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.

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