Alvarez asteroid impact theory

Date June 6, 1980

When scientists Luis W. Alvarez and Walter Alvarez discovered evidence that Earth had been hit by a large asteroid sixty-five million years ago and then linked the asteroid impact with mass extinction, they ignited a firestorm of controversy and the consequent explosion of creative research.

Locale Gubbio, Italy

Key Figures

• Luis W. Alvarez (1911-1988), Nobel Prize laureate and professor emeritus of physics at the University of California, Berkeley
• Walter Alvarez (b. 1940), professor of geology at the University of California, Berkeley
• Frank Asaro (fl. late twentieth century), nuclear chemist at the University of California, Berkeley
• Helen Michel (fl. late twentieth century), nuclear chemist at the University of California, Berkeley
• Eugene Merle Shoemaker (1928-1997), geologist with the U.S. Geological Survey

Summary of Event

The geologic history of the planet Earth is punctuated by a number of mass extinctions in which large numbers of different species disappeared at nearly the same time. These mass extinctions have been recognized for more than one hundred years and are used to mark the major time boundaries of the last 600 million years of geologic history. The most interesting and intensively studied mass extinction was the one that occurred at the end of the Cretaceous period about 65 million years ago. This event has held the fascination of the general public and generations of professional geologists because the great monsters of the past—the dinosaurs—made their last stand at this time. Why did they and many other animals completely disappear from Earth at this time? Many hypotheses have been suggested (supernova explosions, climate change, receding seas, disease, among many others), but a central unifying theory has eluded geologists.

Until the 1980’s, the general consensus among professional geologists was that the mass extinction was the result of significant, but gradual, environmental changes in the oceans and on the continents that were too severe for many animals to survive. In 1980, a hypothesis was put forth by four scientists at the University of California, Berkeley, that revolutionized and revitalized the study of the Cretaceous extinction and reshaped modern thinking about the basic nature of geologic change. Luis W. Alvarez, Walter Alvarez, Frank Asaro, and Helen Michel published a landmark paper in the June 6, 1980, issue of Science that presented data that suggested that the extinction was caused by the impact of a large asteroid.

As so often happens in science, this discovery resulted from research that initially was conducted on an entirely different problem. Walter Alvarez, a geologist with very wide interests, was working on the paleomagnetism of the Cretaceous and Tertiary rocks of north central Italy. He sought to correlate the magnetic changes preserved in the rocks near Gubbio, Italy, with similar changes that had been found in the ocean floors. Alvarez had chosen to study the marine limestones that are very well exposed in Bottacione Gorge near Gubbio. These limestones contained fossils that clearly showed the terminal Cretaceous extinction and were overlain by a peculiar 1- to 2-centimeter-thick clay layer that was formed at the same time as the extinction. Alvarez was naturally curious about the clay layer and about how long it took to be deposited.

Geologists and geophysicists have developed techniques to determine the general ages of rocks and fossils, but none of these methods is precise enough to recognize the boundaries of an event that represents an interval of only a few thousands of years’ duration that occurred 65 million years ago. Another new technique was needed, and Walter Alvarez approached his father, Luis, with the problem. Luis Alvarez was a brilliant scientist of enormous curiosity and drive who had won the 1968 Nobel Prize in Physics for his work on subatomic particles. Captivated by the problem, the elder Alvarez suggested to his son that they have the clay analyzed for meteoritic (extraterrestrial) debris. Luis Alvarez knew that the influx of meteoritic dust was nearly constant and he reasoned that in slowly accumulating sediments such as a marine clay, the amount of dust would be both measurable and a direct indication of the total time for deposition of the layer.

Unfortunately, meteoritic dust is not easily distinguished from normal clays, and Luis Alvarez suggested that they analyze the clay chemically for elements that might indicate the meteoritic contribution to the clay. They enlisted the aid of two nuclear chemists at Berkeley, Asaro and Michel. In order to solve the problem, Asaro and Michel determined the abundances of twenty-eight elements within the clay and the surrounding limestones. The results showed that all but one of the elements had similar abundances among all the samples. The lone exception was iridium, a platinum-like element that was up to 160 times higher in the clay than in the limestones. This iridium anomaly meant that the dating technique could not be accurate (it was far too high for normal influx), but it was the key to the latest Cretaceous event.

