A giant meteorite struck the Earth 65 million years ago, wreaking global devastation. Over a very brief period, an estimated 70 percent of all species died out. While the explosion itself was enormous, the biologic disaster probably resulted from a cascading sequence of events: wildfires, decreased sunlight, acid rain, and, eventually, mass starvation. The extinction of so many species, which marks the boundary between the Cretaceous and Tertiary geologic periods, changed the course of evolution.

The lowest density rocks (indicated in blue), which fill the center of the crater, are made of materials that were frag-mented and melted by the impact.

Serendipity Begets Discovery
In 1980, an American father-son scientific team made an unexpected discovery while studying ancient sediments in Denmark, Italy, and New Zealand. A thin layer of clay at the boundary between Cretaceous and Tertiary rocks contained an unusual amount of iridium a rare element that's far more abundant in meteorites than in Earth rocks. The team's proposal: An enormous meteorite hit Earth 65 million years ago. Later work has confirmed the existence of the iridium-rich layer worldwide.

The light-colored layer of clay in this cross-section marks the Cretaceous-Tertiary boundary. It contains several signs of impact: grains of shocked quartz, spheres of melted rock, and an unusual amount of iridium.

The Search for the Crater
For a decade after the iridium-rich clay layer was found, scientists scoured the globe for a 65-million-year-old impact site. In 1991, they discovered a huge buried crater on the coast of Mexico. Called Chicxulub, the multi-ringed crater is at least 170 km (105 mi) wide. Drill cores from its center contain melted, broken rocks the same age as the clay layer. Glassy impact debris dating from the same period is strewn around the Gulf of Mexico.