Debating the Dinosaur Extinction

Artist's depiction of the Chicxulub impact crater. About 2,225 near-Earth objects (NEOs) have been detected, primarily by ground-based optical searches, in the size range between 10 meters and 30 kilometers, out of a total estimated population of about one million; some information about the physical size and composition of these NEOs is available for only 300 objects. The total number of objects a kilometer in diameter or larger, a size that could cause global catastrophe upon Earth impact, is now estimated to range between 900 and 1,230. Credit: NASA

The dinosaurs dominated the landscape for 160 million years, living over a thousand times longer than modern humans (Homo sapiens first evolved about 150 thousand years ago). During this vast stretch of time some dinosaur species became extinct, but overall the impression is one of an immensely tough class of animals that could endure whatever hardships the planet managed to throw at it. When the end finally came, it came from beyond Earth.

A meteorite impact 65 million years ago is the simple explanation for the extinction of the dinosaurs. The exact details are much more complex, and researchers are still trying to nail down exactly what happened. The Cretaceous-Tertiary (K-T) extinction event is like an ancient tapestry that has become matted and soiled due to time and neglect. There are hundreds of threads of evidence that need to be untangled, smoothed out, and put in their proper place before a clear picture can emerge.

The first, most important thread of evidence is a strip of clay that runs through rocks around the world. Known as the K-T boundary layer, this is the line no dinosaur could cross (although their relatives, the birds, did survive).

In 1980, a team of researchers led by Luis Alvarez and his son, Walter, discovered that the boundary layer contains a relatively high concentration of iridium. Iridium is rare on the Earth's surface but is often found in meteorites. During the molten phase of our planet's formation, most of the iridium of Earth traveled down with iron to form the planetary core. The Earth does receive a light surface dusting of iridium from the occasional meteorites, and some volcanoes can release iridium if their lava comes from a deep enough source. These events give the planet's surface a background iridium level of 0.02 parts per billion (ppb) or less.

Depending on the location of the rocks, the K-T boundary layer has varying amounts of iridium, but all are far above that background level. The section analyzed by Alvarez had 9 ppb. Other sections have upwards of a million times the background level. Luis and Walter Alvarez surmised that a large meteorite rich in iridium must have hit the Earth, and the after-effects of the impact led to the demise of the dinosaurs.

K-T boundary layer Credit: Lunar and Planetary Institute

Later, a large impact crater underneath Mexico's Yucatan peninsula was fingered as the smoking gun. When a meteorite punches the Earth's crust, some rocks and minerals are vaporized, some are flash heated and become molten, while others shatter, or become "shocked" in a distinctive pattern. Samples of the Chicxulub crater had all these features of a meteorite impact. The crater was dated to be about 65 million years old, the same age as the K-T extinction.

The meteorite that made the Chicxulub crater was 10 to 15 kilometers in diameter, or about the size of the island of Manhattan. It screamed to Earth faster than a bullet, smashing open a vast cavern 40 kilometers deep and 100 kilometers across. This crater quickly collapsed under the force of gravity, leaving a hole 180 kilometers wide and only 2 kilometers deep.

Walter Alvarez. Image Credit: Steve Dutch

The energy released by this impact was equal to 100 million megatons of TNT. In comparison, the 1980 eruption of Mount Saint Helens released energy equivalent to just 10 megatons of TNT. The atomic bomb that exploded over Hiroshima released energy equivalent to about 10 kilotons of TNT (or 0.01 megatons).

The impact obviously destroyed life in the immediate area, and the shock wave likely generated huge tsunamis and earthquakes further away from ground zero. Other, longer-lasting effects, such as dust and chemicals from the vaporized rocks, dispersed around the world.

The debate about the K-T extinction was contentious before Alvarez's hypothesis, and the discovery of Chicxulub seems to have done little to stem the often emotional arguments about the extinction event.

At first, some doubted that Chicxulub even was an impact crater. The structure is buried 1 to 2 kilometers under ground - half under land and half under the sea floor - and was only discovered by gravitational and magnetic anomalies from readings taken at the Earth's surface. However, samples from drill cores helped confirm that Chicxulub was formed by a meteorite impact.

Two species greeting each other, separated by epochs.

While most scientists now agree that Chicxulub is an impact crater, not everyone believes it caused the K-T extinction. For instance, some wonder if the Chicxulub impact occurred at the right time. Gerta Keller of Princeton University argues that its true age pre-dates the dinosaur's demise by 300,000 years. However, other scientists contend that Keller's sampling method was flawed, and resulted in an inaccurate date.

Among scientists who agree that Chicxulub was the cause of the extinction, there are disagreements about the tangible effects of the impact. Some scientists think so much dust was sent flying high into the air that the skies darkened for years, halting photosynthesis and killing plants worldwide. Others contend that the dust wouldn't have been so long lasting, since rain would have soon cleared the air. Some have suggested that red-hot impact debris raining back down would have ignited forest fires worldwide, darkening the skies with black soot. Another theory suggests that so much sulfur was sent up into the stratosphere that the rains became like battery acid, poisoning land and sea.

Finally, there are some who believe that while Chicxulub played a role in the extinction, it was not the primary cause. They are seeking answers beyond Chicxulub, wondering if anything else could have contributed to the loss of species. The dinosaurs weren't the only creatures to suffer death and destruction, after all. The K-T mass extinction event killed at least 50 percent of all the world's species. Could a single meteorite impact - even one as large as Chicxulub - have dealt such a fatal blow to life?

Part 2 of this series will discuss whether Earth was hit by more than one meteorite 65 million years ago. Part 3 will look at a controversial crater off the coast of India. Part 4 will cover the debate over whether the K-T extinction was the result of global warming.

Related Web Pages

The Chicxulub crater
BBC's Walking with Dinosaurs
Gerta Keller's Chicxulub Debate
Great Impact: Part I: The Benefits of Hard Bodies
Great Impact: Part II: Much Ado about Nothing?
Great Impact: Part III: The Large and the Small
Great Impact: Part IV: On A Collision Course with Earth
Great Impact: Part V: Encore
Impact Hazards Website
NASA/JPL Near Earth Object Program