Sixty-six million years ago, a global catastrophe affected Mesozoic Earth – which was then dominated by reptiles – transforming it to a Cenozoic Earth brimming with mammals and birds. What caused that catastrophe?
Meteorite impact or volcanic eruption are proposed explanations, but the debate over which of these hypotheses makes more sense is still hot. Let’s take a look.
Meteorite Impact Analyses
The meteorite impact hypothesis came about in 1980, when Nobel laureate physicist Luis Alvarez and his geologist son Walter Alvarez suggested that a meteorite the size of Mount Everest hit Earth 66 million years ago, drastically changing the climate and causing mass extinction. Alvarezes’ evidence came from a clay bed at the K-Pg contact near Gubbio, Italy, which was rich in iridium – a rare find on Earth but common on asteroids. K-Pg contacts in many other parts of the world also show iridium enrichment.
The search for the impact site eventually led to the Chicxulub Crater, a circular feature 180-km wide and buried 2,000 meters beneath Cenozoic sediments in the Yucatán Peninsula. Later drilling confirmed it to be an impact structure, and Mario Fischer-Gödde and colleagues recently published results in Science from ruthenium isotope analysis of spherule samples collected at the Chicxulub impact site. These data indicate that the Chicxulub meteorite was carbonaceous chondrite belonging to the C-type asteroids at the outer edge of the asteroid belt.
Several studies have shown that the relative abundances of platinum group elements (PGEs) at the K-Pg boundary layers are more consistent with chondritic meteorites than volcanic eruptions. High concentrations of PGEs (including ruthenium, rhodium, palladium, osmium, iridium and platinum) have been observed in chondritic meteorite impactors.
A 2025 study published in Global and Planetary Change examined PGEs in a K-Pg layer on Gorgona Island offshore Colombia. Researchers found that these elements were carried around the world with metals such as cobalt, nickel and copper in the form of new crystals embedded in a matrix of iron-rich calcium and silica glass. This indicates a mix of the meteorite and vaporized Earth rock.
Nadir Crater: A New Culprit
Recently, a study by Nicholson and colleagues reported the discovery of another possible K-Pg meteorite impact site offshore Guinea in West Africa. Published in Science Advances and Communications Earth & Environment, the discovery describes a circular, nine-kilometer, fault-bounded structure named the Nadir Crater. It is now buried under 900 meters of water and 300 meters of ocean floor sediments, according to maps on seismic images.
Whether Nadir is an impact structure, or a volcanic feature is yet to be proven by detailed rock sample studies including PGE analyses. An IODP proposal to drill into the Nadir structure, if conducted, will resolve this issue. If both Chicxulub and Nadir are confirmed to be meteorite impacts of the K-Pg boundary, it could mean that the killer asteroid broke up into two or more fragments while heading to Earth.
The Case for Deccan Volcanism
Published by Liu Xuemin in Earth & Planetary Science Letters, analyses of K-Pg boundary samples in the Sichuan Basin in China reveal interesting results. The Sichuan site contains an inch-thick, brick-red layer enriched in PGEs characteristics of meteorites. However, another K-Pg anomalous layer, just 20 inches below the red sand, displays high chemical alteration by acidic fluids, probably related to Deccan volcanic gases.
Some geologists, including Vincent Courtillot, Paul Renne and Gerta Keller, have suggested the flood basalts of the Deccan Traps in India as another potential culprit for the mass extinction. These flood basalts formed 66 million years ago when India, separated from Gondwana, was passing over the Reunion hotspot in the southern Indian ocean.
Two geochronologic studies of the Deccan Traps, published in Science, suggest that more than 90 percent of Deccan volcanism occurred within one million years, from 66.5 to 65.5 Ma. One study by Sprain and colleagues was based on 40AR/39AR dating, and the other study by Schoene and colleagues used U-Pb zircon dating. However, 75 percent of the volcanic volume erupted after the K-Pg extinction.
The fact that both meteorite impact and Deccan volcanism occurred at 66 Ma means that both had tremendous impacts on the environment – from initial global darkness and winter (sunlight blocking) to acid rain, greenhouse effects and ocean acidification. This is a good example of the “multiple working hypotheses method in geologic research. Further studies may continue to shed light on which hypothesis seems more likely.