Dr. Dallas Abbott, of the Lamont-Doherty Earth Institute, and Ann Isley, of SUNY Oswego, assembled an expanded database of terrestrial impacts over the last 4 billion years. They used clues from known craters such as impact spherules created from impact melt, and from impact breccias that are created from shattered debris fused under high temperatures and pressures. They also examined the activity of normal and strong mantle plumes over geological time. Time series derived from this data showed that 10 major peaks in terrestrial impact activity were seen on Earth over this time period. Nine out of 10 of these impact peaks are directly matched by peaks in normal to strong mantle plume volcanism. In addition, there are two prominent lulls in impact activity, also corresponding to periods of lower activity of mantle plume volcanism.

The biggest mystery remaining is the mechanism by which large impacts might intensify volcanism. Abbott and Isley propose three possibilities: impacts may cause cracking and de-stressing of the crust, allowing melts that had been trapped due to tectonic stress and/or impermeable boundaries to rise more easily to the surface; impacts may produce large cracks in the surface of the Earth allowing new plate boundaries to form with consequent thinner lithosphere and longer melt columns; or impacts may produce microdikes at the core mantle boundary, which, if very thin, would allow molten core and mantle material to mix, increasing the amount of heat available for melting the mantle and producing a rapid intensification of existing mantle plumes.

Another question raised by the correlation between impacts and volcanism concerns widely adopted theories that meteorite and comet impacts were the cause of mass extinctions of life on Earth. Was it the impact alone or could major episodes of mantle plume volcanism have contributed to these extinctions?

Dallas Abbott is an adjunct research scientist at The Lamont-Doherty Earth Observatory. Her primary research focus is the thermal history of the earth, and the manner in which heat transport through the crust and upper mantle influences geological processes, both ancient and present-day.
Abbott and Isley's research paper, "Extraterrestrial Influences on Mantle Plume Activity," is appearing in Earth and Planetary Science Letters this month.

The Lamont-Doherty Earth Observatory, a research unit of the Earth Institute, is one of the world's leading research centers examining the planet from its core to its atmosphere, across every continent and every ocean. From global climate change to earthquakes, volcanoes, environmental hazards and beyond, Observatory scientists continue to provide the basic knowledge of Earth systems that must inform the future health and habitability of our planet.

The Earth Institute at Columbia University is among the world's leading academic centers for the integrated study of Earth, its environment, and society. The Earth Institute builds upon excellence in the core disciplines - earth sciences, biological sciences, engineering sciences, social sciences and health sciences - and stresses cross-disciplinary approaches to complex problems. Through its research, training and global partnerships, it mobilizes science and technology to advance sustainable development, while placing special emphasis on the needs of the world's poor.

At the southern end of Madagascar lie four enormous wedge-shaped sediment deposits, called chevrons, that are composed of material from the ocean floor. Each covers more than 100 square kilometers with sediment hundreds of meters deep.

On close inspection, the chevron deposits contain deep-ocean microfossils that are fused with a medley of metals typically formed by cosmic impacts. And all of them point in the same direction - toward the middle of the Indian Ocean where a newly discovered crater, 29 kilometers, or 18 miles, in diameter, lies 3,800 meters, or 12,500 feet, below the surface.

The explanation is obvious to some scientists. A large asteroid or comet, the kind that could kill a quarter of the world's population, smashed into the Indian Ocean 4,800 years ago, producing a tsunami at least 183 meters thigh, about 13 times as big as the one that inundated Indonesia nearly two years ago. The wave carried the huge deposits of sediment to land.

Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the past 10,000 years. But the self-described "band of misfits" that make up the two-year-old Holocene Impact Working Group say astronomers simply have not known how or where to look for evidence.

Scientists in the working group say the evidence for such impacts during the past 10,000 years, known as the Holocene epoch, is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to 1 million years, as astronomers now calculate, catastrophic impacts could happen every few thousand years.

