Departure of the Dinosaurs

Like many children, I was fascinated by dinosaurs from the moment I toddled into my first natural history museum. The hardest thing to accept was that--except for the tyrannosaurs in my nightmares--none of them were left. How could an entire family of creatures that had ruled the earth for millions of years vanish so completely?

The theory popular back then blamed the upstart mammals for eating dinosaur eggs. I didn't buy that, I couldn't see mouse-size beasties gnawing through tough cannonball-size eggs. I blamed volcanoes--chiefly because of a vivid painting showing a family of duckbilled dinosaurs (like those that left fossilized remains in northern Alaska) watching from a verdant bog as great rivers of lava and ash flowed toward them from a line of erupting cones.

The prevalent view now is that dinosaurs were the victims of a celestial collision, and the evidence seems convincing. About 66 million years ago, something stunning happened on Earth. Before then, coniferous plants and giant reptiles dominated the land. Afterward, the dominant forms were mammals and flowering plants. The Cretaceous Period had given way to the Tertiary at what is called the K-T boundary.

The boundary is marked in the geologic record by a thin layer enriched in the platinum-group element iridium. Iridium attaches itself to iron, so nearly all Earth's store of it sank along with the iron to the core early in the planet's history. Normal iridium concentrations in the crust are tens of parts per trillion; at the K-T boundary, they've been found at 3,000 parts per trillion.

The father and son Alvarez team who first identified the iridium anomaly noted that the element is abundant in certain kinds of meteorites. They suggested that the iridium of the K-T boundary layer settled out of the dust cloud left by the impact of a meteorite 10 kilometers in diameter. This dust cloud, they proposed, would have blocked sunlight for years, chilling the dinosaurs (and 75 percent of everything else alive at the end of the Cretaceous) into extinction.

The Alvarezes' wild-sounding hypothesis sent scores of scientists back to the laboratory and into the field as they sought to support or refute the idea. Field workers found the high iridium layer on different continents, at 95 different sites so far. They also found evidence of a terrific shock, such as might be delivered by a monster meteorite slamming into Earth, in particles of transformed quartz.

At the K-T boundary, there are some quartz grains filled with parallel planes along which rows of atoms in the crystal lattice are rearranged. (Properly lit and observed through polarizing filters, a section of shocked quartz under a microscope looks as if it's been sliced repeatedly with a razor blade.) Previously, quartz like that had been found only near A-bomb test or meteorite impact sites. An even more rare form of pressure-transformed quartz called stishovite now is known also from one K-T boundary site in New Mexico.

The theoreticians calculating the impact effects of such a meteorite came up with enough horrible results to do in the dinosaurs ten times over. First, the shock would heat the atmosphere. Most of the debris plume would leave the atmosphere, ripping away part of the atmosphere with it. As ejected particles reentered the atmosphere, they would heat it even more; one scientist compared it to several hours of "a domestic oven set at Broil." What didn't cook immediately might be incinerated by ensuing wildfires; carbon, presumed to be soot, has been found at several K-T sites. The tremendous heat could have caused atmospheric nitrogen and oxygen to combine with water vapor, forming nitric acid: tremendous acid rains would have poured down. Eventually, perhaps, the remaining suspended particles and smoke would have intercepted enough sunlight so that a deep years-long chill would follow the great heat.

And perhaps volcanoes did play a role. The K-T boundary marks a great volcanic outpouring--the biggest pulse in the Deccan Traps, a long-lasting series of flood eruptions that built up a million cubic kilometers of basalt in what is now western India.

All in all, it's more surprising that anything survived than that the dinosaurs died. Our primitive mammalian ancestors of the time must have been terrifically tough--and awfully lucky.