[img]2012q1/ct_etoefm_etoefspeakc_0956_20121[/img] [originaltext]W: Now, class,

W: Now, class, I’m sure you’ve read and heard a lot about the extinction of the
  dinosaurs, which occurred about 65 million years ago. People are fascinated
  by this topic, probably because we have yet to determine its cause. Why did
  these giant reptiles, who had ruled the planet for millions of years, suddenly
  disappear? Well, the theories are endless, but I’d like to talk about two that
  are the most plausible. The first is known as the Impact Theory, and I’d say it’s
  the most widely supported. According to this concept, there was a very large
  asteroid-like object that collided with Earth. The results were catastrophic:
  enormous explosions, earthquakes, tsunamis, raging forest fires...But
  most importantly, an impact of that magnitude would have thrown up large
  amounts of dust into the sky, so much that it would’ve prevented sunlight
  from reaching the surface. According to the theory, plants would have died
  first from the lack of light, and then this decline worked its way up the food
  chain, until the extinction of the dinosaurs was complete. As I said, it’s a very
  well-supported theory, but there’s another one, and it’s called the Volcano-
  Greenhouse Theory. This idea holds that there was a significant increase
  in volcanic eruptions 65 million years ago. These eruptions released large
  amounts of carbon dioxide into the atmosphere, creating a greenhouse effect
  and causing temperatures on Earth to rise. Now, this wasn’t enough to kill
  the dinosaurs immediately. What happened, according to the theory, was that
  the heated environment made it impossible for dinosaur embryos to survive.
  Female dinosaurs were unable to produce offspring, and, well, you can see
  how that would lead to the extinction of species.