Impact Craters On Our Earth

Dennis_De_Bacle

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#1
Unusually Well Preserved Crater Found in North Sea

Stentor Danielson
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.

Seismic Data

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.

Underwater Impact

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.

Multi-Ringed Structure

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.
Sudbury and Vredfort are mentioned in the article, I also know about Chicxulub in the Yucatan Peninsula.

What other impact craters are out there?
 

Mighty_Emperor

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#2
Impact crater fields

Well this is a good set:

Largest ever field of impact craters uncovered

19:00 03 November 04


The discovery of the largest field of impact craters ever uncovered on Earth is the first evidence that the planet suffered simultaneous meteor impacts in the recent past. The field has gone unnoticed until now because it is partially buried beneath the sands of the Sahara desert in south-west Egypt.

Philippe Paillou of Bordeaux University Observatory in Floirac, France, first noticed circular geological structures in the Sahara last year, while analysing radar satellite pictures of the area.

The structures turned out to be part of a huge field of 100 craters spread over 5000 square kilometres near the Gilf Kebir plateau. The craters vary in diameter from 20 metres to 2 kilometres across. The previous largest known crater field covers a mere 60 square kilometres in Argentina.

In February, Paillou led a joint Egyptian and French mission to find the site and examined 13 of the craters, confirming that they were the result of simultaneous impacts. But accurately dating the field has been tricky. Paillou estimates that it is roughly 50 million years old, relatively young in geological terms.

The size of the field suggests that it could be the result of two or more meteors disintegrating as they entered Earth’s atmosphere, the first evidence of a multiple strike, he says.

“Because the field is so big, it can’t have been made by one meteor,” says Paillou. But more information is needed to understand the event and its effects, and Paillou plans to return to the area next month.
http://www.newscientist.com/news/news.jsp?id=ns99996611
 
A

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Re: Impact crater fields

Emperor said:
Well this is a good set:

....

The size of the field suggests that it could be the result of two or more meteors disintegrating as they entered Earth’s atmosphere, the first evidence of a multiple strike, he says.

“Because the field is so big, it can’t have been made by one meteor,” says Paillou. But more information is needed to understand the event and its effects, and Paillou plans to return to the area next month.

http://www.newscientist.com/news/news.jsp?id=ns99996611
Deep frozen, 'icy snowball,' mega-comets, explosively overheating and breaking up as they enter the Earth's atmosphere and spewing out the solid chunks, they contain, as meteors?
 

SoundDust

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#4
[edit]bloody hell Emps, that was quick

rest of post deleted due to duplicating another post
:eek!!!!:
 

rynner2

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#5
More on Silverpit:
North Sea crater shows its scars
By Jonathan Amos
BBC News science reporter

What is thought to be the UK's only space impact crater has been mapped in detail in 3D for the first time.

The so-called Silverpit structure lies several hundred metres under the floor of the North Sea, about 130km (80 miles) east of the Yorkshire coast.

The new pictures show a spectacular set of rings sweeping out around a 3km-wide (1.8 miles) central hole.

Researchers report their description and interpretation of the images in the Geological Society of America Bulletin.

Dr Simon Stewart and Phil Allen detail how the crater's features would have developed from the cataclysmic fall of an asteroid or comet about 60-65 million years ago.

"I'm 99% certain - as certain as you can be - that this is an impact structure," Phil Allen told the BBC News website.

Prof John Underhill, Edinburgh University
"The geomorphology of the crater is absolutely right. If you saw that on Mars or any of the other planetary bodies you wouldn't question it."

But some have - and there is now a lively debate about the origin of Silverpit among those who study the geology of the North Sea.

For their part, Allen and Stewart - who first reported Silverpit's features in 2002 - hope their latest assessment of seismic reflection maps will go a long way to silencing the doubts.

Other worlds

Today, Silverpit is covered by shales and sandstones almost one km deep.

It is only with the seismic data collected by petroleum companies hunting for new oil and gas fields that we know anything about the remarkable features cut into the underlying chalk.

The whole area has been folded over time - stretched on one side, compressed on the other.

Phil Allen, consultant geophysicist
Allen and Stewart say the inner bowl contains a 300m-high central peak, or nipple, typical of impact craters.

This bowl is then surrounded by closely spaced rings, produced by rocks that have collapsed along lines of weakness. The rings stretch out almost 10km from the central point.

