About 66 million years ago, sheet after sheet of lava flooded western India. The Deccan Traps hold about 1 million km³ of basalt in a pile more than about 2 km thick, and may once have covered 1.5 million km², about half the size of modern India. They were already erupting when the Chicxulub asteroid struck Mexico 66.04 million years ago, and they carried on afterwards, which has made them both rival and possible accomplice in explaining the extinction that ended the age of dinosaurs.
Lava without a mountain
There was no great cone. Flood basalts erupt from long fissures, and the runny lava spreads for hundreds of kilometres beneath a solid crust that keeps it hot. Ash is rare, because the lava holds little dissolved gas. Deccan flows ran up to 1 000 km. In a 2021 compilation, individual lobes were most often 8 m thick in some formations and 17 m in others, with 75 to 80 m rare. Today the Western Ghats are not true mountains but the steep edge of a plateau, and the stepped hills gave the rocks their name, from the Swedish for stairs.
Life between the flows
Red, brown or green layers called boles lie between many flows, attributed either to chilling by the next flow or to erosion of the last. Lake beds between the lavas hold frogs, snakes, gar, palms and small mammals such as Deccanolestes. Under the lavas, the Lameta Formation of about 70 to 66 million years ago preserves titanosaurs such as Isisaurus and predators such as Rajasaurus. In 2023 Dhiman and colleagues described 92 titanosaur clutches with 256 eggs from Dhar District. From the same formation comes Sanajeh, a 3.5 m snake found coiled around an egg beside a hatchling about 0.5 m long.
An island on the move
By then India was crossing the ocean as an island. It left Antarctica and Australia together with Madagascar about 125 million years ago, and the two parted 84 to 92 million years ago. One modelling study suggests that the plume now under Réunion thinned India’s base by about 130 km and let it reach about 20 cm a year.
Impact, volcanoes or both
Most researchers treat the impact as the main cause. In 2010 Schulte and colleagues pointed to a single layer of impact debris found worldwide at the boundary, matching the onset of extinction. In 2020 Hull and colleagues found support for major gas release beginning and ending distinctly before the impact, and Chiarenza and colleagues modelled dinosaur habitat: an impact winter suppressed it, while volcanic warming improved it and may have softened the blow. A 2024 study from the western United States found a cooling of about 5 °C, lasting under 10 000 years, about 30 000 years before the boundary, with temperatures recovered before the extinction. A 2025 study of New Mexico fossils found dinosaurs still diverse in the last few hundred thousand years, evidence against a long decline.
The volcanic case is led by Keller, who holds that the impact predates the extinction and that Deccan volcanism was the likely main cause, possibly made worse by the impact. In 2020 her team matched mercury spikes in Tunisia and Israel to eruption pulses and warming, and called volcanism a primary driver. Callegaro and colleagues found the most sulfur in lavas from within 100 000 years before the extinction, pointing to repeated volcanic winters that stressed life before the impact. At Seymour Island in Antarctica, Petersen and colleagues found 7.8 ± 3.3 °C of warming at the onset of Deccan volcanism and two extinction pulses, one with each event. A 2023 model from Dartmouth College suggested that Deccan gases alone could have been enough, with no spike in emissions at the impact.
Even the timing is contested. In one 2019 issue of Science, a team dating zircon found four high-volume pulses, one starting tens of thousands of years before the impact, and favoured both causes together. Another, dating with argon, found that the lava did not come in three discrete large pulses and that about 75 per cent of it erupted after the boundary. Renne and colleagues argued in 2015 that seismic waves from the impact may have set off the largest eruptions, about 70 per cent of the volume, an idea other authors have dismissed as superficial. A proposed second crater off western India, called Shiva, is unlikely to be real.
