How can I teach mass extinctions using the Chicxulub impact?
Use the evidence chain, iridium layer, shocked quartz, spherules and the buried crater, to show how scientists linked an asteroid to the end-Cretaceous extinction.
The extinction at the end of the Cretaceous period, about 66 million years ago, wiped out the non-bird dinosaurs and many other species. The story of how scientists linked it to an asteroid impact is an excellent case study.
The hypothesis. In 1980, a team led by the physicist Luis Alvarez and his son, the geologist Walter Alvarez, reported that a thin clay layer at the boundary between Cretaceous and later rocks, found in Italy, Denmark and New Zealand, contained unusually high levels of iridium. Iridium is rare in Earth's crust but more common in asteroids. They proposed that an asteroid about 10 kilometres across had hit Earth.
More evidence. Other scientists found shocked quartz, which forms only under extreme pressure, and tiny glass spherules in the layer. Tsunami deposits were found around the Gulf of Mexico.
The crater. In 1991, scientists identified the Chicxulub crater, about 180 kilometres across, buried beneath the Yucatán Peninsula in Mexico. Its age matched the extinction. Drilling in 2016 sampled its peak ring.
Effects. The impact would have caused fires, tsunamis and a global cloud of dust and sulfur gases that blocked sunlight, cooling the climate and disrupting food chains.
Debate. Massive volcanic eruptions in India, the Deccan Traps, happened around the same time. Scientists continue to discuss their role.
Activities. Pupils sort evidence cards, build a timeline and evaluate the strength of each piece of evidence.
This topic links geology, biology, chemistry and the nature of science.
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