Chicxulub and Deccan volcanism at the end of the Cretaceous

The end-Cretaceous extinction coincided with two extraordinary geological events: the Chicxulub impact in what is now Mexico and the eruptions that built the Deccan Traps in India. The events overlapped, so asking which one mattered cannot be answered by pointing to a single layer or lava flow. Researchers compare their timing, physical traces and environmental effects with the abrupt disappearance of many species at the Cretaceous–Paleogene boundary.

Two records, two kinds of event

Chicxulub is recorded by a roughly 180-kilometre buried crater, shocked minerals, impact melt and ejecta distributed far from the Yucatán Peninsula. A thin boundary layer with anomalous iridium links global sediments to the impact event. These traces identify a sudden collision very close to the extinction horizon. They do not mean every organism died at the moment of impact; ecological collapse unfolded through environmental changes and food webs.

The Deccan Traps preserve thick volcanic sequences laid down in repeated eruptions over hundreds of thousands of years. High-precision dating shows that major eruptive phases began before the boundary and continued after it. Lava chemistry and climate records indicate that volcanic gases could produce warming over longer intervals and, during some high-flux pulses, shorter cooling episodes. These effects could stress ecosystems before the impact and influence conditions during recovery.

What chronology can distinguish

Dating uses minerals formed during eruptions or crystallisation, as well as stratigraphic order and boundary markers. Methods have improved enough to compare changes on the scale of tens to hundreds of thousands of years, though individual estimates still carry uncertainty. The broad sequence is clearer than it once was: substantial Deccan volcanism preceded the boundary, the impact occurred at the boundary, and volcanism continued afterward.

That order matters. If a proposed trigger must explain a sharp global turnover at a known horizon, the impact has a close temporal and physical match. Volcanism may explain earlier climate fluctuations and contribute to environmental stress, but its extended, pulsed record is not the same as a globally distributed impact layer. A detailed comparison of the wider extinction hypotheses shows why geological timing is central to the debate.

Which explanation fits the biological pattern?

Models of the impact include dust, sulphate aerosols and soot that reduced sunlight, cooled the surface and disrupted photosynthesis. Geological records preserve evidence consistent with rapid environmental change, while the extinction pattern across marine and terrestrial groups tests the scale of the crisis. The precise contribution of each atmospheric material remains an active area of measurement and modelling.

Studies comparing impact and volcanic signals have found that Deccan volcanism did affect climate, but the fossil turnover and crater evidence support Chicxulub as the immediate trigger of the mass extinction. This does not require volcanoes to have been irrelevant. They could have altered the background state of ecosystems and affected regional losses or subsequent recovery while the impact caused the abrupt global shock.

A distinction between trigger and context

“Impact or volcanism” frames the question too simply. The rock record supports both events, while their timing and physical mechanisms differ. The impact provides the strongest match for the sudden boundary crisis; Deccan eruptions provide evidence for environmental change before and after it. How those influences combined varied among regions, habitats and lineages.

This evidence-based distinction is more useful than treating every hypothesis as equally supported or claiming that one event explains every detail. The crater, ejecta, lava sequences, climate proxies and fossil ranges each test a different part of the causal story. Our overview of why non-avian dinosaurs went extinct follows the consequences from the boundary event through collapsing food webs.