The dinosaur's bad year
Around 66 million years ago, a series of volcanic eruptions began in west-central India that was larger than any in human history. This was the Deccan Traps, a large igneous province that erupted a staggering volume of basalt lava—estimates range from 1 to 1.1 million cubic kilometers. This molten rock spread out to cover an area of 500,000 square kilometers, with some geologists estimating the original coverage before erosion at 1.5 million square kilometers, about half the size of modern India. In places, the stacked layers of solidified flood basalt are more than 2 kilometers thick.
The main, most intense phase of these eruptions occurred over a period of roughly 750,000 years, an event that inconveniently straddles the Cretaceous-Paleogene (K-Pg) boundary. This is the precise geological moment when the non-avian dinosaurs, along with about 75% of Earth's plant and animal species, vanished from the fossil record. For decades, the main cause of this mass extinction has been the Chicxulub asteroid impact in present-day Mexico. High-precision dating, however, shows that the Deccan Traps were devastating the global environment at the exact same time, creating a major debates in paleontology.
A crime with two culprits
Scientists use high-precision dating techniques like uranium-lead (U-Pb) and argon-argon (40Ar/39Ar) on minerals like zircon found in ash layers between lava flows to construct a timeline of the eruptions. These methods have revealed that the main phase of Deccan volcanism began approximately 250,000 to 350,000 years before the Chicxulub impact. Some studies suggest the eruptions occurred in four massive pulses, each lasting about 100,000 years. One of these pulses started tens of thousands of years before the asteroid strike, releasing vast quantities of climate-altering gases.
The eruptions would have released large amounts of carbon dioxide, sulfur dioxide, and chlorine into the atmosphere. The sulfur dioxide could have created aerosols that blocked sunlight, causing a rapid "volcanic winter" and acid rain. Following this, the release of CO2 would have driven long-term greenhouse warming, raising global temperatures and acidifying the oceans. Geochemical analysis of fossil shells from this period shows evidence of both ocean warming and high concentrations of mercury, evidence of large-scale volcanic activity.
The debate now centers on whether the volcanoes were the primary killer, with the asteroid delivering a final blow, or if the asteroid was the main culprit, perhaps even triggering the most voluminous phase of the Deccan eruptions through its seismic shockwaves. Some models indicate the Deccan emissions alone were sufficient to cause the extinction, while others suggest the "impact winter" from the asteroid was the more important event. The coincidence of two global-scale catastrophes drives research and scientific argument.