[meteorite-list] Egypt's Craters Pose Geological Riddle

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Oct 6 14:36:45 2006
Message-ID: <200610061835.LAA29690_at_zagami.jpl.nasa.gov>

http://dsc.discovery.com/news/2006/10/06/crater_pla.html?category=travel&guid=20061006103030

Egypt's Craters Pose Geological Riddle
Larry O'Hanlon
Discovery News
October 6, 2006

New research on a few of the more than 1,300 enigmatic
craters found deep in Egypt's western desert has scientists scratching
their heads.

The craters look like they were created by a spectacular ancient rain of
meteors, or perhaps from a vast eruption of steam and gas from inside
the Earth. Or maybe from something else entirely.

"It is a strange and new thing," said Philippe Paillou of the
Observatoire Aquitain des Sciences de l'Univers in Floirac, France.
Paillou is the lead author of a report on an expedition to the remote
landscape of Egypt's Gilf Kebir region in the October issue of the
Journal of African Earth Sciences.

"Right now I cannot tell you I believe this is hydrothermal (explosive
steam) or (meteor) impact," he said.

Hydrothermal vents can look like craters at the surface, Paillou
explained, because eons of wind and rain nibble away at the surface,
exposing more and more of the ice-cream-cone-shaped vent over time.

To find out if that was the case, Paillou and colleagues from France,
Germany, South Africa, Egypt and the United States visited more than 60
of the craters in 2004. Digging was impossible because of the terrain
and remote location, so the researchers used ground-penetrating radar to
peer beneath the craters.

"The problem is that ground-penetrating radar doesn't show this kind of
tube under the surface," Paillou told Discovery News. On the other hand,
the craters, which look like classic volcanic cinder cones, show no
signs of being volcanic.

"When you go there you don't find volcanic rock or ash," Paillou said.
"So it cannot be a classical volcanic feature. That's why we didn't
propose any volcanic hypotheses."

So are they the remains of a meteor that fragmented and peppered the
region with impacts? To test that idea, Paillou and his team looked at
the minerals in the rocks from some of the craters in search of telltale
"shocked" minerals.

They found signs that the rocks has been shocked, alright, but not at
high enough pressures to make a good case for a meteor impact. What's
worse, the shocked minerals don't jive with the hydrothermal hypothesis
either, he said.

"They're clearly not impact craters," said geologist and impact crater
researcher Jay Melosh of the University of Arizona. He recently visited
with some of Paillou's colleagues and learned that the layers of earth
around the rims of the craters are tilted inward - the opposite of what
would be expected from an impact crater, he said.

At the moment, said Melosh, the most likely explanation is some sort of
deeper volcanic activity that perhaps caused widespread hydrothermal
activity in the area. But the specifics are far from clear.

"There's nothing in our current geological literature that describes
them," said Melosh. "It would be most valuable to try to figure it out."

Besides explaining the Gilf Kebir region, the information might prove
useful elsewhere on Earth and beyond. Mars, for instance, was once rich
in both water and volcanism, and today the planet has plenty of
cratered-looking land, Melosh explained. Perhaps some of those famous
pockmarks are not impact craters either.
Received on Fri 06 Oct 2006 02:35:51 PM PDT