T. Rex and the Crater of Doom

Author Walter Alvarez explains that geologists and paleontologists are “Earth historians” who use rocks as clues to understand the formation and evolution of Earth. Early geologists tended toward “catastrophism,” believing that violent, sudden events largely shaped Earth, but over time more professionals in geology embraced “gradualism,” or the theory that the planet developed through small, gradual changes. Today, geologists understand that both theories are correct: Earth has mostly been shaped by small, cumulative changes over long periods of time but has also been profoundly influenced by abrupt, catastrophic changes.

His book, T. Rex and the Crater of Doom, explains how an asteroid impact that occurred 65 million years ago on the Yucatan peninsula of Mexico triggered the extinction of many plant and animal species, including the Tyrannosaurus Rex and other dinosaurs. In addition to the scientific story of the impact and its consequences, the book explains the process of discovery and the controversy within the field of geology. Alvarez believes that the success of the impact hypothesis stemmed from the cooperation of different scientific disciplines, and he argues that scientists must integrate different kinds of knowledge to gain a “holistic understanding of Nature” (xx).

Alvarez describes the landscape of the Cretaceous period, which, like modern Earth, had various landforms and ecosystems such as deserts, mountains, rivers, oceans, and forests spread throughout a geography similar to the modern continents. Much of the land was forested and was populated by small mammals, and larger predators, mostly dinosaurs. These plant and animal species evolved slowly over about 150 million years and would have continued to do so if not for the abrupt and devastating impact of a huge asteroid.

The author explains that the impact that ended the Cretaceous period could have been a comet or asteroid and that it was about 10 km in length and traveled toward Earth at tens of kilometers per second. Comets and asteroids are common parts of the solar system, but most remain in particular orbits; most asteroids are confined to a belt between Mars and Jupiter, and most comets circle the sun. If they enter Earth’s atmosphere, small comets and asteroids burn up. Larger ones can penetrate the atmosphere and impact Earth, however, this is now rare because the early solar system sorted out most of this “debris.” However, on this rare and consequential occasion, the path of a huge asteroid or comet happened to intersect with Earth. Imagining the violent aftermath of this collision is difficult because it was many magnitudes greater than any modern natural disaster. Traveling at about 30 km per second, the 10-km-long asteroid hit Earth, becoming a “cataclysmic weapon” (7) and producing a shock wave in the rock, which was crushed, melted, or even vaporized.

Alvarez breaks down the event, beginning with the “moment of impact” (9). The asteroid cleared Earth’s atmosphere in one to two seconds, creating a sonic boom and flash of light by compressing the air in its way, and therefore heating it, as it descended toward land. The asteroid’s collision with Earth instantly created a massive crater, which the asteroid sank into, and simultaneously created two shock waves, one going forward into the limestone rock and another going backward through the asteroid’s tail end. The impact vaporized the asteroid, along with much of the rock it hit. This vaporized rock then exploded into a “colossal fireball” (10) that blasted rock particles through the atmosphere and into the air, where they were carried around the world before falling to the ground. Because the asteroid had hit limestone, a rock that consists of carbon dioxide and calcium, the impact created a second fireball, which also exploded and sent rocks flying through the atmosphere. Today, the crater is shaped like a bull’s-eye because of the perimeter’s steep outer walls, where rock collapsed inward and created a ringed pattern due to how the granitic crust first rebounded upward in the center of the crater and then collapsed downward again.

The author then examines the “ring of devastation” (10). He refers to the impact area as “ground zero,” explaining that no plant or animal within hundreds of kilometers could have survived the impact. Soon after, the sky became intensely hot and bright as the impact exploded particles into the air, and surrounding forests within thousands of kilometers caught fire, releasing smoke into the atmosphere. Additionally, the asteroid’s impact triggered a huge tsunami, which devastated the forests all along the Gulf of Mexico, and triggered submarine landslides. While North America was the most dramatically devastated by the asteroid, the debris particles and secondary effects had devastating repercussions on the rest of the world.

Alvarez explains why this particular impact triggered a mass extinction event. The impact generated huge amounts of dust, which settled in the atmosphere and therefore blocked the sunlight. Earth suddenly became much colder and darker. After a few months, the dust dissipated. Earth then suddenly became much hotter due to the increase in light, in combination with the carbon dioxide and water vapor that the asteroid impact released. The water vapor and rock dust returned to Earth in the form of noxious acid rain, which dissolved rocks and killed plant life.

In the following years, a mass extinction event ensued, affecting about 50% of all of Earth’s genera. Scientists have inferred that most dinosaurs died because of the darkness and cold that followed the asteroid’s impact. In addition, these conditions killed many plants, thereby removing the food source of herbivorous dinosaurs and, in turn, the carnivorous dinosaurs that preyed on them. Only one kind of dinosaur—the pterodactyl, the ancestor of modern birds—survived this extinction event. Smaller reptiles like crocodiles and turtles, as well as many small land mammals, survived, though it is unclear how. Many species of land plants, marine invertebrates, single-celled marine plants and animals, and the coil-shelled ammonite also died out, disrupting food chains and dramatically altering the balance of life on Earth.

