A comet striking Earth with the power of 300 million Hiroshima-size atomic bombs may have been the cause of mass extinction 11 million years ago, say scientists who blame a bigger blast for killing the dinosaurs eons earlier.

The scientists' new study means that of nine mass extinction episodes in Earth's history, three that occurred about 11 million, 38 million and 66 million years ago now have been linked to comets or asteroids smacking into the planet.The highly controversial theory says such impacts kicked up dust and triggered smoky fires that blocked enough sunlight to freeze many creatures and deprive others of food supplies.

Other new studies bolster support for rival theories that blame mass extinctions on gigantic volcanic eruptions, on changes in sea level or on global climate changes unrelated to objects smashing into Earth.

The studies are being presented at a four-day conference, "Global Catastrophes in Earth History," which opened Thursday at Snowbird.

Scientists from Lawrence Berkeley Laboratory and the University of California's Berkeley and Santa Barbara campuses performed the study that suggests a comet or asteroid impact caused mass extinction of about 25 percent of species 10 million to 11.7 million years ago. The team was led by Nobel Prize-winning UC-Berkeley physicist Luis Alvarez, who died last month.

In 1980, the Alvarez team found thin layers of the metallic element iridium deposited around the world in 66-million-year-old rocks, suggesting a comet or asteroid striking Earth killed about two-thirds of all species, including dinosaurs, at the time.

Iridium is viewed as evidence of such impacts because it is far more common in extraterrestrial objects than in Earth rocks. Iridium in a comet that hit Earth and exploded would have settled from airborne dust.

In the new study, two deposits of iridium were found in 10 million- to 12 million-year-old sea-floor rocks some 6,200 miles apart: in the Weddell Sea near Antarctica and the Tasman Sea north of New Zealand, said Frank Asaro, a nuclear chemist at Lawrence Berkeley Laboratory.

That suggests, but doesn't prove, that a comet or asteroids also hit Earth and triggered extinctions at that time, he said.

The researchers estimated the comet or asteroid would have measured about 1.8 miles across and would have vaporized in an explosion with 300 million times the power of the 13-kiloton atomic bomb that destroyed Hiroshima at the end of World War II, Asaro said.

He said they estimate the 61/2-mile-diameter comet that killed the dinosaurs produced a blast 10 billion times more powerful than the Hiroshima bomb.

In 1982, Alvarez's team said iridium found in 37 million- to 39 million-year-old rocks may have triggered a mass extinction of about 25 percent of species on Earth at that time.

Asaro said evidence that comet or asteroid impacts caused three mass extinctions supports a more controversial hypothesis that extinctions occur at roughly 26 million- to 30 million-year intervals when comets are hurled toward Earth by the gravity of an undiscovered companion star to the sun, nicknamed the "Death Star" or "Nemesis."

Other studies support the theory that huge volcanic eruptions in India 66 million years ago blocked out sunlight and caused mass extinction.

A study by Florida State University scientists suggests "shocked" grains of quartz could have been produced by extremely explosive eruptions. Others say the grains were created by a comet smashing quartz rock. Another team of Florida State and British scientists said volcanic ash found in Weddell Sea rocks suggests volcanoes caused the extinctions.

Some fossil experts object to both comet and volcano theories, arguing that mass extinctions occurred in a "stepwise" fashion, with species disappearing at various times over a few million years as the climate or sea level changed.

"Extinction patterns among vertebrates do not appear to be attributable to any single cause, catastrophic or otherwise," said a study prepared for the Snowbird meeting by J. David Archibald and Laurie Bryant of San Diego State University.

Other studies support a theory that stepwise extinctions occurred because one or more comet impacts killed some species immediately, then caused acid rain and global warming that killed others.