A school of thought within geology, catastrophism holds that violent, sudden events, such as volcanic eruptions and asteroid impacts, largely shaped Earth. This school of thought was popular among early geologists in the 19th century. In the first half of the 20th century, uniformitarianism (a belief that Earth formed via slow, gradual changes over many eons) took precedence, but Alvarez’s impact theory regarding the dinosaurs’ extinction slowly swung opinion back toward catastrophism. Today, scientists understand that both schools of thought are valid in explaining Earth’s history and continuing evolution.

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.