"As far as we're concerned, the structure is pretty near unique - certainly on Earth," said Mr Allen, a consultant geophysicist with Production Geoscience Ltd in Aberdeen.

"We suggest the rings are post impact-deformation. We believe there were two phases. First, during impact, specific areas were weakened - the ring shape was defined during impact, if you like.

"Then, much later - perhaps millions of years later - the rings were produced."


Although nothing quite like Silverpit can be seen elsewhere on Earth or on the other inner planets, Stewart and Allen say the tight rings are a good match for those of impact craters on Jupiter's icy moons, such as Europa and Callisto.

The two researchers think this may have something to do with the type of surfaces being bombarded.

"It goes to what's under the ice in the Jovian examples, which is probably a briny ocean; and what's under the chalk at Silverpit, which are these shales that may transmit the energy. We are beginning to think the layering is important."

Independent lines

To the sceptics, though, there is a more mundane explanation for the Silverpit features which does not require an extraterrestrial impactor.

It relates to a thick layer of salt of Upper Permian (248-256 million years ago) age that lies below the whole area. This layer is well known because it forms the sealing horizon for gas prospecting.


The seismic maps peel off overlying rocks to reveal the chalk features
The salt is highly mobile and, argues Professor John Underhill from the University of Edinburgh, moves in and out of rock regions, influencing the geomorphology above.

He says the crater rings match exactly where the salt has withdrawn.

"Features like this exist whenever you remove material at depth. It's true, for example, if you remove magma at depth; you have a collapse known as a caldera," explained Professor Underhill.

"Likewise, if you have mine shafts collapsing around a central point - if the mass at depth is circular, the pattern of fractures is also circular."

He added: "The best thing about this is that it has stimulated a debate and it is an interesting theory, but I just don't agree with their interpretation."


Similarities exist with impact structures on Jovian ice moons
There are several lines of inquiry that might help settle this argument once and for all.

If, as Stewart and Allen believe, a seven-million-tonne, 120m-wide object struck the Earth at 20km/s, the local rocks should show evidence of melting and metamorphism.

Drill samples pulled up during gas prospecting in the area may find this. They may also give a more tightly constrained age for the Silverpit structure.

In addition, any impact would have thrown material out over a large area. These ejecta, which take very distinctive forms, may yet turn up at locations in the UK and Scandinavia.

And then there are the tsunami deposits. Silverpit was covered by a shallow sea back at the start of the Tertiary. An impact would have sent giant waves surging across nearby land masses. There should be evidence of this disturbance in sediments.
(Pics and map on source page.
http://news.bbc.co.uk/1/hi/sci/tech/4360815.stm )
 

Rubyait

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Huge impact crater found in Egypt

A giant crater made by a meteorite impact millions of years ago has been discovered in Egypt's western desert.
Boston University experts found the 31km (19 mile) wide crater while studying satellite images of the area.

It is more than twice the size of the next largest Saharan impact depression and more than 25 times the size of Arizona's famous Meteor Crater.

The American team that found it says its sheer size may have helped it escape detection all these years.

The structure, which has an outer rim surrounding an inner ring, has been named "Kebira", which means "large" in Arabic and also relates to the crater's physical location on the northern tip of the Gilf Kebir region in southwest Egypt.

"Kebira may have escaped recognition because it is so large," said Dr Farouk El-Baz, director of the Boston University Center for Remote Sensing, where the find was made.

"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."

Desert glass

Water and wind erosion may also have helped hide its extra-terrestrial origin.

The heat from this impact may be responsible for the extensive field of "Desert Glass", yellow-green silica glass fragments found on the desert surface between the giant dunes of the Great Sand Sea in southwestern Egypt.

The crater's vast area suggests the location may have been hit by a meteorite equivalent in size to the diameter of the famous Meteor (or Barringer) Crater in Arizona which is 1.2km across.

The impact would have wreaked devastation for hundreds of kilometres.

The 65 million-year-old Chicxulub crater in Mexico is estimated to be 160 to 240km (100 to 150 miles) wide and is a likely culprit in the extinction of the dinosaurs.

http://news.bbc.co.uk/1/hi/sci/tech/4779482.stm
 

Rubyait

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#7
Space impact clue in Antarctica

Evidence for what may be a large and relatively recent impact crater has been found off the coast of Antarctica.

Scientists say the evidence, if correct, points to a space rock some 5km across having crashed into the Ross Sea about three million years ago.

