K-T boundary

This is probably the most important geological discovery of all time. The K-T boundary is the extinction event 65-66 million years ago that annihilated 75 -90% of all life, both animal and plant species, on the planet Earth. It's most famous for the fact that the dinosaurs were wiped out.  Consequently, this event is responsible for the ascendency of placental mammals on Earth, which subsequently led to the appearance of humans.

This event is the last great extinction event. There have been at least 5 great extinction events over the last 540 million years. The frequency of these events is not regular, which indicates that they're caused by some random catastrophe such as a comet or asteroid collision. All of this is based on fossils and geological finds.

The reason it's called a boundary is because there is a very distinct geological thin white and black rock layer all over the world that marks the disappearance of dinosaur bones. The dinosaur bones are found in vast amounts below the boundary line and are completely gone above it.

It's called the K-T boundary because it marks the separation between the Cretaceous and Tertiary periods (K is the traditional abbreviation for Cretaceous). It's also known as the K-Pg extinction event for the separation between the Cretaceous and Paleogene geological periods.

The reason they know that the K-Pg extinction event was caused by an asteroid collision is because the distinct looking boundary layer has a high concentration of iridium, a metal that's rare on the planet Earth, but is often found in meteorites. The layer also shows a thin section of carbon, indicative of the conflagration that occurred after the collision.
The real trick was finding the location of the collision, which most people now know that it was on the coast of Yucatan Peninsula near the town of Chicxulub. The crater is estimated to be roughly 180 kilometers in diameter. The size of the asteroid that caused the crater is estimated to be 10 kilometers. Luis Alvarez and Walter Alvarez discovered this crater in 1980, and their discovery put to rest the cause of the extinction event.

How did this collision event wipe out so much life on Earth? It's hard to imagine the unbelievable violence that would be caused by a 10 kilometer sized rock impacting the Earth at 40,000 miles per hour. The kinetic energy would be off the charts. The collision would create a blinding fireball much worse than any nuclear weapon or even a huge pile of nuclear bombs. This initial fireball would have rapidly expanded out to incinerate anything in its path. The concussion wave from the explosion would have toppled anything, including trees, and even hills, essentially leveling the landscape for hundreds of kilometers. A resulting Tsunami would have sent waves of water as far up as Kansas. The resulting earthquake would be so powerful as to drive the legs of dinosaurs up into their bodies. Either way you look at it, it would have been a horrible way to die.

But, that would only be the beginning of the terror. The crust at the point of impact would have been melted and heated to thousands of degrees and blown up into space for hundreds of miles, perhaps all the way to the moon. This flaming debris would have rained down all over the planet Earth, immediately setting fire to plants, animals and anything combustible. This would then have created a smoke and dust cloud over the entire Earth and it would have persisted for many years, causing a so-called nuclear winter effect. This would have caused plant life to die off along with most animal life. Only small creatures that lived underground and ate insects would survive. Some creatures that lived in the water would not have survived, but some marine species, especially amphibians, would have survived. However some marine invertebrates like fish may have been partially wiped out. All we know for sure is that the dinosaurs were killed off. The other species extinctions are still up for debate.

The thing to take from this is that we don't want to be like those dinosaurs. We have the capability to change the outcome of a possible asteroid collision. Otherwise, we'll be wiped out just like they were when the next big rock collides with our planet.

Thanks for reading.