Those who regard the K-T extinction--the mass extinction that occurred some 65 million years ago at the boundary between the Cretaceous and Tertiary time periods--as catastrophically fast rather than more gradually occurring tend to invoke a single unusual mechanism. Two of these proposed mechanisms are now popular. The less popular relies on the vast outpouring of volcanic lavas--the Deccan Traps of Western India and the Indian Ocean--which were produced at the end of the Cretaceous times, when the moving Indian plate passed over a rising volcanic plume. The Hawaiian Islands were formed by a more recent event involving the same mechanism, though it is argued that the Deccan explosion was at least thirty times more powerful. Huge volcanic lava flows, some 1.2 miles in thickness, now cover an area of more than a million miles. Proponents of this mechanism argue that this gigantic eruption, which occurred at about the K-T boundary, could have produced a cooling effect, a greenhouse effect, acid rain, atmospheric changes, or some similar effect, or even some combination of these various effects. The best analogy, perhaps, is the eruption of the volcano Tambora in Indonesia in 1815, which produced enough ash and atmospheric dust to make 1816 the "year without summer", leading to widespread crop failures over a vast area. The Deccan eruptions were many, many times more powerful than Tambora and, proponents argue, could have depleted the food sources of the herbivorous dinosaurs and thus, in turn, impacted the food chain of the carmnivores. But whether this Deccan trap plateau volcanism would produce such dire climatic effects is debatable.

The alternative, and currently more popular, hypothesis involves the impact of an asteroid or comet that struck the Earth at the time represented by the K-T boundary. The evidence for this was discovered by Walter Alvarez and others in a well-preserved rock sequence in Gubbio, Italy, that spans this interval. A sharp increase of about 100 times, a "spike", in the iridium content of rocks marks the boundary, and this spike that has since been detected in well over a hundred different localities at precisely the same point in the stratigraphic sequence and in the rocks representing many different depositional environments. Now, iridium is present only in trace amounts in sediments deposited on Earth, but it is present in much higher concentrations in meteorites. Walter Alvarez and his colleagues argued that two other features of the boundary rocks provide further evidence of meteor impact: shocked quartz crystals showing two intersecting sets of fractures, and tiny glass spherules, both known to be characteristic products of the impacts of an asteroid, some 6.2 miles (10 km) in diameter.