history, the moon has been battered by asteroids and comets. You can see the results on a lunar surface entirely pockmarked with craters, said Adam Read, a geologist with the New Mexico Bureau of Geology and Mineral Resources.
Earth has taken the same beating, Read explained, but the geologic processes of mountain building and erosion have erased most of the obvious marks.
At the Santa Fe site, a mountain range has come and gone, and another, the Sangre de Cristo, has been pushed up in its place.
But not all the marks are gone. In the deep rock beneath an impact crater, clues are left about what occurred above.
Reshaped by a massive shock wave that passed through the rock, the clues are called "shatter cones," cone-shaped rocks that are a distinctive signature of a massive impact.
To Elston, after traveling halfway around the world to study such things, the amazing thing is that these are here, in his own geologic back yard, country that he's hiked many times.
"We all feel like idiots," he said with a grin.
While the shatter cones can tell them an impact happened, much work will be needed to figure out how big the resulting crater was. While the crater itself is long gone, evidence of deep cracks in the Earth may remain.
The difficulty, they say, is distinguishing those cracks from the more mundane faulting that has happened as the area's mountains and valleys were built and leveled during the intervening hundreds of millions of years.
The lack of answers delights Elston. If everything was known, he said, scientists would have to find something else to pay the bills.
"If we had the answers," he said, "we'd have to work for a living."