The researchers, who formed the working group after finding one another through an international conference, are based in the United States, Australia, Russia, France and Ireland. They are established experts in geology, geophysics, geomorphology, tsunamis, tree rings, soil science and archaeology, including the structural analysis of myth. Their efforts are just getting under way, but they will present some of their work at the American Geophysical Union meeting in December in San Francisco.

This year the group started using Google Earth, a free source of satellite images, to search the globe for chevrons, which they interpret as evidence of past giant tsunamis. Scores of such sites have turned up in Australia, Africa, Europe and the United States.

When the chevrons all point in the same direction to open water, Dallas Abbott, an adjunct research scientist at Lamont-Doherty Earth Observatory in Palisades, New York, uses a different satellite technology to look for oceanic craters. With increasing frequency, she finds them, including an especially large one dating back 4,800 years.

So far, astronomers are skeptical but willing to look at the evidence, said David Morrison, a leading authority on asteroids and comets at the NASA Ames Research Center in California.

Surveys show that as many as 185 large asteroids or comets hit the Earth in the far-distant past, although most of the craters are on land. No one has spent much time looking for craters in the deep ocean, Morrison said, assuming that young ones do not exist and that old ones would be filled with sediment.

Astronomers monitor every small space object with an orbit close to the Earth. "We know what's out there, when they return, how close they come," Morrison said. Given their observations, "there is no reason to think we have had major hits in the last 10,000 years," he continued, adding, "But if Dallas is right and they find 10 such events, we'll have a real contradiction on our hands."

Peter Bobrowski, a researcher of natural hazards at the Geological Survey of Canada, said that "chevrons are fantastic features" but do not prove that megatsunamis are real. There are other interpretations for how chevrons are formed, Bobrowski said, and it is up to the working group to prove its claims.

Ted Bryant, a geomorphologist at the University of Wollongong in Australia, was the first person to recognize the palm prints of megatsunamis. Large tsunamis of nine meters or more are caused by volcanoes, earthquakes and submarine landslides, he said, and their deposits have different features.

Deposits from megatsunamis contain unusual rocks with marine oyster shells, which cannot be explained by wind erosion, storm waves, volcanoes or other natural processes, Bryant said.

"We're not talking about any tsunami you're ever seen," Bryant said. "Aceh was a dimple. No tsunami in the modern world could have made these features. End-of-the-world movies do not capture the size of these waves."

For example, Bryant identified two chevrons found more than six kilometers inland near Carpentaria in north- central Australia. Both point north. When Abbott visited a year ago, he asked her to find the craters.

To locate craters, Abbott uses sea-surface altimetry data. Satellites scan the ocean surface and log the exact height of it. Underwater mountain ranges, trenches and holes in the ground disturb the Earth's gravitational field, causing sea-surface heights to vary by milimeters. Within 24 hours of searching the shallow water north of the two chevrons, Abbott found two craters.

"We think these two craters are 1,200 years old," Abbott said. The chevrons are well-preserved and date to about the same time.

Abbott and her colleagues have located chevrons in the Caribbean, Scotland, Vietnam and North Korea, and several in the North Sea.

But Madagascar provides the smoking gun for geologically recent impacts. In August, Abbott, Bryant and Slava Gusiakov, from the Novosibirsk Tsunami Laboratory in Russia, visited the four huge chevrons to scoop up samples.

Last month, Dee Breger, director of microscopy at Drexel University in Philadelphia, looked at the samples under a scanning electron microscope and found benthic foraminifera, tiny fossils from the ocean floor, sprinkled throughout. Her close-ups revealed splashes of iron, nickel and chrome fused to the fossils.

When a chondritic meteor, the most common kind, vaporizes upon impact in the ocean, those three metals are formed in the same relative proportions as seen in the microfossils, Abbott said.

Breger said the microfossils appear to have melded with the condensing metals as both were lofted up out of the sea and carried long distances.

About 1400 kilometers southeast from the Madagascar chevrons, in deep ocean, is Burckle Crater, which Abbott discovered last year. Burckle Crater has not been dated, but Abbott estimates that it is 4,500 to 5,000 years old.

