This is a widely-accepted postulation that the ultimate
cause of the
Cretaceous extinctions was a
collision between a
meteor and
Earth. Strong
geological evidence supports the idea that a meteor indeed collided with the earth
65 million years ago. A layer of
iridium separating the
Cenozoic sediments from
Mesozoic sediments in the
crust indicates such and
extraterrestrial arrival; iridium is common in
meteorites but very
rare on Earth. In addition, an
asteroid with an approximately 10km
diameter left a
crater at
Chicxulub on the
Yucatan peninsula at the time of said
extinctions. This
impact could have saturated the
atmosphere with
dust particles for several years, thus cutting off most of the
sun's
energy required for
photosynthesis (and thereby causing the collapse of most
food chains) and causing extreme
acid precipitation.
Based on observations about the shape of the crater, many scientists have concluded that the Chicxulub asteroid struck the Earth at a low angle from the southeast. Such an impact would have created a firestorm on the North American continent that would have killed most of its terrestrial plants and animals within minutes of impact. Evidence does, in fact, suggest that the extinction occurred more rapidly and severely on North America than anywhere else on the planet.
If the so-called First Impact did immediately cause the downfall of the dinosaurs, it was only the coup de grace over many other factors. During this time, the ocean was receding dramatically, the climate was becoming significantly cooler, and extreme volcanic eruptions throughout the world (particularly in India) were clouding the atmosphere.