T. Rex and the Crater of Doom

Alvarez continued to ponder the abrupt change in foraminifera in the Cretaceous and Tertiary layers of rock in Gubbio, Italy, as well as the thick layer of clay that separated them. He consulted with his father, Luis W. Alvarez, a Nobel prize-winning physics professor at the University of California, Berkeley. Alvarez wondered how they should try to date and interpret the clay layer. If the layer was deposited in a short amount of time, he felt that this would indicate a sudden change in Earth and an abrupt extinction event.

Alvarez’s father put him in touch with Richard Muller, a physicist at Berkeley who was an expert on dating rocks. They believed that measuring the isotope Beryllium-10 in the clay would help them date it accurately. Unfortunately, they soon learned that this isotope decayed too quickly to reveal the age of the clay. Alvarez was disappointed by this dead-end; however, this tantalizing opportunity only made him more excited about researching what really happened to cause the KT boundary.

He began a permanent job as a professor at the University of California, Berkeley, and continued to collaborate with his father on researching the mass extinction. Their formal research question was, “Did the clay bed represent a few years, or a few thousand years?” (64). To do so, they needed to find minerals in the clay that had been deposited into the limestone and the clay bed at a constant rate. Meteorite dust constantly settles on Earth, so the Alvarezes looked for the platinum-group elements that could have been deposited from this dust. If the clay bed had formed rapidly, in just a few years, Alvarez guessed it would be free of these elements. If it had formed over thousands of years, however, they would find some meteor-derived platinum-group elements—particularly iridium.

Alvarez consulted with Frank Asaro, a nuclear chemist who also worked at Berkeley, to perform an analysis on the Gubbio rock samples. The results shocked both Asaro and Alvarez: The clay sample contained an unusually high amount of iridium. They wondered what could have caused so much iridium to enter the clay and considered interstellar dust and gas, a supernova explosion, or a meteor impact. To verify that this was a global, not local, phenomenon, Alvarez and Asaro studied the clay from a site in Denmark and found a similar amount of iridium.

The author entertained the possibility that, as Dale Russell suggested, radiation caused by a supernova explosion killed the dinosaurs and other animals. Supernova explosions are sudden and create intense light and radiation. While such explosions were essential to Earth’s formation (given that Earth began as debris from this kind of explosion), if such an explosion occurred near Earth, it would change the climate and dangerously radiate plants and animals (72). This explanation seemed possible to astronomers and physicists but was unimpressive to geologists, who preferred uniformitarianism thinking and did not understand how a supernova’s activities would be preserved in the rock.

Asaro and his colleague Helen V. Michel analyzed the samples for plutonium-244, which a supernova explosion would deposit. Their tests were positive, and Alvarez and his father inferred that this was proof that a supernova explosion had killed the dinosaurs. However, to their great disappointment, in a second analysis with new samples, no plutonium-244 was present, so they ruled out the supernova theory. Alvarez considered the possibility of a giant asteroid impact, but was skeptical about whether this was really enough to cause a global mass extinction event that killed off about half of all species. A year passed, and he and his father continued to discuss the problem. As Alvarez continued his research in Italy, his father began to consider the impact hypothesis more seriously as he thought about how much dust it would generate and the indirect effects it would have on the whole planet.

Alvarez presented his findings on the KT boundary clay bed at a conference in Copenhagen, Denmark; however, he was cautious about articulating their latest hypothesis, the impact theory. He anticipated much criticism and controversy about his ideas. At the conference, Dutch geologist Jan Smit revealed to Alvarez that he, too, was intrigued by the KT boundary clay layer and that he had also done an analysis that revealed high iridium levels in the clay. Alvarez was grateful that Smit was willing to share his knowledge openly and did not rush to publish his own study in order to take credit for the idea. Soon after the conference, more geologists began to publish their own iridium findings, adding support to Alvarez’s findings. Before they could prove that an asteroid impact caused these changes, however, Alvarez and his colleagues had much more work to do.