It would be a great help to the cause if the National Science Foundation sent a ship equipped with modern acoustic equipment to take a closer look at Burckle, said William Ryan, a marine geologist at the Lamont-Doherty observatory. "If it had clear impact features, the nonbelievers would believe," he said.

But they might have more trouble believing one of the scientists, Bruce Masse, an environmental archaeologist at the Los Alamos National Laboratory in New Mexico. He thinks he can say precisely when the comet fell: on the morning of May 10, 2807 B.C.

Masse analyzed 175 flood myths from around the world and tried to relate them to known and accurately dated natural events like solar eclipses and volcanic eruptions. Among other evidence, he said, 14 flood myths specifically mention a full solar eclipse, which could have been the one that occurred in May 2807 B.C.

Half the myths talk of a torrential downpour, Masse said. A third tells of a tsunami. Worldwide, they describe hurricane-force winds and darkness during the storm. All of these could come from a megatsunami.

Of course, extraordinary claims require extraordinary proof, Masse said, "and we're not there yet."

At the peak, people in these favorable locations could see up to 150 shooting stars per hour, or more than two per minute.

"We expect an outburst of more than 100 Leonids per hour," said Bill Cooke, the head of NASA's Meteoroid Environment Office. Cooke notes that the shooting stars during this peak period are likely to be faint, however, created by very small meteoroid grains.

Elsewhere people will see the typically enjoyable Leonid display of a few meteors each hour, weather permitting and assuming dark skies away from city lights.

Ancient debris

The Leonids are bits of debris left behind by repeated passages through the inner solar system of Comet Tempel-Tuttle. Each November, Earth crosses various trails of debris, which have spread out over centuries and millennia. Dense debris trails have caused incredible meteor storms in years, past, notably 1998 through 2002.

Since then the show has been back to normal. But recent computer modeling suggests a brief outburst.

"For parts of Europe, Africa and eastern North America, a far more prolific Leonid show could be in the offing this year," said Joe Rao, Space.com's skywatching columnist.

This year is not expected to be as memorable as some, but well worth a look, astronomers say. The Leonids are known for producing bright fireballs, which could occur at any time.

The Leonids are so-named because they appear to emanate from Leo. The meteors can race across the sky in any direction, but trace each one back and it'll point to Leo.

Other opportunities

Unfortunately for viewers on the U.S. West Coast, the peak occurs before Leo rises. Outside of the expected peak, the best time to watch for Leonids is in the predawn hours, when the constellation Leo is high in the sky.

The Leonids are actually underway already, ramping up gradually to the peak. The event continues for several days after the peak. So any morning during this time could offer up a handful of meteors each hour. Other shooting stars from other sources typically grace the sky at low rates, too.

Flurries of enhanced activity can come at any time. Cooke suggests taking a look in the pre-dawn hours Friday, Saturday and Sunday. Up to 10 shooting stars per hour are possible any of these mornings.

Precise prediction of meteor showers is an infant science, so those in position to observe the possible outburst should plan to head out a half-hour before the predicted peak, allowing eyes time to adjust to the dark, and stay out for up to a half hour after the expected peak.

No special equipment is needed. Telescopes and binoculars are of no use.

A lounge chair or blanket and warm clothes are all you need. Find a dark location with a clear view of the eastern horizon. Lie back, face east, and scan as much of the sky as you can . You never know exactly where a Leonid will appear.

Speaking at the UN Framework Convention on Climate Change, Annan said global warming should be tackled on the same scale as war, poverty and the buildup of weapons.

"While the Kyoto Protocol is a crucial step forward, that step is far too small. And as we consider how to go further still, there remains a frightening lack of leadership," he said.

Delegates to the 12-day conference, which has attracted 165 countries and ends Friday, are discussing ways to reduce greenhouse gas emissions after 2012, when the Kyoto Protocol expires. The U.S. and Australia are the only major industrialized countries to reject the 1997 Kyoto accord, which calls for mandatory cuts in greenhouse gases.