This could have generated a huge tsunami, according to a member of the team investigating the collision.

Details were reported at the Lunar and Planetary Science Conference in Houston, Texas.

Glass hints

Researchers from the Lamont-Doherty Earth Observatory in New York have been studying a 100km-wide depression, known as Bowers Crater, under the Ross Sea.

Team members examined cores drilled from around the area to look for evidence of an impact.

In the cores, they found microscopic glassy grains shaped like teardrops, spheres and dumbbells which are collectively known as tektites.

Some scientists believe these are created when rock fragments are hurled high up into the atmosphere by the impact of a large meteoroid or asteroid, and then partially re-melt as they fall back to the ground.

Other glasses were also found. These are thought to have been formed by cooling of the melted rock and sediment. Similar glasses can be formed through volcanism, but the Ross Sea specimens seem to have a distinct structure under the microscope.

Wave trace

The findings alone do not prove there was an impact in the area a few million years ago, but team member Dallas Abbott says she hopes to search the core material further for a mineral called shocked quartz.

This type of quartz can be distinguished from normal quartz by characteristic lines visible under the microscope which are thought to be formed by the intense pressure of an impact.

The presence of this mineral is considered most diagnostic of a space collision.

Dr Abbott told the BBC News website that an impact in the Ross Sea would have generated a "pretty big tsunami".

The waves could have crashed against the shores of South America; but, she added, the geological history of that continent made it unlikely that evidence of this event would be found.

http://news.bbc.co.uk/1/hi/sci/tech/4816794.stm
 

eburacum

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#8
Incredible that such a large event should have left such a small impact on the geological record of the Earth. Three million years ago was the middle of the Pliocene; there were minor extinction events, but nothing global like the supposed dinosaur killer.
I think that this shows that even large asteroid impacts are not always devastating to the Earth's biosphere.
 

Rubyait

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#9
Thats if it turns out to be a impact crater. I wouldn't be surprised if it isn't as there would surely be an identifiable drop in the general species of the planet with an impact of that size.
 

Rubyait

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Piggy-back mission will hit Moon hard

The silence of a frigid, perpetually dark crater at the Moon's South Pole will soon be shattered. A new mission, announced by NASA on Monday, aims to smash an SUV-sized impactor straight into the crater in late 2008.

The Lunar Crater Observation and Sensing Satellite (LCROSS) was selected as a high-risk, high-pay-off, "piggyback" mission, to share a heavy launch vehicle with the Lunar Reconnaissance Orbiter. LCROSS has a $80 million budget cap. It is designed to help determine the form in which the hydrogen detected by previous orbiters at the South Pole is held, and whether it could used by future human explorers.

Previous observations found evidence of hydrogen in cold craters, but whether that is held as water ice or hydrogen-bearing minerals it not known. The question is important, NASA’s Scott Horowitz, told reporters, because to fulfil President George W Bush's Vision for Space Exploration, "we're going to have to learn how to live off the land".

Both the Lunar Prospector and Clementine missions reported relatively high abundances of hydrogen at the lunar poles, perhaps left from comet impacts in the always-shadowed floors of polar craters.

Heating dirt
Michael Duke, director of the Center for Space Resources at the Colorado School of Mines says "if the hydrogen is there in a useful form and we can get to it, then it can become a resource" to make water or propellant on the Moon.

But he says its usefulness depends on how the hydrogen is distributed. "If it's spread out too thinly, or mixed too greatly with the soil, then to extract it you'd have to heat a lot of dirt to drive off a little bit of hydrogen or water."

Seeking answers about the mysterious hydrogen, LCROSS will use the launch vehicle's upper stage as a 2000-kilogram impactor. The blast should create a crater roughly 5 metres deep and about 30 metres wide, and throw 1000 tonnes of debris as high as 64 kilometres up.

A "shepherding spacecraft" will observe the hit with two cameras and four spectrometers. It will then fly through the plume of tossed-up material, relaying more images and data, before crashing itself 15 minutes later, perhaps 100 metres away.

At least a dozen ground-based observatories will also try to watch the event and its aftermath. Several Moon orbiters will have the opportunity to make follow-up observations, including the newly-arrived LRO and India's Chandrayaan-1 and Japan's Selene missions.