Geologists believe that most of the iridium of the planet was concentrated in Earth’s core when the iron and other chemically similar elements (including iridium) migrated to the center of the planet early in its history. One result of this core formation is that iridium is exceedingly rare in Earth’s crust. To find the relatively high levels of iridium in the clay required an extraordinary explanation. Walter Alvarez initially thought the iridium was produced by the explosion of a nearby supernova. Detailed analysis of the elements and their isotopes, however, showed that this was highly improbable. The Berkeley group then proposed that the iridium was imparted directly to Earth with the impact of a large asteroid, approximately 10 kilometers in diameter.

Most asteroids orbit the Sun in paths between Mars and Jupiter, but some (Apollo objects) do have Earth-crossing orbits. Eugene Merle Shoemaker—the foremost authority on asteroid and meteorite impacts—calculated that the rate of impacting had been fairly constant over the past two billion years and that the probabilities suggest that an impact of a 10-kilometer asteroid would occur on the average of every 100 million years. Shoemaker’s calculations showed that an impact such as that envisioned by the Berkeley group was possible (even highly likely), but the crowning piece of evidence for impact—the crater—was missing. The scientists had calculated that the crater would have been approximately 150-200 kilometers in diameter. Only three craters of this size were then known, and none was of the right age. The missing crater, however, was not a critical flaw in the hypothesis. There was a 70 percent chance that the asteroid hit in the oceans and a 50 percent chance that any crater formed in the Cretaceous seafloor would have been destroyed over the last 65 million years by tectonic processes. Still, a crater of the estimated size and the correct age would have sealed the case for asteroid impact.

Although the Berkeley group had reported only three sites of the iridium anomaly (at Italy, Denmark, and New Zealand), they were confident of their thesis and anticipated that other sites would soon be found. Later, many more sites were located, and their idea of impact was more generally accepted. The main emphasis of the original article, however, was much broader than simple impact. They suggested that the impact was the central cause for the late Cretaceous mass extinction and they proposed a general scenario to describe the event. The asteroid would have blasted both terrestrial and meteoritic debris into the atmosphere and the smaller dust particles would have quickly encircled Earth. The dust would later settle to form Earth’s clay layer. While in the atmosphere, the dust would have darkened the day skies for up to three years (later, it was shortened to a few months), thereby essentially shutting down all photosynthesis in the oceans and on the continents. The loss of photosynthesis would have rapidly collapsed the food chains of the world, and the catastrophic extinction of animals would have followed. Although many other effects of the impact were later recognized, the Berkeley group had made the critical point: The extinction was not gradual but catastrophic.

Significance

In linking asteroid impact with mass extinction, the Berkeley group ignited a firestorm of controversy and the consequent explosion of creative research. Not only were geologists drawn into the controversy but also biologists, chemists, and astronomers became deeply involved in the questions surrounding the impact hypothesis.

The immediate response from the geologic community was to search for evidence to support or undermine the hypothesis. Several groups and laboratories around the world began to search for evidence for iridium anomalies in other Cretaceous-Tertiary boundary clays, and they were soon rewarded by finding more than fifty sites within three years. Studies of known impact craters had shown that the shock was caused by impact-generated fractures throughout many of the minerals (particularly quartz). These sets of fractures are characteristic only of impacts, and similarly “shocked” quartz minerals were found in the boundary clays. Also found in the clays were microtektites, small glasslike beads of material that had been melted by impact and cooled quickly when ejected into the atmosphere.

All these data seemed to indicate definitely that an impact had occurred, but a number of reputable scientists argued that the data were not conclusive and the known evidence was better explained by massive volcanism. Charles Officer and Charles Lum Drake argued that since iridium had been detected in the volcanic eruptions in Hawaii, the iridium-rich layer could have been produced by volcanism on a grand scale. They pointed out that the huge volcanic flows of the Deccan Traps of India are of the right age (about 65 million years old) to have created the anomaly. Nevertheless, shocked quartz has not been found in association with the volcanic flows.

