Gone But Not Forgotten
- Nov 25, 2002
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Sudbury and Vredfort are mentioned in the article, I also know about Chicxulub in the Yucatan Peninsula.Unusually Well Preserved Crater Found in North Sea
National Geographic News
July 31, 2002
While searching for oil beneath the North Sea, British geologists located what they think is a crater caused by the impact of a meteor or comet that crashed into Earth more than 60 million years ago.
Named the Silverpit crater after a nearby seafloor channel, the site—130 kilometers (80 miles) east of the English coast—could give scientists a better look at what happens when an object from space crashes into Earth.
The crater, consisting of a central crater surrounded by ten concentric rings of escarpments, is 20 kilometers (12 miles) wide. It lies under 40 meters (130 feet) of water and a layer of sediment that in places is as thick as 1,500 meters (4,920 feet).
The team that reported on the discovery in the August 1 issue of Nature said the Silverpit crater is remarkably well preserved in comparison with other known craters on Earth that have been eroded by wind and rain, which should make it especially interesting to those who study meteor impacts.
First Evidence for Early Meteorite Bombardment of Earth
Researchers have found the first terrestrial evidence that the Earth was heavily bombarded by meteorites around four billion years ago. Now the scientists are wondering whether the discovery can also be linked to early life on Earth.
"Most craters found on Earth are highly eroded, poorly preserved, and only found on land," the United Kingdom-based authors, Simon A. Stewart of British Petroleum in Aberdeen and Phillip J. Allen of Production Geoscience Limited in Banchory, said in their journal article.
The Silverpit crater, they added, resembles craters seen by astronomers on the Moon and Europa, a moon of Jupiter. Such craters are not subject to the eroding effects of wind and rain.
Stewart said that the undersea crater discovery was "an accident." While Allen was examining seismic data in search of gas fields, Stewart explained, "he noticed out of the corner of his eye some anomalous features" in the shallower layers of the seafloor.
Allen then mapped the unusual area and hung the map in the hope that someone else might know what the features represented. Stewart said he suspected the crater might have been formed by the impact of a meteor or asteroid.
Stewart and Allen ruled out a volcanic origin because there were no magnetic anomalies in the crater. They also eliminated salt intrusions from lower layers of rock, because the underlying Triassic and Permian strata were undisturbed.
The crater is formed in Cretaceous chalk and Jurassic shale, covered by an undisturbed layer of Tertiary sediment. This means the crater was formed between 60 and 65 million years ago, near the end of the age of the dinosaurs.
The team used three-dimensional seismic reflection data, collected in the course of routine oil exploration, to build a map of the crater at a resolution higher than that of other similar craters.
The three-dimensional data will enable scientists to study the internal structure of the Silverpit crater. Most study of multi-ringed craters has been done by analyzing photographs of craters on other planets and moons.
The data didn't come cheap—a seismic survey can cost U.S. million to million. So, while such data is commonly used in fossil fuel exploration, it is generally too costly for academic researchers.
In an accompanying article in Nature, John G. Spray of the Planetary and Space Science Centre at the University of New Brunswick in Canada said: "This sort of claim is important because we know so little about how impact structures are created when meteorites and comets hit planetary bodies that any new example helps."
"Confirmation that the structure was indeed formed by an impact will require further evidence," Spray cautioned.
Analysis of sediments in the crater indicated that at the time of impact, the area was under water of depths from 50 to 300 meters (164 to 984 feet).
"Unlike all of the craters that formed on the shore, this one was formed on a sedimentary basin that was subsiding," Stewart said. Sheltered from the effects of wind and rain, the crater was preserved by "a rain of fine-grained sediment that will fossilize anything on the seafloor," he said.
Stewart said that nearly all meteors and comets that hit Earth are traveling between 20 and 50 kilometers (12 and 30 miles) per second, which suggests the object that created the Silverpit crater was moving that fast.
"What's less well constrained is whether it was a comet or a meteor," Stewart said. "The impactor gets completely obliterated when it hits."
Using established equations, and factoring in the size of the Silverpit crater, Stewart estimated that if it was a meteor that struck, it would have been about 120 meters (394 feet) in diameter and weighed about 2 million tons. If a comet, the object likely would have been larger, as a comet's icy structure is less dense than a rocky meteor.
Stewart said that until a more exact date for the Silverpit impact is determined, he could only speculate on a possible connection between the Silverpit impact and the space object that is thought to have killed off the dinosaurs.
The impact that occurred at the end of the age of the dinosaurs has been dated very precisely to 65 million years ago. The Silverpit crater might have been formed some five million years later.
According to Spray, impact craters show a correlation between size and structure—from simple bowl-shaped craters formed from small impacts to large multi-ringed structures that are left by the largest meteors. The Silverpit crater is among the smallest known multi-ringed craters.
"You've got to go all the way to the moon to find analagous features," Stewart said.
Previously known multi-ringed craters on Earth, such as the ones at Sudbury, Canada, and Vredefort, South Africa, are more than 250 kilometers (155 miles) in diameter. Extraterrestrial craters, such as the Orientale basin on the Moon, may be as much as 2,000 kilometers (1,243 miles) in diameter.
"Development of multiple concentric rings at such a small diameter may not be unusual because, until recently, we have been unable to obtain images with this degree of detail in such a well-preserved example," Spray said.
Geologists are not certain how multi-ringed craters form. "How these rings form is right to the cutting edge of the research aspect," Stewart said. "It's not something that there's an accepted model for."
Spray said one possible explanation is "Bingham fluid behavior," which causes the concentric ripples around a stone tossed in a pond. Faulting of Earth's surface is more likely to be the cause in the case of Silverpit.
Stewart's hypothesis is that seismic energy created by the impact interacted with sediments, forming concentric fractures around the crater. Over a longer period, these fractures developed into fault zones—cracks in the ground that allowed large sections of rock to shift.
Spray said that the discovery of Silverpit highlights what a small sample of craters scientists have available to study—only about 160.
What other impact craters are out there?