I just want to add some more info about this topic.
here is link to sott http://www.sott.net/articles/show/214455-Dinosaurs-Wiped-Out-by-Meteor-Shower-Lasting-Thousands-of-Years- http://www.sott.net/articles/show/214369-Double-Meteorite-Strike-Caused-Dinosaur-Extinction-
Here is some info about The Chicxulub Crater:
The Chicxulub Crater
Chicxulub Crater is an ancient impact crater buried underneath the Yucatan peninsula, with its center located approximately underneath the town of Chicxulub, Yucatán, Mexico. Later investigations suggested that this impact structure is dated from the late Cretaceous, about 65 million years ago. The meteorite's estimated size is about 10 km (6 mi) in diameter, releasing an estimated 4.3×1023 joules of energy (equivalent to 191,793 gigatons of TNT) on impact.
The impact caused giant tsunamis in all directions. The emission of dust and particles caused environmental changes close to a nuclear winter, in which the surface of the Earth is totally covered by a cloud of dust for several years. This timing is in good agreement with the theory postulated by the physicist Luis Alvarez and his son Walter, a geologist, for the extinction of the dinosaurs. The Alvarezes postulated that the extinction of the dinosaurs, roughly contemporaneous with the K-T boundary, could have been caused by the impact of just such a large meteorite. This theory is now widely accepted by the scientific community.
The main evidence is a widespread, thin layer of iridium present in this geological boundary across the world. Iridium is a rare metal on Earth, but abundant in meteorites. It is thought that this impact event may have been partially or wholly responsible for the Cretaceous-Tertiary extinction event.
In early 1990, Alan K. Hildebrand, a graduate student at the University of Arizona, visited a small mountain village named Beloc in Haiti. He was investigating certain K-T deposits that include thick, jumbled deposits of coarse rock fragments, which were apparently scoured up from one location and deposited elsewhere by kilometers-high tsunami, giant sea waves, that most likely resulted from an Earth impact. Such deposits occur in many locations, but seem to be concentrated in the Caribbean basin.
Radar topography (color corresponds to height) of the Yucatán peninsula, revealing the Chicxulub Crater.
Hildebrand found a greenish brown colored clay with an excess of iridium, and containing shocked quartz grains and small beads of weathered glass that appeared to be tektites. He and his faculty adviser William V. Boynton published the results of the research in the scientific press, suggesting not only that the deposits were the result of an Earth impact, but that the impact couldn't have been more than 1,000 kilometers (620 miles) away.
This was particularly puzzling, because no crater of any size was known to exist in the Caribbean basin. Hildebrand and Boynton also reported their findings to an international geological conference, sparking substantial interest.
Evidence pointed to possible crater sites off the north coast of Colombia or near the western tip of Cuba. Then Carlos Byars, a reporter for the Houston Chronicle, contacted Hildebrand and told him that a geophysicist named Glen Penfield had discovered what might be the impact crater in 1978, buried under the northern Yucatan Peninsula.
In that year, Penfield had been working for Petroleos Mexicanos (PEMEX, the Mexican state-owned oil company), as a staff member for an airborne magnetic survey of the Yucatan peninsula. When Penfield examined the survey data, he found buried in the noisy data a huge underground "arc", with its ends pointing south, in the Caribbean off the Yucatan that was inconsistent with what he knew about the region's geology.
Penfield was intrigued, and managed to obtain a gravity map of the Yucatan that had been made in the 1960s and was gathering dust in PEMEX's archives. He found another arc, but this one was on the Yucatan itself, and its ends pointed north. He matched up the two maps and found that the two arcs joined up in a neat circle, 180 kilometers (112 miles) wide, with its center at the village of Puerto Chicxulub.
Penfield was an amateur astronomer and had a good idea of what he was looking at. Although PEMEX would not allow him to release specific data, the company did allow him and a PEMEX official named Antonio Camargo to present their results at a geological conference in 1981. Unfortunately, that particular conference was under-attended in that year, ironically because most geologists were attending a workshop on Earth impacts, and their report attracted very little attention, though it did get back to Byars.