Extra power
LCROSS was selected from 40 ideas to become LRO's piggy-back mission. In January, Horowitz decided for technical reasons that a larger launch vehicle than the Delta II originally slated should boost the LRO into space. The extra power meant an additional payload, up to 1000 kilogrammes in size, could be lifted to the Moon without additional launch costs.

NASA quickly collected the 40 ideas from industry and 19 proposals were submitted by NASA centres for evaluation. In the end, the competition came down to four finalists from which LCROSS, led by project manager Dan Andrews of Ames Research Center in Moffett Field, California, was the winner.

The LCROSS mission will not be the first to deliberately hit the Moon. In July 1999, the Lunar Prospector orbiter was targeted at the surface, having reached the end of its primary mission.

But Butler Hine, deputy program manager for the Robotic Lunar Exploration Program at NASA Ames, says Prospector came in at a low angle, mostly skidding across the lunar surface. In contrast, LCROSS is aiming to crash into Shackleton Crater at an angle of about 75°, and therefore hit the surface harder.

If no water is observed in the plume of the impact, Hine says it would not completely rule out the presence of water. It could mean water ice is distributed in clumps and the impactor missed them.

Duke says for that reason he is more interested in the possibility of a rover that could study many spots within a crater, looking for the hydrogen sources. Hine says landers are planned for later in Robotic Lunar Exploration Program.

http://www.newscientistspace.com/articl ... -hard.html
 

Rubyait

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#12
Big crater seen beneath ice sheet

What appears to be a 480km-wide (300 miles) crater has been detected under the East Antarctic Ice Sheet.
The scientists behind the discovery say it could have been made by a massive meteorite strike 250 million years ago.

The feature at Wilkes Land was found by Nasa satellites that are mapping subtle differences in the Earth's gravity.

"This Wilkes Land impact is much bigger than the impact that killed the dinosaurs," said Prof Ralph von Frese, from Ohio State University, in the US.

If the crater really was formed at the time von Frese and colleagues believe, it will raise interest as a possible cause of the "great dying" - the biggest of all the Earth's mass extinctions when 95% of all marine life and 70% of all land species disappeared.

Some scientists have long suspected that the extinction at the boundary of the Permian and Triassic (PT) Periods could have occurred quite abruptly - the result of environmental changes brought on by the impact of a giant space rock.

It is a similar argument to the one put forward to explain the demise of the dinosaurs at the much later date of 65 million years ago.

A geological structure, known as the Bedout High, in the seabed off what is now Australia, has also been suggested as the possible crater remains from the PT impactor.

But the impact explanation for the great dying is an argument that has struggled to find favour.

The prevailing theory is that several factors - including supervolcanism and extensive climate warming - combined over thousands of years to strangle the planet's biodiversity.

Earth may well have been hit by extraterrestrial objects, but it is unlikely there was some killer punch from space, these other researchers contend.

The Ohio-led team used gravity fluctuations measured by the US space agency's Grace satellites to peer beneath Antarctica's icy surface. Team members were drawn from the US, Russia and Korea.

The crater information was first presented at the recent American Geophysical Union Joint Assembly in Baltimore.

http://news.bbc.co.uk/1/hi/sci/tech/5045024.stm
 
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#13
http://www.sciencedaily.com/releases/2007/01/070118094039.htm


Source: University of Alaska Fairbanks
Date: January 22, 2007
More on: Near-Earth Object Impacts, Early Climate, Fossils, Global Warming, Earth Science, Earthquakes

Geologist Gets To The Bottom Of Chicxulub Impact Crater
Science Daily — About 65 million years ago, a massive disruption led to worldwide extinction of dinosaurs. The impact of a giant asteroid created massive tsunamis and spewed forth a global cloud of carbon gases that altered Earth's atmosphere and blocked the light for weeks, possibly years. In recent years, that impact event has been linked to a 112-mile-wide crater, dubbed Chicxulub, on the coast of Mexico's Yucatan Peninsula.


Since its discovery in the 1980s, the Chicxulub crater has left its own impact on sky-watchers and sci-fi fans worldwide, and impact events have been depicted in Hollywood films such as "Armageddon" and "Deep Impact," as well as countless artistic renditions.

Despite the spotlight on the theories surrounding the impact, Michael Whalen, associate professor of geology at University of Alaska Fairbanks, has managed to stay "out of the limelight, yet into the limestone" with his work sampling the core of the crater. Due to the efforts of Buck Sharpton, UAF Vice Chancellor for Research, Whalen became part of an international effort to correlate seismic data with information obtained from a drill hole that reaches more than 1.2 miles deep, through the impact layer and beyond.

