What is theorized to be the cause of the dinosaur extinction?

The most likely single cause for the extinction of the dinosaurs is an asteroid impact that occurred around 65 million years ago. The asteroid is estimated to have been around 6 miles wide and struck the Earth off the Yucatan Peninsula in what is now Mexico. All life forms within the vicinity were destroyed. Large amounts of debris were thrown into the atmosphere. The dust clouds engulfed the Earth and temperatures changed dramatically. The dinosaurs and many other animals, including marine and flying reptiles, could not cope with the drastic change, and went extinct.

Evidence for this impact is found in an ancient, partially buried crater off the Yucatan Peninsula in Mexico, called the Chicxulub crater, dated to be 65 million years old. There is also a thin layer of iridium within rock layers that also dates back 65 million years. The iridium is significant evidence because the element is rare on Earth but abundant in asteroids. Shocked quartz of the same age has also been found, providing information about the nature of the impact.

However, the asteroid impact is not the sole cause of the dinosaur extinction. Prior to the event, scientists think there was already increased volcanic activity which released poisonous gases into the atmosphere and caused global climate change. By the time their nemesis arrived from space, the dinosaurs, among other animals, were already being stressed by the changing climate, loss of habitat and strained food chain.