Why did dinosaurs go extinct?

Reader's Digest Editors

Could it have been an asteroid impact, erupting volcanoes, or the earth’s changing climate that led to sudden mass extinctions? Here's what science says. 

 For nearly 140 million years, dinosaurs ruled the earth. Many were big, lumbering creatures, though some were small and agile.

Dinosaurs managed to fill almost every ecological niche that the earth provided. Still, 65 million years ago all of the dinosaurs began to die out; over the course of the next few hundred thousand years, the fossil record shows, three-quarters of all the species alive on the earth would become extinct.

What could possibly have caused the extinction of thousands of species of flora and fauna in so little time? Scientists hate gaps—and this was a rather big one.

A number of competing theories have been put forth, some reasonable and others that could be described as outrageous. Among the former are climate changes, volcanic ash, tectonic movement, floods, ice ages; the latter include alien invasion and abduction, and the disappearance of an ancient and nourishing species of ferns.

Any explanation would have to account for the sudden cataclysmic demise of 75 per cent of the earth’s creatures.

 

Supernova or asteroid?

did an asteroid take out the dinosaurs

One interesting (and not altogether implausible) theory, proposed by scientist Dale Russell in 1979, suggests that the explosion of a star— a supernova—might have caused the extinction of 65 million years ago. The massive amounts of radiated energy from these explosions can be equal to the energy generated by 10 million of our suns—as much as every star in the galaxy combined.

A supernova even 50 million light years away would have had drastic effects on the earth. Within hours, the earth would have been bathed by the initial burst of electromagnetic radiation. Between three and 30 years later, a second wave of cosmic rays and extreme levels of radiation would have reached the earth and lasted for about 10 years.

However, the best calculations estimate that such an explosion occurs within 50 million light years of the earth, on average, every 70 million years. Since the dinosaurs managed to survive and thrive on earth for a period of approximately 140 million years, it does not seem likely that radiation from a nearby supernova killed the dinosaurs.

Then what did? In 1980, two scientists at the University of California at Berkeley proposed a theory that has been widely accepted since. Walter Alvarez and his father, Luis, had been studying a layer of clay in 65-million-year-old rock strata that exhibited a curious phenomenon. Concentrations of the extremely rare metal iridium were found to be, in some cases, 30 times the normal levels.

dinosaur

This pattern was found repeatedly in similar studies of different samples found around the world. Where could such high levels of this scarce iridium have come from?

The answer, the two Alvarezes proposed, was from outer space. The main source of the earth’s iridium is from the meteorites and micrometeorites that constantly pelt the earth.

Still, such high concentrations of iridium would have required the simultaneous arrival of about 500 billion tons of material from space. The Alvarezes believed that this could be explained only by the impact of a huge asteroid. An asteroid with this mass would have been 10miles in diameter and would have crashed into the earth with a speed of approximately 100,000 miles per hour. An impact like this would be so cataclysmic as to wipe out much of the life on the planet.

Massive shock waves would have convulsed the land, and vast, dense clouds of ash and dust would have blocked out sunlight, throwing all into darkness. This decades-long period of darkness has been dubbed Meteorite Winter, and probably caused the extinction of most plant life on which most dinosaurs fed. If the asteroid had hit the earth in one of its oceans, tidal waves caused by the impact might have reached an incredible five miles in height.

All in all, the combination of effects would have proved devastating and could have resulted in the extinction of the dinosaurs and most every other living thing. Though the asteroid theory is widely accepted today, it is not without its problems; some scientists are still working on an explanation that agrees more closely with the evidence they have gathered.

Several of these theories are based on the notion that an extinction cycle exists that repeats itself approximately every 60 to 70 million years.

 

The climate question

climate question

The cause of the proposed cycle is in dispute; some researchers attribute it to volcanic action, others to a drop in sea level. Scientists at NASA and the United States Geological Survey have been using the United States’ GOES satellite system to compare weather and water levels over periods of millions of years.

The data shows a significant worldwide drop in ocean levels approximately 65 million years ago. Such a drop in sea levels resulted in a dramatic climate change worldwide, and this drastic change forced living creatures to either adapt or die. The higher temperature and increased levels of carbon dioxide would have made the environment difficult for the dinosaurs to dwell in.

In time they became extinct, allowing mammals—better adapted to warmer climates—to arise and thrive in the new environment.

The cause of mass extinctions of the past is therefore far from settled.

 

The Alvarez theory… fact or fiction?

luis and walter
Walter and Luis Alvarez. Image via Wiki

One of the criticisms of the asteroid theory of extinction has been that such an impact would have been too cataclysmic to recover from.

In other words, the death toll of species would have been too great. Little, if anything, could have survived the impact of a 10-mile asteroid, and the few surviving species would not have had the time to reproduce to allow for the earth’s rapid repopulation.

The Alvarezes’ theory also fails to properly explain the length of time that the extinction is believed to have taken. Their theory holds that the ensuing so-called Meteorite Winter would have laid waste to the earth for mere decades or even for centuries. However, fossil records indicate the mass extinction of the dinosaurs occurred over a much greater span—approximately 140,000 years.

Walter and Luis Alvarez based their theory on the premise that the only source of the high levels of iridium they found in the earth’s crust was unearthly in origin.

Since the majority of the earth’s crust contains little iridium, this seems a reasonable assumption. However, there is another place the curious layer of iridium may have originated—the earth.

Though iridium is scarce on the earth’s surface, the earth’s mantle is rich in iridium.

More recent studies have shown that the iridium content of emissions from a highly active Hawaiian volcano, Kilauea, more closely matches the patterns found in the planet’s iridium-rich clay layer than that in any meteorite known today.