Interestingly enough, unlike other more noticeable craters, the Chicxulub crater spent 55 million years in virtual obscurity, due to the fast infilling that masked its presence. Speedy recovery, which by geologists' standards amounts to about 10 million years, preserved the crater by mantling it with sediment, attracting geologists like Whalen, who studies the effects of extinction events on carbonate layers (also known as limestone) and the organisms that make up those layers.

On Jan. 20, Whalen will be traveling with a team to the Chicxulub site for a week to obtain more core samples in order to get a better understanding of how the crater filled in and how the earth itself recovered from the massive impact. He's also part of an ongoing collaboration that is trying to secure funding to drill two more holes in the crater, one off shore and one through the peak ring.

Note: This story has been adapted from a news release issued by University of Alaska Fairbanks.
 
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#14
'Crater' spied under California
By Paul Rincon
Science reporter, BBC News, Houston



A seismic survey peels away the sediments to reveal the structure
Oil exploration work in California's Central Valley region has uncovered a possible space impact crater.

The 5.5km-wide bowl is buried under shale sediments west of Stockton, in San Joaquin County, and is thought to be between 37 and 49 million years old.

Researchers are continuing to analyse cuttings from oil exploration wells drilled in the structure.

Details of the discovery were presented at the Lunar and Planetary Science Conference in Houston.

Data from a 3D seismic survey of an ancient sea bed clearly shows a circular structure buried 1,490-1,600m (4,890-4,250ft) below sea level.

The Victoria Island structure, as it has been named, has a concentric rim surrounding a "central uplift" - a peak at the centre - which are both characteristic of impact craters.

Co-investigator Professor Jared Morrow, from San Diego State University, said the context of the crater in relation to surrounding rock layers suggested it dated from the middle Eocene Epoch.

Crater pair

Preliminary estimates for the age of the crater suggest it is just too early for an episode of multiple impacts on Earth known as the late Eocene bombardment, which occurred about 35 million years ago.

One of the largest impact craters in the world - the 45km-wide (28 miles) Chesapeake Bay structure on the eastern shore of North America - is from this period.


"It would be interesting if our crater dated to that time, but we can't make that association at the moment," said co-author Jared Morrow, from San Diego State University.

The team plans to look for characteristic geological signatures of impact sites in cuttings drilled from wells in and around the structure.

These include a type of quartz deformed under intense pressure - known as shocked quartz - as well as glass and melt particles, and an enhancement of the element iridium.

Victoria Island is not the first crater proposed for the Central Valley. A 1.3km-wide (0.8 miles) feature to the north known as the Cowell structure, dating to the Miocene (5 to 24 million years ago), has also been put forward as the location of a space impact.

[email protected].

http://news.bbc.co.uk/2/hi/science/nature/6458841.stm
 

rynner2

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#15
UK impact crater debate heats up
By Jonathan Fildes
Science and technology reporter, BBC News

A deep scar under the North Sea thought to be the UK's only impact crater is no such thing, claims a leading geologist.

Professor John Underhill, from the University of Edinburgh, says the Silverpit structure, as it is known, has a far more mundane explanation.

Detailed surveys reveal nine similar vast chasms in the area, he says.

This suggests it was part of a more widespread process, probably the movement of salt rocks at depth, not a one-off meteorite impact, he believes.

"I feel like I'm spoiling a party," said Professor Underhill. "It's a less glamorous explanation, but that's what the scientific data is saying."

Professor Underhill first put forward his theory in 2004 and has spent the last three years collecting evidence to back it up.

I just felt that there was a bit more to the story than met the eye

John Underhill
However, the group that discovered the structure in 2002 stands by its original theory of a cataclysmic asteroid or comet impact about 60-65 million years ago.

"I can't understand why John keeps banging away at an alternative model," said team member Dr Simon Stewart, a geologist with BP.

"The crater interpretation of Silverpit still stands, in my opinion."

Regional view

The 3km-wide (1.8 miles) wide bowl was discovered in 2002 by Dr Stewart and his colleague Phil Allen, of geoscience firm PGL, about 130km (80 miles) east of the Yorkshire coast.

The structure, which comprises concentric, closely-spaced rings, is punched through a band of chalk. Today, it covered by shales and sandstones almost one kilometre deep.

