UPDATE: Dinosaur Debate: What Killed the Dinosaurs?

The question of what wiped the dinosaurs out has been around since... well, pretty much since dinosaurs were first identified. I discussed some of the theories previously, here.

Science, however, being the ever-changing field that it is, has now given us a solid consensus on this issue. Just yesterday, an international panel of paleontologists announced that it was, indeed, an asteroid impact that lead to the K-T boundary extinction. 41 top researchers from around the globe reviewed 20 years worth of evidence pointing to the cause of this mass extinction. Their conclusion? Approximately 65.5 million years ago, a meteor 9.32 miles (15 km) across slammed into the Gulf of Mexico just off of Chicxulub, in Mexico. This lead the a general global catastrophe. There were huge forest fires; tremendous earthquakes, which dwarf the recent ones in Haiti and Chile; continental landslides; massive tsunamis; and so much material was shot into the atmosphere that the planet was plunged into a global winter.

So, why an asteroid? What about the volcanic activity at the Deccan traps? The paleontolgists had several lines of evidence for this decision.

Shocked quartz, from Chicxulub

1. Shocked quartz: This is a rare form of quartz, that exists only at nuclear explosions and meteor impact. It is found world-wide through the layer of ash that marks the K-T boundary. This is a huge failing for the volcanic idea. There is simply no way any amount of eruption would create shocked quartz.
2. Iridium layer: In that same layer of ash that the shocked quartz is found in, there is a huge spike in the iridium content. Iridium is a very rare element on the earth's surface, but shows up in asteroids fairly commonly. Some is also found in the earth's mantle, but not enough for volcanism to cause such a large spike.
3. Speed of the extinction: In geologic time, the K-T boundary is an eyeblink. Dinosaurs and their compatriots, which had survived for 160 million years, were just gone. Poof. No more. Extreme volcanic activity would have significant short-term effects on climate, but it would have taken longer. The researchers found little evidence that many Mesozoic species were on the decline before the asteroid struck.

All this leads up to one conclusion: the Cretaceous-Tertiary mass extinction was caused by an asteroid. The evidence is overwhelming, especially upon reevaluation. One question, answered. Only infinitely many more to go.

Source: Science Daily- Asteroid Killed Off the Dinosaurs, Says International Scientific Panel