Most people have heard that it was an asteroid impact that killed the dinosaurs, but do you know how an asteroid that is only roughly 0.1% the size of Earth can wipe out 75% of the planet? Did you also know that there have been a very large number of paleontologists that have disagreed with this theory? Was it only one asteroid? Let's answer those burning questions shall we?
The event that killed all of the non-avian dinosaurs (I say non-avian, because dinosaurs are still alive today) is known as the K-Pg mass extinction event. This event is one of the 'Big 5' mass extinction events that have occurred in Earth's history. Whilst it wasn't the worst, it was pretty bad. It is estimated that, on land, nothing bigger than a house cat survived this event and many groups of animals were never to be seen again thanks to the event, including the non-avian dinosaurs.
The most widely accepted theory about the cause of what killed the dinosaurs is known as the Alvarez theory, which states that a huge asteroid roughly 12km across struck the Earth in the Gulf of New Mexico around 66 million years ago. This theory came about from observing the Chicxulub crater, as well as a stark change seen whenever the boundary between the Cretaceous and Paleogene is visible, where minerals present mostly in asteroids are in high abundance.
When the asteroid that killed the dinosaurs struck the Earth, it gave of the same energy as several million nuclear bombs detonating simultaneously, vaporising anything within 50 to 100 miles almost instantly. After this initial wave of destruction came the next stage.
Due to how much the impact shook everything up, a large portion of the Earth would have experienced massive tsunamis, volcanic eruptions and earthquakes. If an animal was fortunate enough to make it through all of that, it had to cope with the next phase. An impact like this will cause a lot of debris to be thrown up, much like when you throw a rock into water or sand.
The dust cloud that ended up engulfing the Earth was the final nail in the coffin. Dust and debris was thrown into the stratosphere, carrying around the globe. On top of this, that debris would have been vaporised locally, producing sulphuric fumes that blocked out the sunlight globally. In addition to a global dust cloud that instantly brought on an impact winter, the sulphur would have caused acid rain to fall continuously over the entire planet.
The acid rain was very bad news for marine organisms and the lack of sunlight was bad news for anything on land. In the post going over the big 5 extinction events, a consistent feature was sea acidification, so you can read more about that over there! In the meantime, we'll concentrate on the dinosaurs.
Due to the blocked out sun caused by the asteroid impact, plants were the first to suffer. With their main nutrition source being cut off, the plants soon began to die off. The ecology of any given area has an incredibly delicate balance, so when one thing changes, it will have big affects on everything else. The death of flora is one such example. With little to no plants, all of the herbivores were then cut off from their food source. This means their numbers will start to dwindle. Now that the herbivorous dinosaurs are reducing rapidly, the carnivorous dinosaurs are also now cut off from their food source.
The cause of the extinction that killed off the dinosaurs may have been a single event that happened in a day, but the actual extinction event itself would have been dragged out for quite some time. There were likely dinosaurs at various points around the world that didn't even know anything was out of the ordinary for weeks after the asteroid hit!
Or was it just a single event?...
There have been some recent studies looking back into the extinction of the non-avian dinosaurs, in which scientists have found that there was likely more than one asteroid that struck the Earth at this time.
Discovered around 250 miles of the coast of West Africa, a crater (named the Nadir crater) was recently discovered that dates back to the same time as the Chicxulub crater and is approximately 5 miles across. Something that left a crater like this would have had similar effects to the impact mentioned above, completely vaporising anything within the immediate area and causing extraordinarily high magnitude earthquakes and tsunamis potentially up to around 1000 meters high! Imagine a wave approaching from the sea that is high enough to graze low-sitting clouds!
That's not even as big as the Chicxulub crater, so the effects from two of these impacts are difficult to comprehend. Of course, the jury is out on whether this asteroid broke away from the larger counterpart or was independent. It also begs the question, are there more craters out there?
Of course, as with anything in geology, there are those out there that disagree with this theory. Many have argued that the non-avian dinosaurs were declining anyway and the asteroid(s) just finished them off, others think the asteroid(s) had nothing to do with it altogether.
One of the alternative theories put forward about what killed the dinosaurs is on a similar vein to what is thought to have caused the Permian mass extinction. In West-central India is a layer of rock around 2000 metres thick which is a flood basalt known as the Deccan Traps. This means that, over the course of around 800,000 years, constant volcanic eruptions were spewing out lava over an area of approximately 500,000 square kilometres (200,000 square miles). Those 800,000 years happens to have crossed over the K-Pg boundary, meaning that certain scientists theorised that such a high amount of carbon being released into the atmosphere could have caused the extinction. The creation of the traps certainly wouldn't have helped the conditions, however, certain geophysical models have shown that these eruptions were actually caused by the asteroid impact. If the Permian mass extinction is anything to go by, the effects only seem superficially similar, as marine life would feel these effects much more. There's no doubt the creation of the traps happened and that it would have had some consequences, but it seems doubtful from the evidence that this was anything more than a contributing factor.
A reduction in sea levels has also been attributed to much of the extinction, since most of the marine organisms that died off inhabited shallower waters and a sea level reduction can be seen in the rock record. Of course, this wouldn't have affected life on the land as much, so wouldn't offer a whole picture. Whilst some geologists have theorised that this was thanks to inactive mid-ocean ridges sinking under their own weight, their appears to be no definitive explanation as to why this is. It couldn't have been from growing ice caps, since ice caps weren't even around at the time, and even if it was, this would need to be caused by a drop in global temperature, which would have been caused by...say...an impact winter? Whichever you cut it, it seems likely this was just another contributing factor to an already dire situation.
This was, believe it or not, a topic of hot debate once upon a time. Since the 1980s, however, evidence has come forward that has irrefutably proved the asteroid impact theory. Any other suggestions or events appear to have occurred (likely) as a result of said impact. With the evidence of a second impact though, who knows how many more siblings that asteroid had?...