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.