Penfield didn't give up. He knew that PEMEX had drilled exploratory wells in the region in 1951. One of the wells had bored into a thick layer of igneous rock known as "andesite" about 1.3 kilometers (4/5ths of a mile) down. Such a structure could have resulted from the intense heat and pressures of an Earth impact, but at the time of the borings it had been written off as a "volcanic dome", even though such a feature was out of place in the geology of the region.
Further studies of the archived well cores would have resolved the issue, but unfortunately most of them had been destroyed in a warehouse fire in 1979. Penfield presently flew down to the Yucatan to see if he could find anything out from the "tailings" left by the wellheads. This idea didn't pan out, and in one case Penfield found himself digging through a communal pigsty that had been set up on a wellhead site, a task he described as "unpleasant and unrewarding".
After Hildebrand got in touch with Penfield, however, the two men were able to locate two separate samples from the wells drilled by PEMEX in 1951. Analysis of the samples clearly showed shock-metamorphic materials. Studies by other geologists of the debris found in Haiti at Beloc also showed it to be clearly the result of an impact.
Gravity map of the Chicxulub Crater, Mexico (after Hildebrand et al)
This research was persuasive, and received a major boost when a team of California researchers, including Kevin O. Pope, Adriana C. Ocampo, and Charles E. Duller, conducted a survey of satellite images of the region. They found that there was a nearly perfect ring of sinkholes centered on Puerto Chicxulub that matched the ring Penfield had found in his data. The sinkholes were likely caused by subsidence of the crater's wall.
This evidence was enough to get most of the geological community on the bandwagon, and further studies have reinforced the consensus. Indeed, some evidence has accumulated that the actual crater is 300 kilometers (186 miles) wide, and the 180 kilometer ring is just an inner wall.
In recent years, several other craters of around the same age as Chicxulub have been discovered, all between latitudes 20°N and 70°N. Examples include the Silverpit crater in the United Kingdom, and the Boltysh crater in Ukraine, both much smaller than Chicxulub but likely to have been caused by objects many tens of metres across striking the earth. This has led to the hypothesis that the Chicxulub impact may have been only one of several impacts that happened all at the same time.
The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 proved that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a few days if the comet should collide with a planet. Comets frequently undergo gravitational interactions with the gas giants, and similar disruptions and collisions are very likely to have occurred in the past. This scenario may have occurred on Earth 65 million years ago.
Here is article about Boltysh Impact Crater
Ukrainian Asteroid Impact Craters could be part of explaining Dinosaur extinction around 65 millions of years ago.
Several impact craters are found in Ukraine, however their age ranges from 350 to 65 million years old. So these craters are evidence of older asteroid impacts hitting the earth long time ago. However, these craters plays an important role in understanding the frequencies of asteroid impacts to earth, as well as understanding the critical K/T boundary and associated layers in our geological history.
At least two of the identified impact craters in Ukraine seem to be linked to the age of mass extinction on earth happened around 65 million years ago. This is the age of the K/T boundary found around the world as well.
We will now discuss some of the craters, and begins with the best described of them, the Boltysh Impact Crater.
The Boltysh Crater is an impact crater in the Kirovohrad Oblast province of Ukraine.
The crater is 24 km in diameter and its age of 65.17 ± 0.64 million years, based on argon dating techniques, is within error of that of Chicxulub Crater in Mexico, and the KT boundary. The Chicxulub impact is believed to have caused the mass extinction at the end of the Cretaceous era, which included the extinction of the dinosaurs.
As well as Boltysh, several other impact craters around the world have estimated ages of about 65 million years, leading to the suggestion that the Earth was struck by multiple asteroid impacts at that time. The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 showed that such multiple impacts over a few days are possible.
Boltysh Crater is located in central Ukraine, in the basin of the Tiasmyn River, a tributary of the Dnieper River. It is 24 km in diameter, and is surrounded by an eject blanket of breccia preserved over an area of 6500 km². It is estimated that immediately after the impact, ejecta covered an area of 25,000 km² to a depth of 1 m or greater, and was some 600 m deep at the crater rim. The crater is not visible on satellite pictures as it is part of agricultural land today and covered with sediments.
The crater contains a central uplift about 6 km in diameter, rising about 550 m above the base level of the crater. This uplift currently lies beneath about 500 m of sediment deposited since the impact, and was discovered in the 1960s during oil exploration.
