Cretaceous Extinction

About 65 million years ago, SOMETHING big happened on planet Earth resulting in the mass extinction of 3/4 of all living species - not just dinosaurs, but plants and marine life, too. Evidence uncovered all over the world strongly suggests that a meteorite the size of Manhattan slammed into the planet, setting in motion a cascade of events culminating in the great die-off.

In the late 1970's, father and son geologists Luis and Walter Alvarez discovered a unique layer of clay dating back to this period of major upheaval in Gubbio, Italy. Analysis of this layer - known by geologist as the K-T boundary layer - revealed that it contained a surprising amount of the element Iridium, the presence of which in the Earth's crust signifies two possible events. Iridium may be brought up via volcanic eruption from deep within the Earth's core, or it can arrive here aboard a meteorite.

While a catastrophic volcanic eruption could also account for the extinction, other evidence favors the meteorite hypothesis. Scientists already had calculated that a meteor some six miles across could explain the amounts of Iridium in the K-T layer, when geophysicist Alan Hildebrand found evidence of an ancient impact crater on Mexico's Yucatan Peninsula. The 125-mile-wide ring sits under 3000 feet of rock, and, dramatically, dates back to 65 million years ago.

Scientists calculate that the impact would have exploded with 10,000 times the force of the world's total nuclear weapons arsenal. The cataclysm caused a shock wave that physically altered the rocks around the site, started a global wildfire, triggered a tsunami, caused earthquakes and prompted volcanic eruptions. Together, all these events meant the entire planet was likely covered in darkness for months and veiled in haze for years. Global temperatures must have dipped to freezing and the entire food chain probably collapsed.

It's not the first time - nor probably the last time - a meteor impact shook things up on planet Earth. It was likely this impact, 65 million years ago, that ended the existence of dinosaurs and gave smaller, warm-blooded mammals their chance to find a niche in the world.

For more information, see this summary of theories and evidence of a catastrophic event leading to dinosaur extinction:
Dinosaur Extinction Page
http://web.ukonline.co.uk/a.buckley/dino.htm

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