Research into the processes of mass extinction was stimulated as well. David Malcolm Raup and John Sepkoski, both paleontologists at the University of Chicago, statistically analyzed the occurrences of extinction among families of organisms and recognized that mass extinctions appear to have occurred about every 26 million years. These cycles are of such long duration as to eliminate most, if not all, terrestrial processes and suggest extraterrestrial causes. Raup and others think that impacts could cause cyclic mass extinctions, and the report of iridium anomalies at boundaries other than at the latest Cretaceous strengthened their argument. If impacts are cyclic, then there must be some astronomical process that periodically spins asteroids (or comets) across Earth’s orbit. Astronomers proposed several ideas, including the presence of a dim, distant companion star of the Sun. The search for this star, called Nemesis, has begun at Berkeley, but no good candidates have yet been identified.

Evolutionary theory has also been profoundly affected by the impact hypothesis. Most modern models of evolution state that species gradually change through time by genetic mutations acted upon by natural selection. Extinction caused by asteroid impacts adds another dimension to this view. Impacts cannot be anticipated, they dramatically affect the environment for very short periods of time, and organisms cannot adapt quickly enough to survive. Impacts can, therefore, change the development of life by exterminating otherwise dominant life-forms (dinosaurs, for example) and open up ecological niches for other organisms (such as mammals in the Tertiary).

The basic concepts of geologic change have been altered by the impact hypothesis. Generations of geologists have been strong adherents of uniformitarianism, the idea that geologic change is caused only by processes still at work and that the changes are generally gradual. Impacts are not gradual, and they are now seen as major factors in the history of Earth. Catastrophic events, therefore, came to be thought of as possible and plausible causes of many of the major events of the geologic past.

Bibliography

Alvarez, Luis, Walter Alvarez, Frank Asaro, and Helen V. Michel. “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” Science 208 (June 6, 1980): 1095-1108. The landmark paper that proposed the asteroid impact hypothesis based on the iridium anomaly found at Gubbio, Italy. A technical report that is useful for the interested student of science or the history of science.

Alvarez, Walter. T. Rex and the Crater of Doom. Princeton, N.J.: Princeton University Press, 1997. A very accessible narrative about the scientific discovery of the causes of the mass extinction.

Frankel, Charles. The End of the Dinosaurs: Chicxulub Crater and Mass Extinctions. New York: Cambridge University Press, 1999. The focal point of the book, the Chicxulub Crater, located at the tip of the Yucatán Peninsula, is considered by many to be the “smoking gun” evidence that the Berkeley scientists were looking for. Fairly presents the contending theories about the causes of mass extinction and suggests that other mass extinctions were caused by extraterrestrial impacts.

Glen, William, ed. The Mass-Extinction Debates: How Science Works in a Crisis. Stanford, Calif.: Stanford University Press, 1994. An excellent assembly of essays on the debate between proponents of catastrophism and uniformitarianism.

Goldsmith, Donald. Nemesis: The Death Star and Other Theories of Mass Extinction. New York: Berkley, 1985. An exceptionally well-written account of the asteroid impact theory and Nemesis written by one of the major investigators in the search for Nemesis.

Grieve, Richard A. F. “Impact Cratering on the Earth.” Scientific American 262 (April, 1990): 66-73. An excellent article on the number, nature, and mechanics of impacts on Earth. This well-illustrated article gives geologic evidence of impact at known craters and how it applies to the asteroid impact theory.

Hsü, Kenneth J. The Great Dying. San Diego, Calif.: Harcourt Brace Jovanovich, 1986. Excellent, but somewhat technical, work by a very influential earth scientist. Gives a complete summary of the impact theory and the resulting extinction. Also relates many interesting insights into the lives and thinking of the people researching the Cretaceous-Tertiary extinction. Highly recommended.

Raup, David M. The Nemesis Affair: A Story of the Death of Dinosaurs and the Ways of Science. New York: W. W. Norton, 1986. Well-written account of the asteroid impact theory by the major advocate of periodic extinction. This wonderful book not only gives a clear, historical account of the development of the impact theory but also gives the layperson insights into the process of science itself.

Wilford, John Noble. The Riddle of the Dinosaur. New York: Alfred A. Knopf, 1986. A good popular summary of the modern interpretations of dinosaurs. Several chapters address the extinction of these animals and treat the asteroid impact theory clearly and fairly.