T. Rex and the Crater of Doom

Nonfiction | Book | Adult | Published in 1997

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Summary

Background

Chapter Summaries & Analyses

Foreword-Chapter 2

Chapters 3-5

Chapters 6-7

Key Figures

Themes

Index of Terms

Important Quotes

Essay Topics

Tools

Beta

Discussion Questions

Chapters 6-7Chapter Summaries & Analyses

Chapter 6 Summary: “The Crater of Doom”

Throughout the 1980s, geologists compiled more evidence that supported the impact hypothesis, but Alvarez still had not found an impact site. Still believing it was in the ocean, he looked for signs of a massive deep ocean tsunami from this time period. Initial studies showed that sites in Haiti and Texas contained sediments that pointed to a possible tsunami. In the late 1980s, geologist Jody Bourgeois showed that the Brazos River site in Texas had been the site of a tsunami right at the KT boundary.

While Alvarez researched the Deccan volcanoes and the crater in Manson, Iowa, Canadian geologist Alan Hildebrand began exploring the Brazos River in Texas, and the gulf of Mexico and Caribbean sites. Hildebrand was specifically looking for tsunami evidence and any signs of a crater. Inspired by Hildebrand’s interest in this region, Alvarez considered a new way to look for tsunami damage in the rock record by examining rock layers for a gap, or “unconformity,” that tsunami erosion would have caused. As Hildebrand’s research continued, Alvarez took it more seriously. In 1991, collaborating with other geologists, Hildebrand revealed in a published paper that the Chicxulub crater on Mexico’s Yucatan peninsula was the result of a late Cretaceous asteroid impact.

Alvarez reveals that in the mid-20th century, the Mexican government did some geological work on the Yucatan peninsula while searching for sites that the national oil company, PEMEX, could drill for oil. The team was puzzled by the results of its deep drilling, which turned up crystalline rocks rather than oil deposits. In hindsight, Alvarez explains that those unusual rocks were “impact-melt rocks” (113). In 1981, PEMEX geologists Glen Penfield and Antonio Camargo Zanoguera theorized that this area could be an impact crater, but because they worked for an oil company they did not widely disperse their research and gave only one talk on the subject. Once Alan Hildebrand published his work, however, Penfield and Zanoguera openly shared their own knowledge about the area, providing a detailed report on the KT crater.

After learning about this research, Alvarez traveled to northeastern Mexico and located a “KT boundary outcrop” (115) of rock near Chicxulub to study, hoping to verify that it held signs of an impact-related disturbance. While Alvarez had to return home for other professional obligations, Jan Smit and other colleagues studied the rock layers. At the KT boundary they found deep-sea sediment that had been moved by the tsunami, impact spherules, and target rock limestone. Above that lay coastline sand and wood from the original beaches and forests of the Cretaceous period that the tsunami had moved, and on top of this was clay and fine sand from the successive waves that washed over the region. Importantly, this layer had the same iridium anomaly as the other KT sites.

Researchers were excited to find streaky-colored glass at many such KT sites around the Yucatan. This evidence supported the impact hypothesis because volcanic eruptions tend to make homogenous glass, whereas impact glass, like that found at the sites, freezes quickly and therefore has streaky colors. In another stroke of luck, the oil company PEMEX located the rock samples from its deep drilling and shared them with researchers. These crystalline, melted rocks from the crater provided further evidence of an asteroid impact.

However, not all geologists accepted the impact hypothesis based on this evidence, and Alvarez and his team wanted to provide more definitive evidence to win over these critics. He traveled to northeast Mexico and, with the help of his colleagues and PEMEX geologist Jose Manuel Grajales, located nine more KT boundary outcroppings. They gathered samples from these sites to bring home to the US to study, and Alvarez left this second field trip to Mexico with a sense of completion, feeling certain that these samples could provide more definitive proof of a late-Cretaceous-period impact and its consequences.

Chapter 7 Summary: “The World After Chicxulub”

Alvarez reiterates that the late-Cretaceous-period asteroid impact ended the “stable reign of the dinosaurs” (130) and threw Earth’s ecosystems into chaos, allowing the surviving species of mammals to occupy new ecological niches vacated by extinct species. He reflects on how early humans were among these surviving mammals and ponders why these species survived when the dinosaurs didn’t. He theorizes that perhaps because the mammals were smaller they were also more numerous and thus had better odds of surviving as a species.

Scientists have continued to piece together more information about this giant impact, including its velocity, shock waves, and temperature changes, and are using this information to draw conclusions about giant impacts in general. Since the Chicxulub crater was discovered, geologists have been continually working to understand more about it, performing deep drilling and taking seismic lines and gravity lines. Some of these studies will help them create a 3D picture of the crater. Geologists have tried to find the KT boundary outcropping closest to the crater itself, studying the region of Chiapas in particular. Alvarez and some of his colleagues went to Albion Island in 1995, where they may have found some of the “ejecta blanket,” or sediment that the impact generated, in the rock layers.

