The extinction of the dinosaurs some 65 million years ago, one of the major mysteries of geology, was probably caused by a collison between the Earth and a giant asteroid, according to a team of scientists led by Nobel Laureate Luis Alverez.

The asteroid hit the earth with a force equal to a hydrogen bomb explosion of 100 million megatons, vaporizing so much rock that the dust blockedout the sun's light for as long as five years. Alvarez suggested. The loss of sunlight could have killed off all plant life in the sea and on land that grow by photosynthesis, he said.

"Not only the dinosaurs were made extinct 65 million years ago, all kinds of animal life disappeared at that time." Alvarez told an audience of scientists late Friday at the 146th annual meeting of the American Association for the Advancement of Science. "The result of the main food chain being turned off on earth may have been mass starvation of animals depending on plants for nutrition."

To back up his theory, Alvarez, a physicist, said that he and his geologist son, Walter, collected in four locations on Earth clay samples formed 65 million years ago. They found the clays laced with a rare mix of heavy metals -- like iridium -- found only in meteorities. Alvarez said that exhaustive analysis of the four clays show they could not have come from any event on earth, not even an erupting volcano. t

"As far as we can tell, we've found the first physicial evidence that something strange happened 65 million years ago," said Alvarez, a world-renowned physicist at the University of California who won the Nobel Prize in 1968. "These samples look to us as if the came from something extraterrestrial, not some mundane form of Earth happening."

The four clays analyzed by Alvarez came from ancient rock formations in Italy, Denmark, Spain and a deep-sea core that formed 65 million years ago during the 5,000-year boundary between the Cretaceous and Tertiary periods of geologic time. The clay lay trapped between limestone layers that marked 8,000 years of the Cretaceous and 15,000 years of the Tertiary period. m

The clay taken from the Grubbio Valley north of Rome contained 60 times as much iridium as is found in any part of the Earth's crust. The Danish and Spanish samples held 160 times more iridium than normal.

The dinosaurs thrived all the way through the Cretaceous period, then suddenly disappeared at the start of the Tertiary. The extinction of animal life between the Cretaceous and Tertiary is one of five mass extinctions documented from the fossil record dating back 570 million years to the beginning of the Cambrian period.

Countless theories have been advanced for the disappearance of the dinosaurs, including the onset of an ice age that killed off their food supply, and the appearance on Earth of a poisonous plant that dinosaurs ate with such gusto that it killed them.

The theory that keeps coming back is that there was "supernova," or exploding star, in the nearby heavens that showered the Earth with cosmic rays for hundreds of years, destroying all species of animal exposed to the radiation.

In proposing that an asteroid collision caused the extinction of the dinosaurs, Alvarez set out to prove that the supernova theory was wrong. Alvarez matched the quantities of metals like iridium and osmium found in the 65-million-year-old clays with what one would expect from an exploding star, and calculated that the star would have been 40 times closer to Earth than the nearest star (alpha centauri) is today.

"That's much too close," Alvarez said. "An exploding star that close would have killed everything on Earth, not just dinosaurs."

Before embracing the asteroid collision theory, Alvarez said, he tried out dozens of plausible theories that might also explain mass extinctions.He said his favorite explanation was that Jupiter suffered a partial explosion that bathed the earth in hydrogen, killing the animals by depriving them of oxygen.

"We gave that one up," Alvarez said, "and concluded that the damage had been done by a large asteroid."

Alvarez said the quantities of iridium found in the four clays show that the asteroid would have to be at least seven and as many as 10 kilometers (6.2 miles) across, the size of a city like Pittsburgh.

He said that unfortunately there are no craters left on Earth whose size would match the age of the collision that took place 65 million years ago. s

"There is a chance that the asteroid hit the sea," said Walter Alvarez, "and that whatever crater it made in the ocean floor has already been destroyed by the movements of the plates on the floor of the sea."

Even if it hit the sea the asteroid would have burned off much of its own material into the atmosphere by the time it fell. The force of the collision would have been so great. Alvarez said, that debris 100 times the mass of the asteroid would haveblown into the atmosphere as dust even if the collision took place at sea.

Would that have been enough to shut out the sun's light for five years." Alvarez reminded his audience that the eruption of Krakatoa between Java and Sumatra in 1883 covered the entire Earth in two days with a dust cloud that darkened the skies for more than a week.

"What we're talking about here," Alvarez said, "is an event that put 160 times more debris into the atmosphere than Krakatoa did, an event that was literally capable of turning out the sun's light for at least two and as many as five years."