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Narrowing what killed the dinosaurs down to one single cause is impossible. There were at least a dozen extinction events over the course of history, but the one that most scientists point is as the most significant for its extent is the K-T event.
When asked to point to one thing that would account for what killed the dinosaurs, most would cite the asteroid extinction theory a.k.a the K-T (Cretaceous-Tertiary) theory. It is associated with a geological signature known as the K-T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period and T is the abbreviation for the Tertiary Period (a historical term for the period of time covered by the Paleogene and Neogene periods). Non-avian dinosaur fossils are only found below the K–T boundary, indicating that dinosaurs became extinct immediately before, or during the event. A very small number of dinosaur fossils have been found above the K–T boundary, but they have been explained as reworked, that is, fossils that have been eroded from their original locations then preserved in later sediment layers. The asteroid-impact theory was first proposed in detail in 1978, by a team led by American geologist Walter Alvarez and physicist Luis Alvarez. The Alvarez team analyzed sediment collected in the 1970s from the K-T layer near the town of Gubbio, Italy. The samples showed a high concentration of the element iridium, a substance rare on Earth but relatively abundant in asteroids. Other samples of K-T boundary strata from around the world were also analyzed; excess iridium was found in these samples as well. Using the average thickness of the sediment as a guide, they calculated that a meteorite about 10 km in diameter would be required to spread that much iridium over the whole Earth.
After extensive research into what killed the dinosaurs, scientists believe that the asteroid that impacted the Earth and started their demise created the Chicxulub crater in the Yucatan Peninsula, Mexico. It is approximately 180 km in diameter and 10 km deep. The crater was formed about 65 million years ago by an biolide (an asteroid or a comet) that was roughly 10 km in diameter and it hit with 100 million megatons of force. More than 50% of the worlds different species were killed off because of the climate changes caused by the dust that was thrown into the air. Nearly all of the dinosaurs eventually died from the changes that this impact event had on the Earth’s environment. Acid rain and fires would have finished those that did not die from the initial impact. The dinosaurs would have died in several waves. The first one would have been the initial impact and the resulting tsunami. The next wave would have been acid rain and fires that would have destroyed the plant life. The third wave would have been the herbivores, followed by the meat eaters, then finally the the carrion eaters would have died of starvation. Eventually 90% of the life within thousands of miles would have been dead. It is easy to see how this even could be what killed the dinosaurs.
We’ve also recorded an entire episode of Astronomy Cast all about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.