Fifteen centimeters (6 inches) a year doesn't sound like fast movement. But for an entire continent, this is racing speed and twice as fast as any large plate moves today. Geologists have long been puzzled by the discovery that from 80-50 million years ago, this was India's speed. But now they may know why.
For around 400 million years, India was part of Gondwana, the mighty supercontinent that also included Africa, South America, Antarctica, Australia and the Arabian Peninsula. Gondwana began to break up 180 million years ago, and the landmass which would later become India left 60 million years later.
However, attempts to track its path have revealed something strange. For about 40 million years, the then independent Indian continent moved at about 4-5 cm (2 inches) a year, a typical continental speed. Then for thirty million years, it sped up to three times that rate.
"When you look at simulations of Gondwana breaking up, the plates kind of start to move, and then India comes slowly off of Antarctica, and suddenly it just zooms across -- it's very dramatic," says MIT Professor Leigh Royden.
Royden reasoned that if the main driving force of continental drift is subduction zones, in which the edge of one plate sinks beneath another, movement at double speed might be caused by two zones pulling in the same direction. This has never been observed before, but Royden and colleagues believe they have found the evidence, which they published in Nature Geoscience.
At convergent plate boundaries, one plate will ride over the other. The edge of the lower plate is drawn down into the mantle and the rest of the plate is dragged along behind.
Royden modeled the process that might cause double convergence. He suggested that the northern edge of the Indian plate was drawn beneath an oceanic plate that made up the floor of the Tethys Ocean that lay between Laurasia and Gondwana. At the same time, the opposite edge of the now-vanished plate Royden calls "Kshiroda" was being driven beneath the Eurasian Plate.
Credit: Jagoutz et al, Nature Geoscience. The combined subduction zones moved India north at twice normal speeds.
To test the theory, Royden took 30 students to the Himalayas. They found evidence of two arcs of volcanoes that once encircled the Tethys Ocean, one on each side of the now-vanished plate. They also concluded that shortly before the acceleration occurred the Indian plate separated from Africa. This cut the subduction zone from 10,000 to 3,000km (6000 to 2000 miles) allowing mantle material to be squeezed out the way at either end and hastening the convergence.
Eventually, with the Kshiroda Plate entirely subducted India and Eurasia collided, forcing up the Himalayas.
Credit: Jagoutz et al, Nature Geoscience. The volcanic arcs that marked subduction zones are marked in red.
"In earth science, it's hard to be completely sure of anything," says Royden. "But there are so many pieces of evidence that all fit together here that we're pretty convinced."
Lead author Oliver Jagoutz sees plenty of scope for follow-up research. "There were a lot of changes going on in that time period, including climate, that may be explained by this phenomenon. So we have a few ideas we want to look at in the future."