A widely accepted theory about why the Cretaceous extinction occurred. Originally proposed in 1980 by physicist Luis Alvarez and his son Walter Alvarez, a geologist. It was initially greeted with shocked suprise and disbelief in many quarters - but with eagerness in many others. And over the years the evidence in favor of it piled up.

The theory posits that an asteroid 4-9 miles (6-15 km) in diameter hit the Earth about 65 million years ago. The impact would have penetrated the Earth's crust, scattering dust and debris into the atmosphere, and causing huge fires, increasing already active volcanic eruptions, triggering tsunamis, and severe storms with high winds and causing highly acidic rain. The heat from the impact's blast wave would have incinerated all the life forms in its path. The dust and debris thrust into the atmosphere would have blocked most of the sunlight for months, and lowered the temperature globally, thus forcing organisms that could not adapt to the temperature and light changes to die out.

Major changes in the food chain would result from the event. Because sunlight was blocked, plantlife was likely to be the first to die out. The herbivores (plant eaters) who ate those plants would starve soon after, thus forcing the carnivores (meat eaters) to eat each other, and eventually die out. Their large carcasses must have provided smaller animals with food for quite a while.

There are even a few suspected locations of the impact crater(s) (the most popular site is called Chicxulub, which is found near the Gulf of Mexico). There is also chemical evidence having to do with a layer of Iridium (which is rare on earth but plentiful in meteorites) that was found on the Cretaceous boundry.

While many don't doubt the reality of a large impact around that time, there are doubts that a single impact could produce such a complex extinction event.

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