Cog:
I will bow to the experts on that one.
I was wrong about them finding a comet hit farther north. This the original release. It gives the Lat and Long.
It doesn’t mention a date.
Madagascar would be the closest land, but the chevrons seem to be on all Indian Ocean coasts.
Note the locations of the other craters they have found using this technique..
2006 Philadelphia Annual Meeting (22–25 October 2006)
Paper No. 119-20
Presentation Time: 1:30 PM-5:30 PM
IMPACT CRATERS AS SOURCES OF MEGATSUNAMI GENERATED CHEVRON DUNES
ABBOTT, Dallas H.1, MARTOS, Suzanne1, ELKINTON, Hannah1, BRYANT, Edward F.2, GUSIAKOV, Viacheslav3, and BREGER, Dee4, (1) Lamont Doherty Earth Observatory of Columbia University, Oceanography 103A, 1000 Rt. 9W, Palisades, NY 10964,
dallas@ldeo.columbia.edu, (2) School of Geosciences, University of Wollongong, Wollongong, NSW 2522, Australia, (3) Tsunami Laboratory, Institute of Computational Mathematics and Mathematical Geophysics, Novosibirsk, 630090, Russia, (4) Dept. of Materials Science, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104
Chevron dunes are not formed by wind. Chevron dunes are not oriented in the direction of the prevailing wind, they can form where there are no beaches, and they contain grains larger than 2 mm in diameter. Chevrons are produced by megatsunamis originating from point sources, i.e. landslides, impact craters, and volcanic explosions. We have assembled data on chevrons worldwide. Most are best explained as the result of tsunami generated from large impact cratering events. We now have data confirming an impact origin of two chevron sources. In the Indian ocean, chevron dunes in Western Australia, India, and Madagascar point towards the 29 km Burckle Crater at 30.865S, 61.365E. The impact ejecta from Burckle crater contain meteorite fragments, impact glass, oceanic mantle fragments, and impact spherules. The impact spherules are >200 microns in diameter, consistent with a 29 km crater. The impact glasses have no K and cannot be continental in origin. In the Gulf of Carpentaria, we found impact ejecta that contain impact glass and meteoritic material: merrillite, high Ni metal, and probable melted carbonaceous chondrite [1]. We also found abundant magnetite impact spherules with a bimodal size distribution [2]. This implies two source craters for the chevrons: the 18 km Kanmare (Serpent) crater at 16.58S, 139.057E and the 12 km Tabban (Rainbow) crater at 17.125S, 139.86E. In the Mediterranean, a megatsunami source near the Rhone delta is of undetermined origin. All other sources are impact crater candidates and require more study. We found the following: the 1 km Judge crater candidate in Long Island Sound at 41.17N, 72.405W, the 10 km Quetzalcoatl crater candidate in the Caribbean at 22.04N, 96.32W, the 18 km Grendel crater candidate in the North Sea at 58.16 N, 5.86E, the 5 km Kangaroo crater candidate at 39.0465S, 141.285E and the 4 km Joey crater candidate at 39.16S, 141.21E.
[1].Elkinton et al., 2006-this volume. [2] Martos et al., 2006-this volume
2006 Philadelphia Annual Meeting (22–25 October 2006)
General Information for this Meeting
Session No. 119
Impact Craters: Structures, Drilling, Ages, and Geophysics (Posters)
Pennsylvania Convention Center: Exhibit Hall C
1:30 PM-5:30 PM, Monday, 23 October 2006
Geological Society of America Abstracts with Programs, Vol. 38, No. 7, p. 299
Here are 2 other hit discoveries that didn’t get much play.
No date for the Egyptian one.
Huge crater carved by space rock found in Egypt
By Robert Roy Britt, Space.com
Scientists have discovered a huge crater in the Sahara desert, the largest one ever found there. The crater is about 19 miles (31 kilometers) wide, more than twice as big as the next largest Saharan crater known.
It utterly dwarfs Meteor Crater in Arizona, which is about three-fourths of a mile (1.2 kilometers) in diameter.
In fact, the newfound crater, in Egypt, was likely carved by a space rock that was itself roughly 0.75 miles wide in an event that would have been quite a shock, destroying everything for hundreds of miles. For comparison, the Chicxulub crater left by a dinosaur-killing asteroid 65 million years ago is estimated to be 100 to 150 miles (160 to 240 kilometers) wide.
The crater was discovered in satellite images by Boston University researchers Farouk El-Baz and Eman Ghoneim. El-Baz named the crater "Kebira," which means "large" in Arabic and also relates to its location on the northern tip of the Gilf Kebir region in southwestern Egypt.
"Kebira may have escaped recognition because it is so large—bigger than the area of 125 football fields, or the total expanse of the Cairo urban region from its airport in the northeast to the Pyramids of Giza in the southwest," El-Baz said Friday. "Also, the search for craters typically concentrates on small features, especially those that can be identified on the ground. The advantage of a view from space is that it allows us to see regional patterns and the big picture."
The crater has two rings, a common configuration. Over time, it has been eroded by wind and water to make it unrecognizable to the untrained eye.
"The courses of two ancient rivers run through it from the east and west," Ghoneim said.
The timing of the impact has not been determined.