It can only be seen on seismic data, collected by petroleum companies hunting for new oil and gas fields.


Two studies by Dr Stewart and Mr Allen, the latest of which mapped the structure in 3D, concluded that it was the result of a space impact. But Professor Underhill has never been convinced.

"I just felt that there was a bit more to the story than met the eye," he told BBC News.

To establish whether the feature was unique, he examined a 3,750-sq-km-area around the structure.

"I decided to throw a more regional view at it, and ended up finding a whole load of these features with very similar cross sections," he said.


Click here to see the geology of the Silverpit area

Along with a colleague, Dr Zana Conway, he has identified at least nine major bowl-shaped depressions, known as synclines, and over 15 subsidiary structures including Silverpit itself. He says that more have also been identified elsewhere.

Salt push

He says that the swarm of structures is the result of movement of a thick layer of salt of Upper Permian (248-256 million years ago) age that lies below the whole area.

The salt is highly mobile and flows between areas of high and low pressure.
sed the crater-like Silverpit structure, argues Professor Underhill.

"The key observation is that every single syncline is exactly coincident with where the salt has thinned or withdrawn," he said.

"There is an absolute one-to-one correlation between these two levels."

In addition, Dr Conway has examined the coastlines of Denmark and the east of England for evidence of tsunami deposits of the right age.

If a space object did crash into the shallow North Sea, the argument goes, it would have caused great waves to dash the coastlines of surrounding countries. In addition, it would have left a layer with high levels of an element known as iridium in the rocks.

"There is a lack of any independent evidence for a meteorite impact for the time that they say in the place that they advocate," said Professor Underhill.

Missing links

Dr Stewart is un-moved. He points to a 300m-high central peak, or nipple, in the centre of the inner bowl, typical of impact craters.

In addition, he argues the seismic surveys show areas of undeformed rock underlying the crater.


The Silverpit structure has been mapped in 3D


Enlarge Image


He explained it was like finding a hole in the roof of your house at the same time as you were digging in the basement.

"With only this information, one might conclude your roof collapsed because of subsidence into the hole you made in the basement," he says.

"But if you then point out that the first floor is intact, undeformed, we would conclude the roof hole was unrelated to the basement hole and indeed was most likely to be caused by something dropping through it."

Professor Underhill is unconcerned by this argument. He says that different rocks are mechanically stronger than others and will react in different ways when the salt withdraws.

Conclusive proof

The debate has drawn in other researchers from the geological community.

Impact expert Dr Gareth Collins from Imperial College London has also examined the evidence and says the circular structure is geometrically similar to other craters, particularly those found on other planets.


Similarities exist with impact structures on Jovian ice moons


North Sea shows its scars

"On balance an impact origin is the simplest and most likely explanation," he says. "But to qualify that - it has absolutely not been proven to have an impact origin."

To unequivocally show Silverpit is a crater, he says, geologists would have to drill through its centre and look for evidence of minerals, such as shocked quartz, catastrophically altered by the crushing forces of the impact.

"The rocks and minerals affected by the impact would have been changed in a way which is absolutely diagnostic of high pressures that happen over a very short period of time," he said.

Other geologists with experience of the North Sea say that the large number of similar structures found by Professor Underhill strongly favours salt withdrawal.

"Given the abundance of these features and their distribution, it looks more like a salt withdrawal phenomenon than an impact, unfortunately," said Professor John Gluyas, of the University of Durham and co-founder of North Sea oil firm Fairfield Energy.

"On balance, I think John has it at the moment; but I think I'd like to see more evidence before I side with one camp."

Professor Underhill's and Dr Conway's work will be presented at the annual American Association of Petroleum Geologists meeting in Long Beach, California, in early April.

http://news.bbc.co.uk/1/hi/sci/tech/6503543.stm
 

rynner2

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#16
Meteorite made big impression on Ullapool
Mark Henderson

The largest meteorite to hit the British Isles struck near the Scottish town of Ullapool about 1.2 billion years ago, new research suggests.

Scientists from the University of Oxford and the University of Aberdeen have found evidence that rock formations in northwest Scotland, once thought to have been created by volcanic activity, may be debris left by a huge meteorite strike.

The material is scattered over an area about 31 miles across, according to a paper published in the journal Geology.Tests on the rocks have identified traces of iridium, a metal abundant in asteroids and meteorites.

