Chapter 23: The Asteroid Theory

      The island is a tropical paradise covered with lush vegetation. The sun has just set, and the sky toward the west is a beautiful canvass of reds, pinks, and purples. Quietly at first, meteors begin to streak across the evening sky, catching the attention of an Iguanodon and several lemurs. Soon a meteor shower lights up the sky, as thousands of meteors plunge to the earth. A flock of birds is disturbed and takes flight; a lemur sniffs the air, sensing that something is wrong.

     Suddenly, an enormous chunk of rock is seen plunging toward the earth, glowing white-hot, trailing fire and a cloud of smoke. There is a blinding flash of white light, then a mushroom cloud, as though there had been a nuclear explosion. On impact, the asteroid disintegrates into millions of smaller pieces, which shoot out in all directions, glowing and trailing smoke, landing like a scattered artillery barrage. The circle of devastation around the impact site expands in waves, as when a stone is tossed into a calm pond.

     Soon the first shock waves hit the animals, knocking the lemurs off their feet. From the site of the impact, a tidal wave and a wall of fire are racing toward the island. The Iguanodon runs for his life. He runs across the island and plunges into the sea on the side opposite the cataclysm. The Iguanodon and a few of his lemur friends survive the initial impact, but the world that emerges is not the tropical paradise that existed before. The post-impact world is a desert, almost devoid of plant life and drinkable water.  

If you have young children, you may have recognized what we just described as one of the early scenes in the Disney animated feature, “Dinosaur.”[1] It reflects the currently fashionable belief that a giant asteroid struck the earth 65 million years ago, wiping out the dinosaurs.

The demise of the dinosaurs is a mystery for which many solutions have been proposed. Paleontologist Robert T. Bakker kept a file of published explanations for the dinosaur extinction. The dinosaurs disappeared—according to these varied theories—because:

§        The weather got too hot

§        The weather got too cold

§        The weather got too dry

§        The weather got too wet

§        The weather became too hot in the summer and too cold in the winter

§        The land became too hilly

§        New kinds of plants evolved which poisoned all the dinosaurs

§        New kinds of insects evolved which spread deadly diseases

§        New kinds of mammals evolved which competed for food

§        New kinds of mammals ate the dinosaurs’ eggs

§        A giant meteor smashed into the earth

§        A supernova exploded near the earth

§        Cosmic rays bombarded the earth

§        Massive volcanoes exploded all around the earth.[2]       

The asteroid theory is one of the more recent explanations. It grew out of research conducted by Walter Alvarez, a geologist with the University of California. Alvarez and his colleagues were looking for geological evidence of shifts in the Earth’s polarity. While working near Gubbio, Italy, they noticed a layer of reddish-gray clay a couple of centimeters thick right at the boundary between the Cretaceous and the Tertiary. Alvarez told his father, Luis Alvarez, a Nobel prizewinning physicist also at Cal-Berkeley, about the strange layer of clay.

The elder Alvarez was an adventurous scientist who had once tried to x-ray the pyramids of Egypt to reveal hidden chambers. He tackled the problem of the boundary clay with relish. While testing the clay to see if it could be radiometrically dated, Alvarez discovered it had thirty times the iridium content of the rocks above and below it. Because iridium is vastly more abundant in meteorites than in rocks formed on the earth, the Alvarezes began to theorize that the extra iridium had resulted from the explosion of a supernova.[3]

They obtained samples of the boundary clay from several other locations, as far away as New Zealand and Denmark, and found that these also contained high levels of iridium. They abandoned the supernova theory and, in 1980, published their conclusion that a giant asteroid had struck the earth at the end of the Cretaceous period. The resulting cloud of dust had blocked out the sun, they theorized, killing the plants. The lack of plant food killed the herbivorous dinosaurs, and the lack of prey animals killed the carnivorous dinosaurs.[4]

The news media responded very favorably to the asteroid theory, implying that the great dinosaur mystery had at last been solved. Even the normally reserved scientific community was unusually open to and interested in the theory. Almost immediately, a number of scientists from a variety of disciplines began working out the details of the asteroid model. A large crater at Chicxulub, in the Yucatan peninsula of Mexico, was claimed as the remnant of the fatal impact. The asteroid theory has now become the most widely accepted theory to explain the “mass extinction” at the Cretaceous-Tertiary boundary, although it is probably not accepted by even a majority of scientists.

The first thing to note about the asteroid theory is that it is not a uniformitarian theory. It relies upon a postulated global catastrophe the like of which has never been observed. Charles Lyell established uniformitarian geology by insisting that only presently observable causes, acting at presently observable rates, could permissibly explain the geological facts. To invoke catastrophes was unscientific, Lyell insisted. He berated the geologists of the seventeenth and eighteenth centuries for thinking in terms of catastrophe:

They employed themselves in conjecturing what might have been the course of Nature at a remote period, rather than in the investigation of what was the course of Nature in their own times. . . . It appeared to them far more philosophical to speculate on the possibilities of the past, than patiently to explore the realities of the present; . . . the claims of each new hypothesis to credibility appeared enhanced by the great contrast, in kind or intensity, of the causes referred to and those now in operation. Never was there a dogma more calculated to foster indolence, and to blunt the keen edge of curiosity, than this assumption of the discordance between the ancient and existing causes of change.[5]

Thus did Lyell dismiss the efforts, however scholarly, of a