Alvarez observes that this incredible event “truly marked the end of a world” (16) as the dinosaurs suddenly disappeared after a 150-million-year existence as Earth’s main land animals. Because of the sudden absence of dinosaurs, mammals filled new niches and evolved into larger animals. The chapter concludes with a reminder that these events are factual and can be verified by examining the information encoded in Earth’s rock.

Humans have used written language for only about 5,000 years, so written documents detail a relatively tiny fraction of human history and the history of the planet. Therefore, geology, or the study of rocks, helps in understanding prehistoric times before the invention of writing. Geology itself is a relatively new discipline, invented only a couple centuries ago. Alvarez laments that rocks are somewhat underappreciated as subjects of study and sources of information because they seem less dynamic than plants and animals; however, rocks are the best sources of information about the prehistoric past. Rock layers generally show strict stratigraphy: Younger layers are closer to the surface, while older layers are deeper down.

Alvarez points to the town of Gubbio, Italy, as an example of how valuable information can be embedded in rocks. In this mountainous town are layers of “Scaglia Rossa,” a pink, sedimentary rock largely made of calcite, or limestone. Within this rock are tiny microfossils of an extinct, shelled marine species called foraminifera. Alvarez explains that their presence in the rock indicates that this limestone was once deep-sea sediment, the best for recording natural history because little erosion occurs on the ocean floor, and therefore layers can build up undisturbed. Over time this rock layer was pushed upward until it became land.

Like historians, geologists use both specific dates and general ages to refer to events in Earth’s history. The author provides a chart that shows the eras of life on Earth, from its beginning 4,600 million years ago to the present. Within each of the five eras are smaller periods. The fifth mass extinction marked the end of the Cretaceous period, the third and final period of the Mesozoic era, and the beginning of the Cenozoic era, which is the current one. Because Earth has existed for so long, geologists’ typical unit of time is millions of years. From a geological perspective, the Cretaceous mass extinction was a relatively recent event.

The stratigraphy of Earth’s rock layers makes it fairly easy for geologists to establish a chronological sequence of events. The law of superposition dictates that younger layers form on top of older layers. Determining the numerical age of rocks is much more difficult. Scientists can use radioactive minerals to date igneous rocks by examining how the radioactive atoms in these rocks have decayed. This provides a “reliable clock” because “radioactive decay takes place at an unvarying rate” (33). Dating sedimentary rocks is more challenging: They cannot be numerically dated with accuracy unless they have some mineral content, such as from volcanic ash that settled on them. However, fossils formed in sedimentary rocks provide valuable clues to understanding the chronology of those layers.

In the 1800s, geologists observed that fossils in different layers of sedimentary rock slightly differed from each other. These differences were a mystery until Charles Darwin’s theory of evolution and natural selection explained how animals evolve over time. With little technology to help them, 19th century geologists relied on invertebrate fossils, such as ammonites, as dating tools because they were plentiful and large enough to see without a microscope. Geologists still rely on such fossils to understand changes in time periods and evolution. Foraminifera, tiny shelled single-celled marine organisms, are particularly helpful for this because they are fossilized in the sedimentary rock in ocean floors around the world. Limestone from the ocean floor often has neat layers of rock and fossils because these layers have accumulated undisturbed for tens of millions of years. These sources are not necessarily still underwater: For instance, Gubbio, Italy, has an excellent pelagic (oceanic) limestone in its Bottaccione Gorge.

In the 1960s and 1970s, as Alvarez was beginning his career as a researcher, geologists were beginning to understand more about Earth’s magnetic field. Mineral grains in rock record the direction of Earth’s magnetic field at the time they become embedded in rock. This phenomenon creates what scientists call a “fossil compass.”

When geologists realized that rocks indicated different magnetic directions, this helped them prove that Earth’s tectonic plates have not been stagnant but have actually been constantly moving. Interested in the movement of plate tectonics in the Mediterranean, Alvarez teamed up with geologist Bill Lowrie to prove the rotation of microplates in that region. While they could not find specific evidence to support their theory, the two did find proof of magnetic reversals encoded in the rock. Intrigued, Alvarez and Lowrie collaborated with other professionals from Princeton to record the magnetic reversals in the Scaglia rock in Italy, creating a timeline of “geomagnetic polarity history” (38). Earth’s liquid iron core, which the author compares to a magnet bar, creates the planet’s magnetic field. A magnetic reversal occurs when the directionality of Earth’s magnetic field switches from north to south and back again. Scientists still don’t understand why this periodically happens.

In doing this research, Alvarez became more familiar with the “KT Boundary,” which is the distinction between the Cretaceous rock layer and the subsequent Tertiary rock layer. He became more interested in why a one-centimeter-thick clay layer between these rock layers and noticed that every sample contained many more foraminifera in the Cretaceous layer than in the Tertiary layer. Learning that this abrupt shift in rock layers coincided with the dinosaurs’ extinction, Alvarez began to wonder how the two might be connected, feeling that it was a “world-class scientific problem” (41). The author’s geology education had steeped him in “the doctrine of ‘uniformitarianism’” (41), yet he wondered if the abrupt extinction of many foraminifera at the KT Boundary indicated a major catastrophe at that time.