It's almost as if the universe had it in for the dinosaurs (birds aside). Even before the asteroid hit, enormous volcanic eruptions were changing the climate and bringing to an end the great beasts' planetary dominance. Now, evidence has emerged that these eruptions were the result of two, rather than one, gigantic upwellings of magma, each of which alone would have altered the climate.
Towards the end of the Cretaceous era, volcanic eruptions unleashed astonishing quantities of lava in what is now central India to form the basalt province known as the Deccan Traps. Although the eruptions have been attributed to a plume from the deep mantle, the details of how this occurred are not clear.
Many surface clues associate the eruptions with a hotspot that now lies under Reunion Island. Movement of the Indian continental plate has shifted the traps far to the north-east, but around 66 million years ago, the continent and hotspot were aligned. However, attempts to map currents within the Earth's mantle have found no sign of an upwelling at the appropriate location.
Dr Petar Glišović of the Université du Québec à Montréal and Dr Alessandro Forte of the University of Florida have modeled convection currents within the mantle, reversing the timing to start from modern conditions and going back 70 million years. They argue in Science that their method is more reliable and fits with what we see in the geologic record.
Glišović and Forte's model revealed a hot upwelling from deep in the mantle near the current location of Reunion, but added two other features, which they referred to as “plume-like anomalies”.
One of these anomalies didn't significantly breach the crust, but the other produced an upwelling near the site of the modern-day Comores Islands. The authors suggest that this upwelling and the Reunion plume both fed the Deccan traps, helping explain why the basalt province is so large. The combined effect of the volcanic gasses emitted from each source cooled the planet rapidly enough to make life difficult for less adaptable species.
The plumes didn't simply stop once the largest species were extinct. Instead, one lasted 30 million years and the other almost 50, although for most of that time they were in the deep ocean, India having left them in its wake.
Passage over the two plumes caused the Indian plate to lift and sink again, leaving a legacy in the Seychelles Plateau, which is raised far above the rest of the floor of the Indian Ocean. Moreover, the remnants of the Reunion plume continue to contribute to driving India northwards, where its collision with Asia is forcing up the Himalayas.