How Did the Dinosaurs Die?

September 17, 2010 | In: Random interesting fact

What killed the dinosaurs? It’s like a classic Sherlock Holmes mystery. Bodies have been found. There are all kinds of clues. Something killed a great many victims. But what?

The murders occurred 65 million years ago. The victims included the dinosaurs. In fact, not only were the dinosaurs killed off, but so were most other animals and plants. Fortunately, science often produces a Sherlock Holmes to solve its greatest mysteries, and one of them seems to have solved the mystery.

This modern detective was Nobel prize-winning physicist Luis Alvarez. His theory seems to explain many of the clues the killer left behind, buried with the dinosaurs in the rocks beneath our feet. Alvarez’ theory is highly controversial, but it seems to be the best theory around.

Luis Alvarez’s investigation of the case of the dead dinosaurs began several years ago, when his son, Walter Alvarez, a geologist, pointed out a puzzle. There was an unexpected rare metal, iridium, found in some rock samples.

Iridium, a whitish-yellow metal, is the Latin word for “rainbow,” and was named because of the colors it gives to liquids. Iridium is quite rare on Earth, yet was strongly present in the geologist’s samples. Why?

The samples had been taken from the thin slice of Earth that occurs right above the layer where the dinosaurs died. Over Earth’s billions of years, soil and rocks have rolled down mountains, been blown by wind and been carried by streams, depositing layer upon layer in valleys.

They found that, all over the world, the story seems to be the same: Below the iridium layer, dinosaurs and many other kinds of life flourished. Just above it, most of them are gone. Something weird must have happened when the iridium layer formed. Something that killed off most life on Earth.

But what could have done it?

Many theories have been proposed over the years. Some scientists think that Earth’s climate turned cold, killing off most life. This was the most popular theory prior to Alvarez. But why did the climate change? Some think there was a tremendous eruption of volcanoes that polluted the air and water.

Luis Alvarez proposed a radically different theory. When his son discussed the iridium layer with him, Alvarez pointed out that there is one place where iridium is fairly common. It’s not on Earth — it’s in space.

How do we know iridium is more common out there? Because meteorites have lots of it. Earth rocks usually have little.

Most geologists are used to dealing with normal rocks, not space rocks. But Alvarez was used to thinking about space, so like Sherlock Holmes seeing the one clue that all the other detectives had overlooked, he deduced a connection between the dinosaurs and outer space.

Not only that, he was used to the idea of big explosions. He worked on the atomic bomb during World War II. If a meteorite killed off dinosaurs all over the world, it had to have been a big one. He knew that space is littered with big ones. Many of them are far bigger than the one that created Meteor Crater in Arizona, 1.2 kilometers (almost a mile) in diameter.

He, his son and other scientists began thinking about the details. How big would the meteorite have to be? What are the odds against such a big rock hitting the Earth in 65 million years? What would be the effect of the explosion?

And suppose it hit the ocean instead of land? What would it do to the atmosphere? How would it kill — by heat, poisoning or some other way?

Above all, they had to compare the theory with the facts. Were there any known facts that contradicted the theory? If so, might the “facts” be wrong?

The scientists sat down and computed the effects of the biggest explosion the world had ever seen.

The blast must have been big enough to throw dirt thousands of miles. They figured that any meteorite much smaller than about ten kilometers (6 miles) in size would probably not be big enough to do the job. They estimated that there are enough ten-kilometer asteroids — about the height of Mount Everest — so that one of them ought to hit us every hundred million years or so. Asteroids much bigger than that are far rarer, so are not likely to have done the dirty deed.

What would happen if Mount Everest came crashing to Earth as fast as a typical meteorite (around 40 kilometers per second or 90,000 mph)?

It would be like millions of hydrogen bombs going off at once. Whether it hit land or water, it would shoot millions of tons of debris high into a high layer of the atmosphere, the stratosphere, where lighter particles float for years. Sunlight would be cut off as on a badly overcast day. The world would turn cold.

Since plants use sunlight to make food that animals eat, the plants would die and animals would starve to death. So that is how the dinosaurs probably died: Starvation. The mighty brontosaurus, the fierce tyrannosaurus, the armor-plated stegosaurus — they all died in a cold, dark world, their bellies crying for food.

Why didn’t all life die out? Some seeds remained in the ground, waiting for the Sun to return. Some animals survived by eating the carcasses of other creatures.

In 1980, the Alvarez group published their ideas in the journal “Science” and triggered a storm of controversy. Many geologists, who normally ignore astronomy, thought it was ridiculous. Astronomers, on the other hand, welcomed the theory with open arms. They’re always happy to find that their research applies to Earth.

The debate has continued. More and more evidence has been found supporting the Alvarez theory, and now even many geologists accept it. Astronomers have come to prefer the idea that comets killed the dinosaurs, rather than meteorites. They would have roughly the same effect, but Earth has probably been hit by more comets of this size than meteorites, so it’s still basically the Alvarez theory.

Where’s the crater? There’s evidence that it’s in the Yucatan province of Mexico, largely covered up by dirt and jungle. This is often called the “smoking gun” of the Alvarez theory — the evidence that could convict the astronomical dinosaur killer.

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