Throughout the 1980s, the impact hypothesis intrigued geologists, hundreds of whom weighed in by publishing their own papers and theories. Professionals from other disciplines, such as geochemists, paleontologists, and astronomers, were also interested in this mystery and brought their own expertise to the research. In 1981, Alvarez participated in a meeting in Utah where earth science professionals from these different backgrounds convened to discuss the research and learn how to communicate with each other. Alvarez found this collaboration enjoyable and necessary to solve the impact question. In the broader field of geology, most of the debate about the impact hypothesis was civil, but the theory’s controversial nature sparked conflict among some geologists. Because of the media interest in the research, people outside the field became more aware of the ongoing research and debate.

Instead of presenting the events through the lens of who was right and who was wrong, Alvarez prefers to focus on the mystery that geologists and other professionals solved together: the location of the impact crater. He compares nature to a mystery writer who left tantalizing but often confusing clues about the location. One critic of the theory was Bill Clemens, who did field work in Montana and specialized in dinosaur remains from the late Cretaceous period. Clemens sent in samples from this region that, like the other samples, revealed iridium at the time of dinosaurs’ extinction. Alvarez noted that no dinosaur bones were found above the KT boundary layer, indicating that the event that deposited the iridium in the clay must have killed the dinosaurs. He and Clemens disagreed about what this indicated. Similarly, geologist Chuck Pillmore found dinosaur footprints in New Mexico below the KT boundary but none above it. The fossil record of other creatures, such as ammonites, likewise shows that they were abundant before the KT boundary and disappeared afterward.

By 1980, geologists had identified over 100 craters on Earth, and Canadian geologist Richard Grieve had made a list of craters that asteroid impacts definitively caused. Alvarez was puzzled because none of these craters seemed big enough to fit his hypothesis; he estimated that such a crater should be 150-200 km wide. Earth’s existing craters are all relatively recent, because early impact sites have long been obscured through the constant movement of Earth’s plates and changes on its surface. Alvarez believed that such a huge crater should be obvious if it was on land, so he inferred that it was either covered by ice near the poles, sunk in the ocean, or destroyed by plate tectonic movements. An asteroid impact would create totally different effects and debris depending on whether it landed on a continent or an ocean, since their crusts have different minerals and chemicals.

Jan Smit found debris from the “target rock,” that is, the rock that the asteroid hit, in samples from Caravaca, Spain. He called these little white, round grains “spherules,” and he was surprised to discover that they contained a rare mineral called sanidine and had a feathery structure. In addition, Smit’s analysis revealed that the target rock was oceanic crust, so Alvarez deduced that the crater should be in the ocean. He wondered if the crater could be in a remote and unexplored part of the ocean floor, but because about 20% of the ocean floor of the Cretaceous period has since been subducted, or “swallowed up into the deep Earth” (95), Alvarez realized that they may never locate the crater at all. Then, other geologists found quartz that the impact’s shock waves had affected, a discovery that pointed toward continental impact.

In hindsight, Alvarez admits that nature had “fooled” him and that he had come to an obvious conclusion without considering other factors. He explains that the asteroid actually landed on a piece of continental crust in the Yucatan peninsula and melted this crust together with sedimentary rocks, creating a chemical makeup similar to oceanic crust.

In the 1980s, geologists fell into one of two camps: those who believed that the mass extinction was caused by an asteroid impact, and those who blamed volcanic activity, namely from the Deccan traps in India. Some experts further muddied the waters by suggesting that mass extinctions happen at periodic intervals, potentially because of an unknown companion star to the sun whose activity could trigger comet showers at regular intervals. However, such a hypothetical companion star has never been found.

Alvarez’s research continued, and he and his team responded to numerous challenges and criticisms of their theory. By sampling more rocks, Alvarez confirmed that the iridium deposits at the KT boundary were anomalous. They then entertained the idea that they might need to find several small craters rather than one large one, reflecting the possibility that a comet shower could impact Earth via multiple asteroids over the course of a million years. Around this time the team located a crater site about 35 km across in the town of Manson, Iowa, that dated to the late Cretaceous period. Alvarez then believed that the mass extinction could have been triggered by two impact hits, the one in Manson, and one in an ocean crust that had since disappeared.