Ken Amor, of the Department of Earth Sciences at Oxford, said: “We found more evidence when we examined the rocks under a microscope: telltale microscopic parallel fractures that also imply a meteorite strike.”

There are 174 meteorite craters known on Earth, and the newly identified one is the largest in and around the British Isles. “If there had been human observers in Scotland 1.2 billion years ago they would have seen quite a show,” said Dr Amor.

“The massive impact would have melted rocks and thrown up an enormous cloud of vapour that scattered material over a large part of the region around Ullapool. The crater was rapidly buried by sandstone, which helped to preserve the evidence.”

http://www.timesonline.co.uk/tol/news/u ... 628516.ece
 

EnolaGaia

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#17
Earth's oldest known impact crater found in Greenland

A 100 kilometre-wide crater has been found in Greenland, the result of a massive asteroid or comet impact a billion years before any other known collision on Earth.

The spectacular craters on the Moon formed from impacts with asteroids and comets between 3 and 4 billion years ago. The early Earth, with its far greater gravitational mass, must have experienced even more collisions at this time – but the evidence has been eroded away or covered by younger rocks. The previously oldest known crater on Earth formed 2 billion years ago and the chances of finding an even older impact were thought to be, literally, astronomically low.

Now, a team of scientists from the Geological Survey of Denmark and Greenland (GEUS) in Copenhagen, Cardiff University in Wales, Lund University in Sweden and the Institute of Planetary Science in Moscow has upset these odds. Following a detailed programme of fieldwork, funded by GEUS and the Danish 'Carlsbergfondet' (Carlsberg Foundation), the team have discovered the remains of a giant 3 billion year old impact near the Maniitsoq region of West Greenland.

"This single discovery means that we can study the effects of cratering on the Earth nearly a billion years further back in time than was possible before," according to Dr Iain McDonald of Cardiff University's School of Earth and Ocean Sciences, who was part of the team.

Finding the evidence was made all the harder because there is no obvious bowl-shaped crater left to find. Over the 3 billion years since the impact, the land has been eroded down to expose deeper crust 25 km below the original surface. All external parts of the impact structure have been removed, but the effects of the intense impact shock wave penetrated deep into the crust - far deeper than at any other known crater - and these remain visible.

However, because the effects of impact at these depths have never been observed before it has taken nearly three years of painstaking work to assemble all the key evidence. "The process was rather like a Sherlock Holmes story," said Dr McDonald. "We eliminated the impossible in terms of any conventional terrestrial processes, and were left with a giant impact as the only explanation for all of the facts."

Only around 180 impact craters have ever been discovered on Earth and around 30% of them contain important natural resources of minerals or oil and gas. The largest and oldest known crater prior to this study, the 300 kilometre wide Vredefort crater in South Africa, is 2 billion years in age and heavily eroded.

Dr McDonald added that "It has taken us nearly three years to convince our peers in the scientific community of this but the mining industry was far more receptive. A Canadian exploration company has been using the impact model to explore for deposits of nickel and platinum metals at Maniitsoq since the autumn of 2011."

The international team was led by Adam A. Garde, senior research scientist at GEUS. The first scientific paper documenting the discovery has just been published in the journal Earth and Planetary Science Letters.

SOURCE: http://www.eurekalert.org/pub_releases/ ... 062812.php
 

kamalktk

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#18
I figure this is the most relevant thread for this.

http://geology.com/meteor-impact-craters.shtml
===================
Explore fifty of earth's most obvious asteroid impact sites with satellite images and maps. Use the buttons in the upper left to navigate and change your zoom level. To navigate just grab the image and drag it around. Switch between maps and satellite images using the buttons in the upper right. To recenter, double click on the map/image.

