For decades, the sudden disappearance of the dinosaurs at the end of the Cretaceous period remained one of geology’s most stubborn enigmas. The dominant paradigm favored a slow, gradual decline driven by long-term climatic shifts and volcanic activity. This consensus was abruptly challenged in 1980 by Luis and Walter Alvarez, who proposed that a giant asteroid impact was the primary driver of the mass extinction.
The strength of the Alvarez hypothesis lay not just in its revolutionary claim, but in the meticulous sequencing of its supporting evidence. The argument began with a singular, anomalous finding: a thin layer of clay at the Cretaceous-Paleogene (K-Pg) boundary in Gubbio, Italy, which exhibited iridium concentrations thirty times higher than normal. Because iridium is rare in Earth’s crust but abundant in chondritic meteorites, the authors posited an extraterrestrial source. To establish that this was not a localized anomaly, they quickly followed this finding with data from Denmark and New Zealand, confirming the global distribution of the iridium spike.
However, a global iridium layer was merely circumstantial evidence of an impact; it did not prove that the impact caused the extinction, nor did it locate the collision site. To tighten the causal chain, the narrative of evidence shifted from chemical anomalies to physical shock dynamics. In the late 1980s, geologists identified shocked quartz grains—crystals showing microscopic deformations characteristic of high-pressure shock waves—within K-Pg boundary sediments worldwide. These mineralogical signatures could not be produced by volcanism, effectively narrowing the causal mechanism to an impact event.
The final piece of the argument required locating the physical crater. Proponents of the theory pointed to deposits of tektites—glassy spherules formed from cooled rock melt—that grew progressively thicker in sediments surrounding the Caribbean basin. This spatial gradient directed researchers toward the Yucatan Peninsula, where the buried Chicxulub crater was finally confirmed in 1991. By sequencing the evidence from chemical anomalies, to shock-deformed minerals, to spatial geological gradients, the Alvarez team built a cumulative case that successfully shifted the scientific consensus from gradualism to catastrophism.
Which of the following best describes the transition in the sequence of evidence from the global iridium layer to shocked quartz grains?
- establishing a global chemical anomaly that suggests an extraterrestrial origin, to presenting physical mineralogical proof that rules out a competing volcanic explanationAnswer
- Blocating the precise impact site of the asteroid, to tracing the global path of shock waves that distributed mineral debris across multiple continents
- Cproposing the primary claim that mass extinctions are driven by volcanic activity, to offering geological details that demonstrate gradual climatic shifts
- Ddescribing the general biological impact of the extinction event, to isolating the specific chemical components of chondritic meteorites