Alvarez has made calculations about the “ballistic trajectories” of the ejecta in order to predict where most of it would have landed. He consulted Susan Keiffer, an expert in fast-moving geological processes such as meteor craters and volcanic eruptions. Keiffer helped Alvarez understand how the Chicxulub impact would have created an initial fireball of vaporized rock, followed by a second fireball of C02 vapor.

Like the plate tectonic revolution, the discovery and study of the Chicxulub crater has revolutionized geology. Geologists now understand that most of Earth’s development and change is gradual, but that catastrophic events can also significantly shape the planet. Scientists continue to study craters and their ejecta in rock layers around the world. In fact, another impact crater may be implicated in a different mass extinction, but so far no substantial physical evidence supports this idea.

Another factor in Earth’s changes that has always interested geologists is volcanism. Unanswered questions remain about how the volcanic activity in the Deccan traps region of India may have coincided with the extinction event that ended the Cretaceous period. Similarly, an extinction event in the earlier Permian-Triassic period coincided with massive volcanic activity in the Siberian Traps region. This has puzzled geologists, who are still working out the connections between volcanic activity, extinctions, and impacts. In addition, geologists have participated in documenting impact craters on the moon and even observing known impacts on other planets, all of which helps build their understanding of Earth’s significant impacts.

Chapters 6-7 Analysis

In the book’s final chapters, Alvarez brings the mystery of the Yucatan impact to a satisfying conclusion, providing insight into each specific clue that he and his colleagues encountered. In doing so, he advances the theme of Rocks as Historical Records. His detailed explanation of each rock component gives readers, even those without a geology background, a basic understanding of the various elements inside rock layers. For instance, he describes “spherules,” or tiny, round white objects in the rocks, as important indicators of the gases and rock content that combined during the asteroid impact:

Our samples were packed with spherules, but every one we looked at was altered. We were impressed with the tiny bubbles that occurred in most of the spherules and could still be seen, despite the alteration. We suspected that they were due to the carbon monoxide gas that would be given off when limestone and dolomite, the surface layers in the target rock at Chicxulub, are shocked by an impact (121).

Similarly, the melted glass found in rock layers at the crater confirmed that it resulted from an impact rather than a volcanic event: “This alone argued for an impact event, because impact melts quickly freeze, whereas volcanic melts stay liquid for a long time and are almost always homogenous” (121).

Additionally, the details about how numerous professionals combined their expertise to understand the physical evidence of the asteroid impact help emphasize the author’s firm belief in the value of interdisciplinary collaboration. For instance, volcanologist Susan Kieffer helped Alvarez understand how the impact generated two fireballs and how their contents spread over Earth. Through her insight, Alvarez was better able to understand the impact’s aftermath: “Sue’s calculations gave just the dynamics Philippe and I had inferred from the pattern of the ejecta. It was satisfying to see details of the impact event fall into place so neatly” (138). Likewise, interdisciplinary collaboration helped Alvarez learn more about the Yucatan as a possible impact site. He humbly acknowledges that his focus on ocean sites had sidetracked his search for the crater, and he credits geologist Alan Hildebrand with locating the correct crater site. He praises Hildebrand’s creative thinking and solid research:

At that point, what was needed was someone absolutely focused on finding the source of the Brazos tsunami—someone who was persuaded that the Brazos sand bed was the fundamental clue and who would not rest until the culprit was tracked down […] It was Alan Hildebrand who would be that relentless detective” (109).

Once Alvarez and a team of geologists began to research Mexican sites more thoroughly, they benefited from the insight of the local geologists with whom they collaborated. For example, Manuel Grajales realized that the name of one village meant “paving stone” in Spanish, which indicated that it was on a “thick bed of sandstone” (126). This clue helped the researchers find another sandstone outcropping that yielded evidence from the KT boundary. The author recalls how trading knowledge with local experts enriched his research experience: “Every evening over dinner, Ricardo, Pedro and Eduardo would tell us about the geology and the history of northeastern Mexico, and we would tell them about the KT mass extinction and the search for the site of the impact” (127).

In addition, these passages help support The Role of Catastrophes in Earth’s History as a theme. The author builds this theme by noting that the research on the Yucatan impact provoked an important shift in the field of geology. Having finally discovered numerous types of persuasive evidence supporting the impact hypothesis, Alvarez and his colleagues finally convinced their more uniformitarian colleagues that catastrophes, while only occasional events, nevertheless have a massive influence in the development of Earth’s geology, geography, and ecosystems. Alvarez and his colleagues showed that a single catastrophe, the Yucatan impact, had the power to completely change Earth’s evolutionary trajectory by triggering a mass extinction event. The author emphasizes the consequential nature of this ancient catastrophe:

After the impact at Chicxulub, 65 million years ago, life on Earth was changed forever. The long-standing and stable reign of the dinosaurs had been destroyed by a chance event. The new world was inherited by a different cast of characters, and the previously insignificant mammals came to dominate life on the land” (130).

Alvarez hints that more will soon be known about catastrophic events, since more scientists are dedicating their time to understanding impacts and volcanic eruptions.