In 1980 Luis and Walter Alvarez of the University of California at Berkeley announced that they had found evidence that a giant meteor had hit the Earth at the time in geologic history that marked the transition from the Mesozoic to the Cenozoic Era.  Because the end of the dinosaurs is also associated with this transition, the Alvarezes suggested that it was the impact of a giant meteor that had brought a sudden end to the dinosaurs.


According to the Alvarezes, a giant asteroid—about the size of the island of Manhattan—had slammed into the Earth, sending up huge clouds of debris that encircled the planet.  That debris can now be found as a thin layer at many places around the world, for example, along the Purgatorie River near Trinidad, Colorado.  The crater produced by the impact has also been found:  a hundred-mile-wide circular feature known as Chicxulub that lies beneath much of the Yucatan Peninsula.  But could such a huge impact had produced a major mass extinction?  Probably not.


Study of the Chicxulub crater shows that the impact came from the south.  That means the greatest devastation from the impact was to the north across North America.  Dinosaur species that were confined to North America, such as Tyrannosaurus rex, probably went extinct as a direct result of the impact.  But, elsewhere in the world, a detailed examination of the rock record across the Mesozoic-Cenozoic transition has shown that the mass extinction occurred hundreds, thousands, or perhaps, tens of thousands of years after the impact—during a time period when massive sheets of lava were being erupted in India.


Known as the Deccan Traps, the eruptions that produced these massive sheets of lava began a few tens of thousands of years before the Chicxulub impact.  But the main phase of the eruptions began within a few tens of thousands of years after the impact, pouring huge amounts of carbon dioxide into the atmosphere, causing the planet to heat and producing the extinction.


And so did the impact or the eruption kill the dinosaurs?  It seems that it was both, the two events acting in concert.  An impact on a scale that could produce the Chicxulub Crater happens once every few hundred million years.  An eruption on the scale of the Deccan Traps happens one or twice every hundred million years and lasts less than a million years.  And so the chance that two rare events would occur during the same few tens of thousands of years is low—the Chicxulub impact, in some way, triggering—or, at least, intensifying—the eruption of the Deccan Traps.