RENO, NEV.--The Chicxulub Crater, sprawled across the Gulf of Mexico and the Yucat¿n Peninsula, is an approximately 180-kilometer-wide remnant of the impact of a 10-kilometer-wide meteorite. It has been called the smoking gun in the extinction of the dinosaurs between the Cretaceous and Tertiary periods 65 million years ago. Some geologists, though, are starting to believe the meteorite didn't act alone. Volcanic phenomena known as superplumes may have been accomplices in that and other mass extinctions. "The general idea is that plumes are strengthened by impacts," says Dallas Abbott, a researcher at Columbia University's Lamont-Doherty Earth Observatory. At the Geological Society of America meeting in Reno last November, she showed a correlation between the timing of purported superplumes and large impact events--and their possible association with mass extinctions.

A plume can be visualized as a rising glob of liquid in a slowly warming lava lamp: material hotter than the surrounding rock of the earth's mantle pushes toward the surface in a concentrated stream. The funnel ends below the earth's outer crust, where the plume material spreads and ponds. If the molten rock erupts through the earth's surface, it releases gas and particulates into the air and produces lava flows. A superplume may be a gathering of small plumes, the size of those under the Hawaiian Island chain and Iceland, or one very large plume.

   
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