About 66 million years ago the Cretaceous–Paleogene (K–Pg) extinction event wiped out about 80% of life on Earth, and no one is exactly sure what caused it. Many species, most famously the dinosaurs, were annihilated, while many of the survivors underwent radical transformations. Scientists have long debated the cause of one of the biggest mass extinctions in history, and while they’ve reached some general consensus no one is 100% certain which of the following is the cause.
Many of the other theoretical causes of extinction stem from an asteroid impact, and an impact event is the leading theory as to what killed the dinosaurs. Called the Alvarez hypothesis, it was developed by Nobel Prize winning physicist Luis Alvarez, his geologist son Walter Alvarez and two chemists, Frank Asaro and Helen Michels. The team discovered incredibly high levels of iridium in the sedimentary layers where the Cretaceous–Paleogene boundary is found. Iridium is uncommon in the Earth’s crust, but it is common on asteroids.
There was only one problem — if an asteroid hit Earth, where was the crater? Well in 1990, 10 years after the theory was published, researchers found the Chicxulub crater in Mexico. It’s believed that is was made by an asteroid that was about 110 miles wide and about six across — roughly the dimensions that Alvarez and his team had calculated.
It’s believed that after the asteroid hit it sent dust up into the atmosphere that blocked the sun for decades. This would have caused massive climate change. Adding to the darkness was ash and smoke, as wildfires would have started from debris re-entering the atmosphere. All of these factors would come together to make life impossible for dinosaurs.
9. Global Firestorm
While most scientists believe that it was an asteroid that wiped out the dinosaurs, there’s much debate as to what exactly the asteroid did when it hit Earth. One theory is that it started an unstoppable wall of fire that engulfed the entire planet. The argument is that when the asteroid hit, it sent vaporized rock particles above the atmosphere. There, the particles would have formed into grains of sand and then re-entered the atmosphere. The particles would have been incredibly hot, turning the sky red and making the atmospheric temperature 2700 F. This would have made the air above the point of impact act like a broiler, and everything below it would have caught fire in a blaze that would proceed to spread around the entire world. The researcher who came up with the theory said it would have been like a one-megaton bomb going off every four miles all over the Earth. Only animals that could burrow or go underwater would have been saved.
Using a computer simulation, a MIT Professor theorized a super hurricane (known as a hypercane) could have theoretically occurred if an asteroid caused a 40-50 mile area of the ocean to heat to 120 F. This would have created hurricanes so powerful that winds would have reached speeds of over 600 mph. For comparison, the most powerful storm known to human history was Typhoon Tip, which made landfall on October 12, 1979. Wind speeds reached 190 mph, less than a third of the wind speed during a hypercane.
As long as the conditions exist to create them, hypercanes will keep forming, meaning the dinosaurs possibly contended with a number of these storms. However, the real danger is the height of the storm, which would have been 40 miles. This would have brought water into the stratosphere and deteriorated the ozone, which would have killed off any creatures that couldn’t find shelter until the ozone reformed.
7. Gradual Extinction and Mammal Competition
A less dramatic theory is that the dinosaurs weren’t killed off with one single blow, but died off gradually over the course of millions of years thanks to competition from mammals. Mammals, which were starting to become more complex beings, could have been better at finding food and dealing with changes in the environment.
A major difference between mammals and dinosaurs is that mammals don’t lay eggs. A newborn dinosaur was small but could grow to a massive size, meaning they would need a lot of food. Conversely, mammals didn’t need much food and could therefore flourish much easier. And since mammals carry their children in a womb, it would have been much safer than laying an egg that would be vulnerable to predators. This theory would explain why dinosaurs died out but mammals survived the extinction event.
6. Continental Drift
The dinosaurs are thought to have first appeared during the Mesozoic era, which was 248 to 65 million years ago. Scientists divide that era into the Triassic, Jurassic and Cretaceous periods. During the Triassic, all the continents were in one giant landmass known as Pangea. During the Jurassic, Pangea broke in half and started drifting apart. By the time the Cretaceous rolled around, which is when the K-Pg extinction occured, the two continents had broken up even more, getting closer to the continents we know today.
The drifting of the continents would have been devastating to the dinosaurs, as their habitats would have changed drastically. It also would have caused a change in climate, and ocean cycles would have been altered. This would have led to an increase in violent storms that would have wrecked havoc on the environment, further limiting the dinosaurs’ ability to survive.
5. Changes in Water Levels
There have been five mass extinctions in Earth’s history. A professor of geology at the University of Wisconsin-Madison believes that there’s one culprit responsible for all five — change in sea levels. Since Earth formed about 4.54 billion years ago, it’s been constantly changing. As a result, land masses and oceans have evolved quite a bit. And any time a mass extinction happened, it’s correlated with a change in sea levels. While ocean life would have been affected the most, creatures on land would have been devastated as well. In some areas there would have been massive flooding. On a global scale, ocean cycles would have changed radically and drastically affected the environments where land creatures would have lived, ate and drank.
From looking at mosquitoes and ticks stuck in amber, Dr. George Poinar from Oregon State University said that germs started to emerge at the same time the dinosaurs were dying out, and it’s possible the mighty dinosaurs were brought down by a microscopic plague.
Disease would have been devastating to dinosaurs, because they wouldn’t have built up an immune system and they bred very slowly. Bugs would have simply transferred the disease to all the dinosaurs until they were wiped out, but mammals were able to survive simply because the diseases didn’t affect them.
A theory that stretches back to 1980 is that the death of the dinosaurs was caused by a different kind of impact event — one created by a comet. A comet is made up of ice, dust, rocky materials and organic compounds, while asteroids are usually made up of metal and rock. Comets are also smaller, and move a lot faster.
Detractors say that a comet wouldn’t have been big enough to cause the Chicxulub crater. However, researchers said that if the comet was moving fast enough, it could have caused a crater of that size. Not only that, but it would have sent more dust and debris into the air than an asteroid. The end result would have been the same either way, but it’s still a question worth debating.
The most metal theory on our list is that a volcanic region known as the Deccan Traps, located in what is now India, erupted. The eruption would have spewed out sulfur into the air for 10,000 years — 10 times the emissions put into the air by an asteroid. This would have caused incredible global warming and the acidification of the oceans.
This decades old theory was given more credence in 2009, when oil companies drilling below the ocean surface off the east coast of India found eons old lava filled sediments. They analyzed the sediment and found fossils from the time of the K-Pg extinction.
1. Combined Event
Many of the events listed here could have come together within a relatively short time span. While researchers debate what exactly was the culprit, they don’t deny that these other cataclysmic events could have happened as well. For example, there’s evidence of an asteroid or comet hitting the Earth and volcanic eruptions happening around the same time. Also, if an asteroid did hit the Earth, hypercanes and firestorms could have started. If there were survivors, disease and changes in sea level would have been devastating, and they might not have been able to compete against mammals. With so many things going wrong in such a short period of time, these small-brained creatures were simply overwhelmed by devastation.