When first identified, the age of the crater could only be roughly constrained between the age of the impacted rocks (the target) and the age of overlying sediments. The target rocks date from the Cenomanian (98.9 to 93.5 million years ago) and Turonian (93.5 to 89 million years ago) epochs. Bore samples of sediments overlying the crater contain fossils dating from the Paleocene epoch, 65 to 54.8 million years ago. The age of the crater was thus constrained to between 54.8 and 98.9 million years.
Subsequent radiometric dating reduced the uncertainty. The concentration of uranium-238 decay products in impact glasses from the crater were used to derive an age of 65.04 ± 1.10 million years. Analysis of argon radioactive decay products yielded an age of 65.17 ± 0.64 million years. These ages are similar to that of Chicxulub Crater.
Although the ages derived for Chicxulub and Boltysh are the same to within their statistical errors, it does not necessarily follow that they formed at exactly the same time. At the estimated rate of impacts on the Earth, it would not be extremely unusual for a Boltysh-sized crater to be formed within half a million years of Chicxulub. The dating of these impact craters is not yet accurate enough to establish whether the asteroids arrived thousands of years apart, perhaps as part of a generally elevated rate of impacts at that time, or were almost simultaneous, like the impacts of the fragments of Comet Shoemaker-Levy 9 on Jupiter in 1994.
The discovery of the unconfirmed Silverpit crater and the early report of its age as 65 – 60 million years initially gave greater weight to the hypothesis that the Earth was struck by multiple asteroids at this time, however, the age estimate has now been broadened to 74 – 45 million years.
The controversial Shiva crater is claimed to have formed around the same time, but its status as an impact crater is disputed.
One hundred and two core boxes containing over 400m of core from the Bolytsh impact crater in the Ukraine have arrived in Aberdeen as part of the NERC funded project into the environmental effects that resulted from this 65 million year old impact crater. CEPSAR scientists Dr Jon Watson, Prof. Simon Kelley and Dr Iain Gilmour joined their
University of Aberdeen colleague Dave Jolley for an exciting day doing an initial sampling of the core. Core recovery is over 95% providing the team with a near complete geological record starting from the impact rocks of the crater floor through nearly 400m of sediments from the lake that filled the crater after the impact.
The initial sampling comprised some 200 samples that will form part of the detailed geochemical and palynological examination that the team will be undertaking in the coming months. The project is examining the possibility for several impacts at the 65 million year old Cretaceous-Tertiary boundary and their separate and combined consequences for life on Earth at the time.
Another impact crater north of the Bolytsh impact crater is the Rotmistrovka crater with a diameter of approximately 2.7 kilometers and is assumed to be of 120 +/- 10 million years old. So this crater is of a much older date and is not part of the potential mass extinction multiple impact scenario around 65 million years ago which Bolytsh impact crater seems to belong to.
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Seleny Gai impact crater just south of the Bolytsh crater is of 80 million +/- 20 million years old, so it could be part of the multiple impact event where Bolytsh belongs.
Ilyinets impact crater with a diameter of 8.5 kilometers and an age of 378 +/- 5 million years is located some 230 kilometers west of Bolytsch crater. This crater is not exposed to surface today.
Around 70 kilometers north of Ilyinets crater we find the Zapadnaya Impact Crater with a diameter of 3.2 kilomters. The age of this crater is estimated to 165 +/- million years. Also this crater is not exposed to the surface today.
If you have further interest in knowing more about impact craters around the world, The Earth Impact Database could be a good place for you to start learning more about where these are.
The The Silverpit crater
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.
"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. $2 million to $3 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.
Indian Impact Crater Hints of Another Dino-KillerA mysterious basin off the coast of India could be the largest, multi-ringed impact crater the world has ever seen. And if a new study is right, it may have been responsible for killing the dinosaurs off 65 million years ago.
Off the west coast of India, there is a suspicious basin called Shiva. It forms a rough ring over 500 kilometers (311 miles) in diameter and has a central underwater peak the size of Mt. McKinley. Where it sweeps on shore, the land appears shattered and riddled with faults and geothermal hot springs.
