ASTEROID THEORY HAVING GREAT IMPACT : 65-MILLION-YEAR-OLD COLLISION MAY EXPLAIN FOSSIL COUNT IN NORTH AMERICAN.

Byline: Keay Davidson San Francisco Examiner 

The North American continent and many of its creatures were fried 65 million years ago in a nightmarish ``corridor of incineration'' - an immense, white-hot jet that gushed from an asteroid crash in Mexico, scientists say.

The conclusion, based partly on laboratory experiments at the National Aeronautics and Space Administration's Ames Research Center in Mountain View, Calif., may solve an old puzzle: Why did the asteroid impact cause more extinctions in North America than anywhere else?

In recent years, most earth scientists have accepted the idea that a falling asteroid wiped out the dinosaurs and many other species 65 million years ago. The asteroid fall - which apparently occurred in Mexico's Yucatan Peninsula - triggered a global climate change that destroyed plant and animal life around the world.

But a mystery lingered: Fossil records showed that the cataclysm caused an unusual number of extinctions in North America. If the extinction was truly global, then why was North America so hard hit?

A simple yet dramatic solution appears in this month's issue of Geology, in an article by planetary geoscientist Peter H. Schultz of Brown University in Providence, R.I., and paleoceanographer Steven D'Hondt of the University of Rhode Island.

They cite geological evidence that the asteroid didn't strike the Yucatan head-on. Rather, it approached from the southeast and hit at an angle, perhaps 20 to 30 degrees from horizontal.

The low-angle impact forced the searing debris toward the northwest - into a parabola-shape kill zone over western and central North America.

This ``corridor of incineration,'' as Schultz and D'Hondt call it, perhaps ranged beyond the Pacific shore and the Appalachian Mountains and even all the way to Siberia.

``San Francisco is in the kill zone,'' Schultz notes puckishly.

Schultz and D'Hondt's work has intrigued the world of impact scientists, who study how asteroid falls have skewed the evolution of life.

The corridor of incineration hypothesis ``is going to be the prevailing theory from now on. It's been well-received and has no real detractors within the impact community,'' says one of their admirers, Peter Sheehan, curator of geology at the Milwaukee Public Museum.

If humans had existed in North America 65 million years ago, they might have witnessed a cataclysm worthy of biblical lore: First, a brilliant flash in the southeast as the asteroid rammed into the Yucatan Peninsula, gouging out a crater about 120 miles wide and vaporizing the upper crust.

Then - if Schultz and D'Hondt are right - a brilliant, hot plume of vapor and incandescent ``sun-bright'' (Schultz's phrase) debris would have arced across the sky at about 7 to 10 miles per second, then crashed onto North America.

The scalding heat would have killed countless land-dwelling plants and animals. For example, some 90 percent of known types of leaf-bearing trees and plants died out, according to fossil records.

Afterward, a slower-moving yet still high-velocity cloud of dust, debris and molten material swept over North America, ``adding more insult,'' Schultz said. ``Then an hour or more later, more dust began to fall from the sky as material was dispersed around the globe - and that may have gone on for days.''

A few fish and other aquatic creatures may have survived in cool rivers, lakes and coastal waters, Schultz adds. ``Animals that were able to hide themselves in the muck or water would have had a much better chance to survive.''

Schultz mimicked the asteroid impact in lab experiments at NASA-Ames. He used a hypervelocity gas gun, which fires projectiles (such as quarter-inch metallic spheres) at 4 miles per second at low angles toward ``targets resembling the Yucatan surface - literally dirt or carbonates.''

The collisions generated extremely hot plasmas of ionized gas - ``as hot as the sun's surface, up to 10,000 degrees Fahrenheit.''

Schultz and D'Hondt also cite geological evidence of the kill zone in North America: so-called shocked quartz, fragments of quartz that have been subjected to extraordinarily high pressure. Shocked quartz has been found all over Earth, but the largest fragments exist in North America.

Also, Schultz says, the outer ring of the Yucatan crater, dubbed Chicxulub, is horseshoe-shape, and its inner core is oblong and aligned southeast to northwest. As his laboratory demonstration showed, that's just the shape one would expect if the asteroid struck at an angle from the southeast.

Impact science dates from the late 1970s, when several Bay Area scientists - including the son-father team of Walter and Luis Alvarez - proposed that a falling asteroid triggered a climate change 65 million years ago, wiping out the dinosaurs and many other species. That mass slaughter is technically known as the Cretaceous-Tertiary or ``K-T'' extinction. (Scientists use ``K'' instead of ``C'' because ``C'' had previously been used to refer to a different extinction.)

Few sciences are more interdisciplinary than impact science. Decades ago, paleontologists, geologists, space scientists and military experts assumed they had little to say to each other.

But now they pack impact-science conferences and exchange Niagaras of e-mail, just as Schultz uses the space agency laboratory to mimic the Chicxulub impact.

``I'm a paleontologist and I'm dealing with nuclear ballistics people at meetings!'' Sheehan said with a laugh. ``I've never (before) been at meetings where people are so closely associated with so many different fields and disciplines.''