Brian Dredge from Feilding asks :-

What do you think happened to wipe out the dinosaurs?

Duncan Steel, a Visiting Researcher at the Australian Centre for Astrobiology at the University of New South Wales, responded.

There is no doubt that there was an impact on Earth by a large (10 to 15 km) asteroid or comet at very close to the time of the well-known dinosaur extinction 65 million years ago. The scar in the Earth's crust - a heavily-eroded crater over 200 km in diameter - has been identified on the Yucatan Peninsula in Mexico. This phenomenal explosion distributed dust and rock around the globe. The initial evidence for the impact, identified before the crater was mapped, came from an anomalously large amount of the element iridium being found in the geological strata laid down at the time of the disappearance of the dinosaurs, the iridium being detected first in rocks in Italy, Denmark, and New Zealand. Iridium is common in meteorites, but not in Earth’s crustal rocks.

Over the past decade or so other terrestrial craters of about the right age (65 million years) have been found and dated, implying that there may have been several impacts. This may have been due to an asteroid or comet breaking apart before various large fragments struck our planet.

The situation is confused, however, by the fact that a huge set of volcanic eruptions also occurred at about that time. These have left a type of rock called flood basalt over an area of thousands of square kilometres of India. These are known as the Deccan Traps.

The volcanism and the impacts may have been simply coincidental, producing a double-whammy that killed the dinosaurs, or else it has been suggested that a really large extraterrestrial object perhaps 50 km across may have struck India and smacked a hole into the crust deep enough such that volcanic flows followed. Geologists speak of that impact being 'self-erasing' in that the eruptions, which continued for millennia, wiped out the local evidence for their origination.

What killed the dinosaurs? Many of them were grilled to death as the sky lit up from the meteors produced by the atmospheric re-entry of rocks thrown out by the impact(s). Those that survived froze to death as dust blanketed the atmosphere for some years. Then there was the highly-acid rain resulting from the vapourised sulphate and carbonate rocks. Next the global temperature was elevated for millennia because of the carbon dioxide from those carbonates. And lots of other nasty knock-on effects: a calamity like a big impact really upsets the terrestrial environment for an extended period of time, causing geological boundaries that are obvious in the rock strata.