If Sankar Chatterjee of Texas Tech University is right, Shiva is the largest impact crater on the planet, the scar leftover from a cataclysm that had a hand in killing the dinosaurs.
The basin is a bonanza of oil and gas resources, and energy companies have been drilling in it for decades. Information on its rocks has trickled out slowly from company vaults, but Chatterjee has kept close tabs, suspecting for years that the region has a dramatic story to tell about a long-lost cosmic catastrophe.
A three-dimensional reconstruction shows the submerged Shiva crater, a 300-mile-wide depression off India's western coast.
Sankar Chatterjee, Texas Tech University
So far the evidence he has assembled is mostly circumstantial. The bedrock that lines Shiva is rife with mantle rocks, as though Earth's crust was simply obliterated across a huge area. And the areas dating to the moment of the suspected impact 65 million years ago are rich in iridium -- a typical fingerprint of impacts.
Major questions about the area abound, though.
For one, a massive spate of volcanic eruptions known as the Deccan Traps overlaps the eastern edge of Shiva. The traps also appear to have erupted around 65 million years ago, just as the Cretaceous period -- and the reign of dinosaurs on Earth -- was coming to a crashing halt.
Were the two events related? Did one, or both contribute to the mass extinction that followed? And what about the Chicxulub impact basin in Mexico, which has already been blamed as the dinosaurs' killer?
"The idea of Shiva as an impact basin is still very speculative," Chatterjee admitted. "But it tends to explain so many things."
For one, the Chicxulub asteroid is thought to be between 8 and 10 kilometers (5.0 and 6.2 miles) wide, a pipsqueak by Shiva's standards.
Chatterjee thinks the two would have to combine, perhaps with the Deccan Traps eruptions in order to induce a global extinction event.
But the idea, which Chatterjee presented Sunday at the annual meeting of the Geological Society of America in Portland, Ore., has been met with deep skepticism.
"There is no need to call on Deccan or Shiva to extend the trauma of the Cretaceous-Tertiary extinction," Steven D'Hondt of the University of Rhode Island said. "We can explain it in the context of the Chicxulub impact alone."
D'hondt allowed that if Siva is ever definitively shown to be an impact crater, it will force scientists to rethink the devastation of 65 million years ago.
But he said, "Nobody has yet demonstrated that this feature is an impact feature."
The interesting thing to me is Iridium, that is why I set it in bold.
Here is some info about it from wiki:
Iridium (play /ɨˈrɪdiəm/ i-RID-ee-əm) is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.
Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named the iridium for the goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium as well as the only stable isotopes; the latter is the more abundant of the two.
The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in industrial catalysis, and in research. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-end spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Iridium radioisotopes are used in some radioisotope thermoelectric generators.
Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. For this reason the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 65 million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.
Also what interested me was osmium.
I search for the map of the world with a locations of Iridium, but I didn't find. I find one, but it seems to me that this map didn't show the full picture of where is Iridium located. (_http://www.mindat.org/min-2045.html) I think it will be interesting to look in which areas Iridium located in high quantity, it could be the locations of meterites impacts.
IMO, it already enough proof that the dinosaurs extintion was cause not by one meteorite impact. The meteor that cause the Shiva crater seemes to be the biggest of them.
I wrote about these ones because they are seemed to of the same dates, according to the data that was given.
Here is also article on sott about meteorite traces, that were discovered by Russian scientists, deep in the norh-west of Pacific Ocean.:_http://www.sott.net/articles/show/130444-Meteorite+Traces+Deep+In+The+Ocean+
You can find a lot of articles on sott. This one I found to be very interesting to me: http://www.sott.net/articles/show/210216-Deeper-Impact-Did-Mega-Meteors-Rattle-Our-Planet-
Deeper Impact: Did Mega-Meteors Rattle Our Planet?
On the west coast of India, near the city of Mumbai, lies a tortured landscape. Faults score the ground, earthquakes are rife, and boiling water oozes up from below forming countless hot springs.
These are testaments to a traumatic history. Further inland, stark mountains of volcanic basalt provide compelling evidence that this entire region - an area of some 500,000 square kilometres known as the Deccan traps - underwent bouts of volcanic activity between 68 and 64 million years ago.