Annan also announced a UN plan to help Africa get funding for clean development projects, such as wind power and renewable energy.

Canada's environment minister, Rona Ambrose, is expected to speak on Wednesday.

Ambrose has been criticized for the Canadian government's new environmental position - a focus on clean air and smog reduction rather than the wider problem of climate change.

But after arriving on Tuesday, she told reporters there is "no bad guy" on climate change. Delegates from around the world are at the meeting to engage in serious talks, and all efforts should be encouraged, she added.

"They're all here to make progress. It's really important to start talking about inclusivity."

In a news conference on Monday, Canadian opposition parties and environmental groups said the Harper government's position does not reflect the concerns of many Canadians.

Canada and Australia were cited by an environmental group, the Climate Action Network, as having contributed the least to progress during the talks, receiving the group's Fossil of the Day award.

The Harper government's proposed clean air act, introduced in October, aims to cut greenhouse gas emissions by between 45 and 65 per cent from 2003 levels by 2050.

Under Kyoto, however, Canada agreed to reduce emissions to six per cent below 1990 levels between 2008 and 2012, and Prime Minister Stephen Harper has indicated Canada will not likely meet those targets.

The conference is the second meeting of the Kyoto-backed countries, and the first UN climate summit in sub-Saharan Africa.

Scientists with the institute selected 5,000 glaciers in the region and used remote sensing and geographic information system methods to monitor their changes in the past 50 years, said Liu Shiyin, an expert with the institute, who participated in the monitoring program.

The 5,000 glaciers account for one-fourth of the total acreage of glaciers in western China.

Liu said only a small number of glaciers were expanding and about 82 percent of the monitored glaciers shrank by 4.5 percent in the past 50 years.

Glaciers in the central and northwestern parts of the Qinghai-Tibet Plateau shrank slowly and those in the neighboring areas were shrinking faster, Liu said.

Liu said, 95 percent of the 170 glaciers on the northwestern slope of the Qilian Mountains have shrunk by 4.9 meters each year on average, and only 10 glaciers expanded during the 1956-2000 period.

Almost all the glaciers on the northern slope of and 69 percent of glaciers on the southern slope the Tianshan Mountains, northwest China's Xinjiang Uygur Autonomous Region, are receding, and the glacier acreage shrank by 10 percent on the Pamirs in the past 50 years.

Glaciers on the northern slopes of the Kunlun Mountains and the Himalayas were also shrinking, Liu said.

Monitoring results show that water flow in some rivers in northwest China's dry regions has been increasing, a result possibly brought about by melting glaciers, Liu said.

However, Liu warned, if glaciers continued to melt fastly, water sources for local rivers would reduce and that would impose serious impact on local production and the life of local people.

A 16-inch wave hit the port of Nemuro on Hokkaido at 9:29 p.m., and live footage from the area showed calm seas. The agency warned that larger waves could follow.

The Pacific Tsunami Warning Center in Hawaii says a tsunami warning also had been issued for Russia. A tsunami advisory has been issued for Hawaii, but there is no threat of a destructive wave.

"When they are really deep they are felt widely but don't cause any damage. This one is over 300 km deep," said John Bellini, a geophysicist at with the US Geological Survey National Earthquake Information Center in Golden, Colorado.

He added there was no risk of a tsunami.

The quake occurred at 11:20 p.m. local time (1950 IST), 300 km south of Ambon, the provincial capital of the Mollucas islands. (AP)

No casualties and damage to property were reported.

Philvolcs said the epicenter of the undersea quake was located 61 kilometers northwest of Mamburao town in Mindoro Occidental, some 170 kilometers southwest of Manila.

Phivolcs also said the quake registered at Intensity 4 in Manila, Intensity 3 in the cities of Makati, Quezon and Pasay and Intensity 2 in Batangas.

Phivolcs Director Renato Solidum said seismic activity affected the Manila Trench with a depth of 95 kilometers below the surface.

He said there was no danger of a tsunami. (PNA)

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