The Dinosaurs' Demise

Talk about a terrible day.

It starts out like any warm, spring day 66 million years ago. Dinosaurs go about their dino business: a T. rex sniffs the air in search of its next meal, herds of beak-billed Maiasaura munch on tall ferns, a Dakotaraptor sinks its sickle-shaped claw into its unlucky prey. The air buzzes with insects, sharks prowl the oceans, and little mammals scurry through the dense plants.

Suddenly, a bright red fireball streaks through the sky and … BOOM! An asteroid the size of a mountain slams into Earth along the coast of what is now Mexico’s Yucatán Peninsula. The asteroid destroys the rocky surface where it hits, shooting ash, soot, dust and gasses high into the sky. It leaves a scar in the Earth: a crater 18 miles deep and more than 100 miles across. Intense heat from the crash burns everything nearby and even sets grass, trees and shrubs on fire.

But that’s just the beginning of the destruction. The asteroid impact affects the Earth like a big rock thrown into a still pond, creating circular ripples that flow outward. Only in this case, the “ripples” are violent blasts that rocket through the air, land and seas.

Massive waves (tsunamis) as tall as skyscrapers wash over parts of the American continents. A blast of wind as fast as a passenger jet rips through the region. Earthquakes more powerful than anything humans have ever experienced shake the land. As earthquake expert Rick Aster explained in National Geographic magazine, “[An] event of this size would be the equivalent of all the world’s earthquakes for the past 160 years going off simultaneously.”

Any animal close enough to see the asteroid impact dies instantly.

A horrible, new world

The bad news doesn’t end there. Like a row of dominoes tumbling one after another, the asteroid impact starts a chain of events that ends up wiping out more than three-fourths of Earth’s animals.

The ash, soot, dust and gas thrust into the air by the asteroid’s impact fill the sky. Rain provides no relief. Instead, the drops mix with harmful particles and the gasses in the air and come down as muddy “acid rain,” killing many marine animals swimming in the shallow waters of oceans, lakes and rivers.

For the first hours after the asteroid impact, the sky is almost totally dark. But even after it lightens up, it looks like twilight or a very cloudy day — and it stays that way for months, maybe even years, according to planet scientist Gareth Collins.

The darkened sky changes Earth’s climate and environment in a big way. Without the warmth of the sun, temperatures plunge for several years, causing what scientists call an “impact winter.” Even in the tropics — the warmest regions of Earth — the average temperature drops from 81 degrees Fahrenheit to a chilly 41 degrees.

The cold temperatures and reduced sunlight make it difficult for plants to grow. Plant-eating animals can’t find enough food to survive. Many die within weeks, leaving meat-eaters with little to eat. They suffer the same fate. The food chain collapses, taking down those non-avian dinosaurs and many other animals.

For the animal species that survive the asteroid impact, fires, tsunamis, acid rain, years of darkness, and impact winter, a new problem arises. After the impact winter, Earth begins warming rapidly and stays hot for the next 100,000 years.

After ruling Earth for 174 million years — much, much longer than the 6 million years humans have been around — the only dinosaurs to survive the mass extinction 66 million years ago are the avian dinosaurs, which evolved into today’s birds. All other dinosaurs go extinct. They are not the only casualties of the mass extinction. The fearsome serpent-like sea reptiles (plesiosaurs, mosasaurs and ichthyosaurs) also die out, as do the pterosaurs, the mighty winged reptiles that ruled the skies during the dinosaur era.

Some animals do survive the mass extinction. Besides the birds (avian dinosaurs), other survivors include some mammals, amphibians, fish, and reptiles, such as turtles, lizards, snakes, and crocodilians. But the overall loss of animal diversity is extreme, with more than three-quarters of animals wiped out.

Detective work

How did we learn all of this? Well, humans did not live during the time of dinosaurs — no matter what movies and TV shows might have you think — so we didn’t witness their demise (probably a good thing, too). And so, for nearly two centuries, it was a mystery why the dinosaurs went extinct.

Paleontologists — the scientists who study life long ago — brainstormed lots of possible explanations. Did dinosaurs die of a terrible disease? Did they run out of things to eat? Did little mammals eat too many dinosaur eggs? Did dino bodies grow too large to be controlled by their little brains? But the scientists could not find any evidence to support these ideas. In fact, the evidence showed they were wrong.

Then, in the 1970s, geologist Walter Alvarez discovered a strange layer of clay in the Earth. It was much different from the layers above and below it, and it dated back to when the dinosaurs became extinct. He and his father, the noted physicist Luis Alvarez, discovered that the clay contained unusual things: some rare minerals that only form only under extreme heat and pressure, and an element called iridium that is common in space — but rare on Earth. In 1980, the Alvarezes and their scientific partners Frank Asaro and Helen Michel came up with the asteroid-impact explanation (or, in scientific terms, “theory”) for the dinosaurs’ demise.

The theory was so different from earlier ideas that, at first, a lot of other scientists thought it was silly. But over the years, more and more evidence has supported the asteroid-impact theory. In the early 1990s, scientists discovered the Chicxulub crater off Mexico’s coast, the site where the asteroid slammed into Earth. And in the past decade, paleontologists working in North Dakota found fossils from that awful time: a fish that sucked in debris from the asteroid blast, a turtle speared by a stick during the floods that followed the impact, and possibly even a piece of the asteroid itself preserved in a bit of amber (fossilized tree resin).

Today, most scientists accept the asteroid-impact theory as the cause of the mass extinction 66 million years ago. But not all scientists. Some think climate change doomed the dinos. Huge volcanic eruptions right before the dinosaurs’ extinction could have filled the air with gasses and changed the climate and habitats in such a way that the dinosaurs and many other animals could not survive. Some scientists blame both the volcanoes and the asteroid.

Paleontologists are still searching for the answers to many questions: Was the asteroid impact the only cause of the dinosaurs’ demise or just one cause? Were the dinosaurs already dying out before the asteroid impact? If the asteroid hadn’t hit Earth, would the non-avian dinosaurs have survived?

Maybe there’s many more answers to come.