We don't know why. The Deccan traps lie far away from any tectonic plate boundaries, those fractures in Earth's crust through which lava usually forces its way up from the planet's interior. No volcanism on the scale implied by the Deccan traps occurs on Earth now. However, smaller, equally mysterious "hotspots" dot the globe away from plate boundaries - the smoking volcanoes of the Hawaiian islands, for example, or the bubbling geysers of Yellowstone National Park in Wyoming.
Geologists have generally thought that the history of such features can be traced through the slow churnings and contortions of rock under pressure in Earth's mantle. But it seems there is more to it than that. Sometimes volcanic activity needs - and gets - a helping hand from above.
It was in the late 1960s that oil companies prospecting off India's western coast found something odd in the rocks beneath the ocean floor. Sediments laid down on an ocean bed over millions of years generally form rocks resembling a layer cake, with the layers getting older the deeper you delve. That was true in the boreholes drilled off the coast near Mumbai, to a point. But some 7 kilometres down, in a layer of rock deposited 65 million years ago, the neat progression abruptly stopped. Beneath it was a layer of shattered rock, followed by a layer of solidified volcanic lava up to 1 kilometre thick.
Something equally dramatic lurked onshore in the layered lava flows of the Deccan traps. These flows are interrupted by intermediate layers of sedimentary rocks, indicating that the volcanic activity that shook and remodelled the area from about 68 million years ago was not continuous. It was also not catastrophic; fossils found in the sedimentary layers suggest that dinosaurs had coexisted with this activity reasonably well.
But rooted in layers of lava dating from 65 million years ago - around the time dinosaurs disappeared from Earth's fossil record - are colossal spires of lava of a fundamentally different composition. These spires are up to 12 kilometres high and 25 kilometres across at their base, so that their tips appear as surface hills. The lava they are made of is highly alkaline and rich in iridium, an element rare in the Earth's crust but which commonly occurs in meteorites.
The lava in the Deccan traps is rich in iridium, an element rare in the Earth's crust but which commonly occurs in meteorites.
To palaeontologist Sankar Chatterjee of Texas Tech University in Lubbock, all of this was telling a story. In 1992, he recounted it to the world: the entire basin area off the coast of Mumbai, he claimed, was a huge undersea impact crater, some 500 kilometres across, formed when a meteorite 40 kilometres in diameter slammed into Earth 65 million years ago and convulsed its surface. He named the crater Shiva, after the Hindu god of destruction and renewal, and touted it as the big brother of Chicxulub, a crater 180 kilometres across under the Yucatán peninsula in Mexico, which dates to the same time.
This claim was bound to stir controversy. The aftermath of the Chicxulub impact supposedly did for the dinosaurs and many other species that disappeared in a wave of extinctions around that time. If Chatterjee was right, Chicxulub was unlikely to be the whole story.
Most geologists were unconvinced. For a start, the Shiva crater was simply too large. Whereas massive impacts were common in the rambunctious early days of the inner solar system, the absence of recent large craters on Mercury, Venus and Mars strongly suggests that those days are long gone. "These surfaces demonstrate that objects larger than 30 kilometres have not produced impacts in the last three billion years," says planetary geologist Peter Schultz of Brown University in Providence, Rhode Island.
Chatterjee responds that there are still objects of the right size out there, for example the near-Earth object 1036 Ganymed that NASA is monitoring closely, although it is happily not on a collision course with Earth. Moreover, he says that studies off the Indian coast by oil companies in the 1990s revealed gravitational anomalies that add weight to his arguments.
The exact strength of the gravitational pull an object feels at the Earth's surface differs from place to place. It is weaker in areas dominated by low-density granite rocks, for example, and stronger where high-density basalt rocks dominate. If you cross from one side of the posited Shiva crater to the other, the gravity signal weakens towards the centre before reversing and becoming much stronger again towards the proposed rim.
That, says Chatterjee, squares with the idea that a meteorite hit what is now the Mumbai coast from the south-east at an oblique angle of 15 degrees to the horizontal, obliterating the crust entirely and scraping away a portion of the upper mantle, too. The impact would have thrown up a granite peak 50 kilometres high that collapsed back down through a pool of rock below that had been melted in the impact.