There are many other craters. We have selected the ones that are easiest for the inexperienced viewer to recognize. Featured craters include: Meteor Crater in Arizona, USA; Chicxulub on the Yucatan Peninsula of Mexico; Aorounga in Chad; Roter Kamm in South West Africa / Namibia; Mistastin Lake in Newfoundland and Labrador, Canada; Manicouagan in Quebec; Clearwater Lakes in Quebec; Deep Bay in Saskathchewan; Bosumtwi in Ghana; Grosses Bluff in Northern Territory, Australia; Kara-Kul in Tajikistan; New Quebec in Quebec, Canada; Goat Paddock Crater in Kimberley Plateau, Northwestern Australia; Gweni Fada Structure, Chad; Acraman Structure, South Australia; Vredefort Dome in South Africa; Teague / Shoemaker Structure in Western Australia; Ouarkziz Structure in Algeria; and Ramgarh Crater in Eastern Rajasthan, India; Lumparn in Finland; Janisjarvi in Russia; Brent in Ontario, Canada; La Moinerie in Quebec, Canada; Goyder in Northern Territory, Australia; Mien in Sweden; Liverpool in Northern Territory, Australia; Lappajarvi in Finland; Chiyli in Kazakhstan; Haughton in Northwest Territories, Canada; El'gygytgyn in Russia; Aouelloul in Mauritania; Tenoumer in Mauritania; Tabun-Khara-Obo in Mongolia; Pretoria Saltpan in South Africa; Lonar in India; Nicholson in Northwest Territories, Canada; Serra Da Cangalha in Brazil; Gow in Saskatchewan, Canada; Riachao Ring in Brazil; Oasis in Libya; B.P. Structure in Libya; Mount Toondina in South Australia; Tin Bider in Algeria; Chukcha in Russia; Upheaval Dome in Utah; Talemzane in Algeria; Monturaqui in Chile; Wolfe Creek in Western Austsralia; and Amguid in Algeria.
Many people refer to these structures as "Meteorite Craters". "Meteor" is the proper term for the object as it streaks through Earth's atmosphere. "Meteorite" is the term used for the object after it has fallen to Earth's surface. "Meteor crater" is the most common useage for craters produced by a meteor impact.
 

EnolaGaia

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#19
This May 2017 item is interesting because it raises the possibility the Permian 'Great Dying' involved an impact similar to the K-T Boundary event ...

Falkland Islands basin shows signs of being among world's largest craters
A basin in the Falkland Islands exhibits traits of a large impact crater, according to a new analysis by a team of scientists. The structure measures approximately 250 kilometers, or more than 150 miles, in diameter and is described in the latest issue of the journal Terra Nova. ...
The researchers ... acknowledge that samples from the site are necessary to confirm the conclusions of the analysis.

The basin is situated on the Falkland (Malvinas) Plateau to the northwest of West Falkland (Gran Malvina) Island. Seen in seismic-reflection profiles, and in gravity and magnetic surveys, it has traits that are consistent with impact craters ...

The scientists estimate the age of the basin to be from the late Paleozoic Era -- approximately 270 to 250 million years ago.

"If the proposed crater turns out to be 250 million years old, it could correlate with the largest mass extinction ever -- the Permian extinctions, which wiped out more than 90 percent of all species" ...
SOURCE: https://www.sciencedaily.com/releases/2017/05/170504131658.htm
 

EnolaGaia

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#20
There are clues suggesting a geological feature under the Greenland ice sheet is an impact crater of significant size ...

Greenland ice sheet hides huge 'impact crater'
What looks to be a large impact crater has been identified beneath the Greenland ice sheet.

The 31km-wide depression came to light when scientists examined radar images of the island's bedrock.

Investigations suggest the feature was probably dug out by a 1.5km-wide iron asteroid sometime between about 12,000 and three million years ago.

But without drilling through nearly 1km of ice to sample the bed directly, scientists can't be more specific. ...
FULL STORY: https://www.bbc.com/news/science-environment-46181450
 

gerhard1

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#21
There is one near me in Ames, Oklahoma. It is said to be more than 300 million years old.
 
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#25
The oldest impact may have had a beneficial effect.

BARCELONA, SPAIN—Barlangi Rock, an ancient hill in the outback of Western Australia, is dimpled by the quarries of Aboriginal people who chiseled its fine-grained rocks into sharp tools. Now, geologists have added a much deeper layer of history to those rocks by showing they were forged 2.229 billion years ago, when an asteroid crashed into our planet. The finding makes Yarrabubba crater, the 70-kilometer-wide scar left by the collision, Earth’s oldest.

The geologists who reported the date last week, here at the Goldschmidt geochemistry conference, also point out a conspicuous coincidence: The impact came at the tail end of a planetwide deep freeze known as Snowball Earth. They say the impact may have helped thaw Earth by vaporizing thick ice sheets and lofting steam into the stratosphere, creating a powerful greenhouse effect.

https://www.sciencemag.org/news/2019/08/shock-and-thaw-earth-s-oldest-asteroid-impact-may-have-helped-lift-planet-out-deep
 
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