We all know the science fiction movie Jurassic Park where dinosaurs thrive in human time. But did anyone wonder how all of a sudden, in the Cretaceous period, the big creature disappeared? Scientists have always regarded the “impact theory” as the best possible explanation of the disappearance — a hypothesis subject to revision based on further evidence gathered from around the globe.

The impact theory rests upon the fact that a celestial impact near the Yucatan peninsula (Chicxulub Crater on Mexico's Gulf Coast) on one awful day, some 65.5 million years ago wiped out many species, including the dinosaurs on the earth’s surface. The review published online March 4 in Science re-evaluated the whole picture, according to Kirk Johnson of the Research and Collections Division at the Denver Museum of Nature and Science and co-author of the paper.

There has been other competing hypothesis as well. This inc­ludes volcanism and smaller, multiple asteroid strikes — ra­ther than a single asteroid strike. The volcanism theory supports the view that volcanism that produced the great Deccan Trap formation in western India at the end of the Cretaceous period might have produced enough sulfur and carbon dioxide to initiate a massive shift in climate.

But scientists argue that the exact pinpoint of time when the heavy volcanism occurred is sketchy and it most likely kicked off some 400,000 years before the extinction event. Johnson noted in a March 3 conference call: “The emissions from these volcanoes are likely to have warmed the planet slightly, actually making life easier for many animals and encouraging diversification and dispersion over wider geographical areas.”

Other scientists have pointed to multiple layers of impact residue as evidence that there was more than one asteroid responsible for the extinction. Johnson says there was “no evidence for multiple impacts,” and sites that had turned up these various layers were very close to Chicxulub itself. Thus, the layers could be a result of the single asteroid strike in Chicxulub.

Another assertion is that the impact occurred hundreds of thousands of years before the extinctions. But this assertion also failed to hold with the new research. Evidence of Cretac­eous period shells on top of the impact crater are not a sign that the animals persisted after the impact, but rather that they got “washed into the hole,” Johnson noted.

Researchers assessed about 350 sites all over the globe that had evidence of the impact and all of these cases could be traced back to the Chicxulub location. In some areas near the crater on the Mexican coast, the impact layer was 80 metres thick, pointing to one single devastating day for life on the planet.

Based on the size of rock material and the crater diametre, researchers have estimated the dinosaur extinction asteroid was about 10 kilometres in diameter. And when it struck — at about 20 kilometres per second — it created an instant crater about 100 kilometres wide and 25 to 30 kilometres deep “almost piercing the crust of the Earth,” Johnson noted.

The final crater that formed after the initial impact was about 180 kilometres across and two kilometres deep, which is still close to the depth of the Grand Canyon, Johnson pointed out. The asteroid strike would have spurred massive earthquakes of about magnitude 11, tsunamis and landslides. Perhaps most devastating, however, the crash would have caused acid rain and darkness, as particulate matter blocked sunlight, prohibiting photosynthesis in both land and water ecosystems, effectively shutting down large swaths of the food chain.

“This is not geologic time — this is instant time,” Johnson said, acknowledging that it is a very tricky task to pin down a single event from 65.5 million years ago. But, judging from the chemical, geochemical and geo-chronological evidence, he said, “The Chicxulub Crater really is the culprit.”

The write is doctoral candidate, Carnegie Mellon University, USA; and Knowledge Editor, Financial Chronicle