That would explain not only the anomalous area of lower gravity under the ocean, but also the odd geology of the Deccan traps. As the granite peak collapsed it too melted, causing the impact crater to overflow and creating enormous melt ponds of alkaline, iridium-rich lava in the charred surroundings. Meanwhile, the shock of the impact caused the moderate Deccan volcanic eruptions, already occurring nearby, to go into overdrive. "A lava trickle became a torrent," says Chatterjee. This torrent of normal lava enclosed the iridium-rich lava overflow from the impact, producing the stunning enclosed spire architecture seen in the Deccan layers today.
That is at best half an answer: it does not explain where the Deccan volcanic activity came from in the first place. Many palaeoscientists, including Chatterjee, think this was linked to a hotspot currently active under the island of Réunion in the Indian Ocean. This hotspot may well have been beneath the area of the Deccan traps 68 million years ago, before continental drift moved them apart.
Even so, it is a contentious claim: to suggest that impacts can amplify volcanic activity is to give them a far greater influence on Earth's recent geological history than has conventionally been allowed. The effects might not just be volcanic, either. According to Chatterjee's calculations, the force of the impact could have been enough to open up a new rift in Earth's crust to the west of the crater, causing a tiny sliver of western India to migrate out into the sea as new oceanic crust forced its way up. The most obvious sign of such a detached sliver today lies almost 2800 kilometres south of the Indian mainland - the island group of the Seychelles.
Comparison with other impact sites shows that if the Shiva crater exists and if it is as big as proposed, the impact would indeed have released enough energy to have such effects. "The physics of the process is undeniable," says geophysicist Adrian Jones of University College London. Even if the Shiva impact never happened, in a startling twist it seems an impact could well have caused the massive Deccan eruptions.
To understand how that might be requires an abrupt change of scene, to the icy permafrost of northern Siberia. This region contains a huge expanse of volcanic rock just as curious as the Deccan traps - and, at some 2 million square kilometres, roughly four times the size. These Siberian traps contain slabs of lava up to 3 kilometres thick that were formed in a single event 251 million years ago.
For geochemist Asish Basu at the University of Rochester in New York, this was fascinating, not least because the lava's date tallies with the largest mass extinction known, the Permian-Triassic extinction, in which over half the existing animal families died out.
Where did so much lava come from over such a short period? When Basu analysed the chemical composition of the rock to find out, it threw up a surprise. The lava showed abnormally high concentrations of the isotope helium-3, generally a signature of rocks from far down in Earth's interior. "Something was causing the deep mantle to come up, but we did not know what," says Basu.
A hole punched by an impact, perhaps? Basu was aware of Chatterjee's work, and it was tempting to float a connection between two huge unexplained lava flows, each dating from the same time as a mass extinction. So Basu travelled to India to do his helium analysis on the rocks there, too. He came up with the same anomalous result.
For Basu, that only deepened the mystery. For one thing, there was no noticeable impact site anywhere near the Siberian lava flows. For another, he was not convinced that the Shiva site was actually an impact crater.
His brainwave was that it didn't matter. "A big impact anywhere would have shaken the planet and created pressure that might have amplified deep-mantle volcanic activity already in progress," he says. If that was so, whether Shiva was an impact crater or not was irrelevant. An impact anywhere in the world could have been the trigger for the Deccan volcanism; arguably, it could even have been the well-documented Yucatán impact.
Shaken and stirred
Basic physics says that is plausible. "The idea of volcanic activity being primed and increased by energy waves sent through the mantle by impacts elsewhere on the planet is a reasonable one," says Jones. Pressure waves from earthquakes travel extremely well through the inner layers of the Earth: seismographs in Europe and the US routinely pick up tremors thousands of miles away in China, for instance. A superpowerful pressure wave such as one created by a huge impact could well have done enough to rattle volcanic plugs and stir lava domes, activating otherwise mild or dormant volcanism.
A superpowerful pressure wave created by a huge impact from space could rattle volcanic plugs and activate dormant volcanism.