The impact that carved Kebira might have created an extensive field of yellow-green silica fragments, known as desert glass and found on the surface between the giant dunes of the Great Sand Sea in southwestern Egypt, the researchers said.
They seem to think this one is real old.
Science Daily — Planetary scientists have found evidence of a meteor impact much larger and earlier than the one that killed the dinosaurs -- an impact that they believe caused the biggest mass extinction in Earth's history.
The 300-mile-wide crater lies hidden more than a mile beneath the East Antarctic Ice Sheet. And the gravity measurements that reveal its existence suggest that it could date back about 250 million years -- the time of the Permian-Triassic extinction, when almost all animal life on Earth died out.
Its size and location -- in the Wilkes Land region of East Antarctica, south of Australia -- also suggest that it could have begun the breakup of the Gondwana supercontinent by creating the tectonic rift that pushed Australia northward.
Scientists believe that the Permian-Triassic extinction paved the way for the dinosaurs to rise to prominence. The Wilkes Land crater is more than twice the size of the Chicxulub crater in the Yucatan peninsula, which marks the impact that may have ultimately killed the dinosaurs 65 million years ago. The Chicxulub meteor is thought to have been 6 miles wide, while the Wilkes Land meteor could have been up to 30 miles wide -- four or five times wider.
"This Wilkes Land impact is much bigger than the impact that killed the dinosaurs, and probably would have caused catastrophic damage at the time," said Ralph von Frese, a professor of geological sciences at Ohio State University.
He and Laramie Potts, a postdoctoral researcher in geological sciences, led the team that discovered the crater. They collaborated with other Ohio State and NASA scientists, as well as international partners from Russia and Korea. They reported their preliminary results in a recent poster session at the American Geophysical Union Joint Assembly meeting in Baltimore.
The scientists used gravity fluctuations measured by NASA's GRACE satellites to peer beneath Antarctica's icy surface, and found a 200-mile-wide plug of mantle material -- a mass concentration, or "mascon" in geological parlance -- that had risen up into the Earth's crust.
Mascons are the planetary equivalent of a bump on the head. They form where large objects slam into a planet's surface. Upon impact, the denser mantle layer bounces up into the overlying crust, which holds it in place beneath the crater.
When the scientists overlaid their gravity image with airborne radar images of the ground beneath the ice, they found the mascon perfectly centered inside a circular ridge some 300 miles wide -- a crater easily large enough to hold the state of Ohio.
Taken alone, the ridge structure wouldn't prove anything. But to von Frese, the addition of the mascon means "impact." Years of studying similar impacts on the moon have honed his ability to find them.
"If I saw this same mascon signal on the moon, I'd expect to see a crater around it," he said. "And when we looked at the ice-probing airborne radar, there it was."
"There are at least 20 impact craters this size or larger on the moon, so it is not surprising to find one here," he continued. "The active geology of the Earth likely scrubbed its surface clean of many more."
He and Potts admitted that such signals are open to interpretation. Even with radar and gravity measurements, scientists are only just beginning to understand what's happening inside the planet. Still, von Frese said that the circumstances of the radar and mascon signals support their interpretation.
"We compared two completely different data sets taken under different conditions, and they matched up," he said.
To estimate when the impact took place, the scientists took a clue from the fact that the mascon is still visible.
"On the moon, you can look at craters, and the mascons are still there," von Frese said. "But on Earth, it's unusual to find mascons, because the planet is geologically active. The interior eventually recovers and the mascon goes away." He cited the very large and much older Vredefort crater in South Africa that must have once had a mascon, but no evidence of it can be seen now.
"Based on what we know about the geologic history of the region, this Wilkes Land mascon formed recently by geologic standards -- probably about 250 million years ago," he said. "In another half a billion years, the Wilkes Land mascon will probably disappear, too."
Approximately 100 million years ago, Australia split from the ancient Gondwana supercontinent and began drifting north, pushed away by the expansion of a rift valley into the eastern Indian Ocean. The rift cuts directly through the crater, so the impact may have helped the rift to form, von Frese said.
But the more immediate effects of the impact would have devastated life on Earth.
"All the environmental changes that would have resulted from the impact would have created a highly caustic environment that was really hard to endure. So it makes sense that a lot of life went extinct at that time," he said.
He and Potts would like to go to Antarctica to confirm the finding. The best evidence would come from the rocks within the crater. Since the cost of drilling through more than a mile of ice to reach these rocks directly is prohibitive, they want to hunt for them at the base of the ice along the coast where the ice streams are pushing scoured rock into the sea. Airborne gravity and magnetic surveys would also be very useful for testing their interpretation of the satellite data, they said.
NSF and NASA funded this work. Collaborators included Stuart Wells and Orlando Hernandez, graduate students in geological sciences at Ohio State; Luis Gaya-Piqué and Hyung Rae Kim, both of NASA's Goddard Space Flight Center; Alexander Golynsky of the All-Russia Research Institute for Geology and Mineral Resources of the World Ocean; and Jeong Woo Kim and Jong Sun Hwang, both of Sejong University in Korea.
When you think about it, the Moon is really pock marked with hits.
Even allowing for the protection of an atmosphere, if you have something as much bigger as the Earth is, with a greater gravitational pull, hits should not be that surprising.
