Final answer:
Geologists tested the Alvarez hypothesis by investigating the global iridium spike at the K-Pg boundary, examining extinction patterns, and identifying shocked quartz and other high-impact minerals in the clay layer. The 1991 discovery of the Chicxulub crater provided critical support for the asteroid impact theory as the cause of the iridium layer and mass extinction event.
Step-by-step explanation:
Geologists tested the Alvarez hypothesis that an asteroid impact caused the formation of a 66-million-year-old layer of clay containing high levels of iridium, by looking for evidence that could support or refute this idea. They examined the global iridium spike within the sedimentary layer at the K-Pg boundary, considered unusual due to the rarity of iridium in Earth's crust. Researchers believed that a sudden influx of iridium from space to the atmosphere—likely from a large asteroid—led to this spike.
Further supportive evidence included the disappearance of many species at the K-Pg transition, indicative of a mass extinction event. The presence of shocked quartz and other minerals that form under high temperature and pressure, consistent with an asteroid impact, was found in the boundary clay layer. In 1991, the discovery of the Chicxulub crater in the Yucatán peninsula provided a crucial piece of evidence supporting the asteroid impact theory. This crater matched the age and the massive energy release needed to disperse iridium and other asteroid-related elements globally.