To lend credence to the idea, what Basu needed was evidence of a meteorite impact 251 million years ago - not in Siberia, but anywhere. That had him stumped until 2003, when he and his colleagues were handed a 251-million-year-old rock sample from near the Beardmore glacier in Antarctica. Within the rock, they found inclusions with an odd chemical composition that looked for all the world like meteorite fragments. They published a paper detailing the exciting discovery and its possible implication: that the two largest volcanic events in the past billion years could have been caused by meteor impacts (Science, vol 302, p 1388).
The claim caused a considerable stir, and many geologists dismissed the Antarctic finding out of hand. "A lot of criticism came because folks figured it wasn't possible for meteorite fragments to last so long," says Eric Tohver of the University of Western Australia in Perth. Meteorites are mostly metal and would usually be expected to rust away into nothingness over 100 million years, even if buried. The fragments must be modern, said the critics, and somehow have infiltrated the sediments.
Undeterred, Basu and his colleagues pressed on with their exploration. In March this year, at a conference of planetary scientists in Houston, Texas, they presented what they consider to be a smoking gun: more meteorite fragments, this time enclosed in clay containing fossils that date them to 251 million years ago. Clay readily absorbs water, drawing off moisture and preventing meteorite fragments from rusting away.
Scepticism remains. "Small meteorites fall from the sky all the time," says Schultz. "Just because these meteorite fragments are the same age as the Siberian lava does not mean they and the Siberian lava flows are related."
As debated as Chatterjee's and Basu's ideas are, the concept that extraterrestrial bodies might have direct geological effects is now more widely accepted. "The idea of impacts causing volcanism is absolutely plausible," says Vicki Hansen, a planetary geologist at the University of Minnesota, Duluth: modelling shows that impacts can readily melt a planet's surface layer where it is relatively thin.
The question is what sorts of volcanic activity that might generate. Might impacts help to explain the hotspots of Hawaii and Yellowstone, for example? Hansen is open-minded, but sceptical. "There can be little doubt that an impact could spawn a type of hotspot given the right conditions," she says. The crust beneath Hawaii, though, seems relatively intact, and the hotspot looks to be the result of a bulge of superheated mantle, or "plume", forcing its way up for reasons unknown. We know less of what underlies Yellowstone; there is no evidence yet that an impact played a significant part there.
With other hotspots it is a different story. The Ontong Java plateau lies beneath the western Pacific, north of the Solomon Islands, and it is a hotspot that was active some 125 million years ago. The upper layers of the mantle are uplifted there, but not as much as under Hawaii. A likely explanation is that an impact fractured the crust, allowing melt from below to rise and spill out as an eruption. The escape of so much melt material would reduce the density of what was left behind, causing the mantle bulge seen today (Earth and Planetary Science Letters, vol 218, p 123).
How long such impact-induced fireworks might have lasted is another area of debate. Tohver thinks not so long - a few hundred thousand years, perhaps a few million. "It is a lot like dropping a spoon into thick pea soup," he says: the initial large disturbance would quickly die down. Schultz agrees, on the basis of studies of other solar system bodies. "Theoretical models concluded that impacts could not trigger sustained eruptions," he says.
Jones begs to differ, arguing that better modelling will show that sustained eruptions can result from impacts. "A major difference between the Earth and our neighbouring planets is that Earth is still very hot and geologically active, so may be much easier to melt with impacts," he says.
The debate will rage on, but one thing seems certain: accumulating evidence means the days of thinking about geology without considering influences from above are numbered. "Geologists don't typically consider impact hypotheses, perhaps for psychological reasons," says Hansen. "We have been trained to consider things that come from within our planet." Being forced to consider the effects of random meteorite strikes adds another complexity to an already involved subject. But in the end, says Hansen, "We are never going to get anywhere if we keep trying to understand our planet with our hands over our eyes and ears."
According to Cassiopaean timeline (http://cassiopaea.org/forum/index.php/topic,13920.0.html
27 million years ago – Fourteen cometary bombardments trigger major dying of dinosaurs. Close to this time a close orbit of Mars. Earth also previously had a slightly different gravitational field.
Also I think that there may be a few of craters in Oceans, in the areas in which is hard to make a research.
Edit: a little correction Mod's note; SOTT links have been activated.