Dinosaur Debate: What Killed the Dinosaurs?

Like the warm versus cold blooded debate, this argument has been around since the first dinosaurs were identified. It's pretty easy to tell when the dinosaurs went extinct: dinosaurs are found in rocks older that 65.5 million years, and are not found in rocks younger than 65.5 million years (with the exception, of course, of birds. But, for the purpose of this post, assume that dinosaurs are non-avian dinosaurs). But what killed them is a much more challenging question. There are several popular hypotheses, with varying scientific support, as to the cause of this mass extinction.

Volcanism

As I've discussed previously (here and here), volcanoes can be very destructive forces. A single volcano, however, is not enough by itself to cause 60% of all life on Earth to die out. Not even the Yellowstone caldera will do that much damage. On the other hand, a long stretch of volcanic activity - a few million years - could be long enough to put enough soot and greenhouse gases into the air to cause pretty severe climate change. This climate change may have put a lot of pressure on the dinosaurs, as well as other organisms living during that time, which could have lead to their extinction.

• Support: Near the end of the Cretaceous, there was a huge volcanic system in India, known as the Deccan traps. These volcanoes were active for several million years, which would have been long enough to emit an enormous amount of soot, gas, and iridium-rich lava.
• Contradictions: Volcanism is linked tightly to plate tectonics. While the tectonic shift in the late Cretaceous was significant, it was not enough to suggest a mass extinction. In fact, tectonic movement alone has never been the cause of a mass extinction. Also, the iridium layer found in rocks 65.5 million years old is pretty consistent, and is not characteristic of volcanic activity.

Meteor or Asteroid Impact

This is by far the most widely accepted hypothesis. Many scientists believe that, around 65.5 million years ago, a meteor about 10 km (6.2 miles) in diameter smashed into the Earth. It would have vaporized anything within at least 100 miles of it, created a truly massive tsunami, and spewed debris into orbit. This traveled across the globe and fell back to Earth. It would have changed climate and set of a chain reaction the dinosaur couldn't handle.

• Support: The reason why this idea is so widely accepted is because there is a lot of support for it. First off, there is the world-wide iridium layer at 65.5 million years ago. It exactly boundaries where dinosaurs stop showing up in the fossil record. Iridium is very rare on the Earth's surface, but can be found in the mantle, or in many meteors. This particular iridium layer is relatively uniform, suggesting it came from an extraterrestrial impact. Second, there is shocked quartz and small glass "beads" at this layer as well. These are formed from an impact, as stone and quartz are melted, thrown up into the atmosphere, then fall to the ground again. Third, we have an impact crater. Usually, the Chicxulub Crater, off of the Yucatan Pennisula in Mexico, is cited as the one that killed the dinosaurs. It is the right age and size for the meteor. Recently, however, a new, larger crater from about the same time was found off the coast of India. It may have been either of these, or both, that led to the dinosaurs' demise.
• Contradictions: There aren't many problems with the meteor theory, but one that commonly comes up is that it seems unlikely that a single meteor impact killed all of the dinosaurs. What about ones that burrowed or were just really small or could scavenge off the dead? Why didn't they survive? Climate change helps explain some of this, but isn't fully satisfactory either.
  

These are the two most accepted hypotheses of what caused dinosaurs to go extinct. There are a few others, such as simply gradual climate change or mammals eating dinosaur eggs, which are not very well supported. Still, no hypothesis is perfect by itself. The geologic record tells us that there were several large meteor impacts, a huge volcanic system, major climatic change, and significant reconfiguring of the continents during the late Cretaceous. The stress caused by just one of these could hurt life on the planet; all of them together could easily cause a mass extinction. Therefore, while the general consensus is that a meteor was the tipping point, many factors led the the Cretaceous-Tertiary mass extinction and the death of the dinosaurs.

For more information about the K-T boundary extinction, visit this website.