The extinction of non-avian dinosaurs except birds at the end of the Cretaceous has intrigued paleontologists for more than a century. In the 1900s, numerous speculative hypotheses were proposed. One was that dinosaurs just got too big, but the largest dinosaurs, such as Apatosaurus, lived long before the end of the Cretaceous.
An asteroid, about 10 km across
Then in 1980, Walter and Luis Alvarez discovered that a bed of marine clay in Italy, right at the Cretaceous-Tertiary boundary, contained abnormally high levels of iridium, an element rare in rocks on the Earth's surface but more common in asteroids and comets. Thus, they argued that an asteroid, about 10 km across, impacted the Earth, generating massive tsunamis, with impact debris cutting off sunlight for months, stopping photosynthesis, and causing freezing temperatures. Chemical reactions in the atmosphere caused acid rain and long-term global warming, all of which extinguished non-avian dinosaurs. Rocks from an impact crater called Chicxulub, buried beneath the shore of the Yucatan Peninsula, were dated radioisotopically at 65 million years, right when the extinctions occurred, confirming an impact.
Massive lava flows
However, at the same time, massive lava flows erupted across what is now southwest India. These layers of lava, called the Deccan Traps, are up to 2,400 m thick and cover an area as large as California. They represent the second largest episode of continental volcanism in the Earth's history. The eruptions occurred over a 500,000 year period spanning the end of the Cretaceous, and probably caused many of the same effects as the impact: reduced sunlight, acid rain, short-term cooling, long-term greenhouse warming.
Although most scientists believe that the impact represented the final blow for non-avian dinosaurs, the fact that both events occurred when these dinosaurs went extinct suggests that both events, with their similar "killing mechanisms," could well have played a role.