T. Rex and the Crater of Doom

In the area now known as the Yucatan peninsula of Mexico, an asteroid collided with Earth 66 million years ago, forming the Chicxulub Crater. This crater is about 200 kilometers in diameter and 20 kilometers deep. The book explains how Alvarez and other geologists used the scientific process to conclusively establish that this crater represents the impact that effectively ended the Cretaceous period and the Mesozoic era and resulted in the extinction of the dinosaurs.

The third and last period of the Mesozoic era, the Cretaceous period began 145 million years ago and ended 66 million years ago with the asteroid impact that formed the Chicxulub Crater on what is now Mexico’s Yucatan peninsula. The book explains how this impact ended the Cretaceous period by triggering a mass extinction event that abruptly changed Earth’s species and ecosystems.

The plural form of foraminifer (or foram), foraminifera are single-celled, shelled marine creatures. These primitive species have lived for millions of years and continue to exist today. There are tens of thousands of species of foraminifera, both extinct and living. Alvarez explains that foraminifera are important sources of information to geologists because their fossils are encased in sedimentary rock. By examining the types of foraminifera in rock, geologists can date the rocks to certain periods. When Alvarez observed that there were many foraminifera in the rock layer dating to the Cretaceous period but very few in the rock layer dating to the subsequent Tertiary period, he hypothesized that a catastrophic event may have abruptly reduced the species. This hypothesis was the beginning of his impact hypothesis.

Igneous rocks containing mineral content can reveal the direction of Earth’s magnetic fields at the time that the rock formed, effectively functioning as fossil compasses. Such fossil compasses were critical in helping scientists understand plate tectonics, since their varying directions showed that Earth’s plates had rotated. Alvarez was initially interested in using fossil compasses to research Mediterranean plate tectonics before developing his impact hypothesis.

The plural form of genus, genera are categories of classification for living things. A genus is more specific than a family of plant or animal but not as specific as a species. For example, the genus Panthera, within the family Felidae, contains the five big cat species: lions, tigers, jaguars, leopards, and snow leopards.

See uniformitarianism.

Impactor is another term for an asteroid or comet that impacts Earth. The text postulates that an impactor caused an extinction event affecting most life on Earth at the end of the Cretaceous period, the last period of the Mesozoic era.

The word Mesozoic literally means “middle life.” The Mesozoic era of Earth’s history began 252 million years ago and ended 66 million years ago. Within the Mesozoic era were three periods: the Triassic, the Jurassic, and the Cretaceous. The Mesozoic era is significant in Alvarez’s work because the Yucatan impact occurred at the end of the Cretaceous period, thus ending that period and the Mesozoic era as a whole.

A school of thought within geology, uniformitarianism (also known as gradualism) holds that Earth was formed through small, gradual changes over long periods of time. Alvarez explains that his impact hypothesis ran counter to uniformitarian thinking, which was dominant in the field in the 1970s and 1980s, because it proposed that a single catastrophic event could have changed the whole planet forever.