Washington | Half the original mass of India and Eurasia disappeared into the Earth’s interior before the two landmasses began their slow-motion collision about 60 million years ago, a new study has found.
The study by researchers at University of Chicago in the US has important implications for our understanding of when the continents grew to their present size and how the chemistry of the Earth’s interior has evolved.
It examined the collision of Eurasia and India, which began about 60 million years ago, created the Himalayas and is still in slow progress.
The scientists computed with unprecedented precision the amount of landmass, or “continental crust,” before and after the collision.
“What we found is that half of the mass that was there 60 million years ago is missing from the earth’s surface today,” said Miquela Ingalls, a graduate student who led the project.
After considering all other ways the mass might be accounted for, the researchers concluded that so huge a mass discrepancy could only be explained if the missing chunk had gone back down into the Earth’s mantle – something scientists had considered more or less impossible on such a scale.
According to plate tectonic theory, the surface of the Earth comprises a mosaic of about a dozen rigid plates in relative motion.
These plates move atop the upper mantle, and plates topped with thicker, more buoyant continental crust ride higher than those topped with thinner oceanic crust.
Oceanic crust can dip and slide into the mantle, where it eventually mixes together with the mantle material.
However, continental crust like that involved in the Eurasia-India collision is less dense, and geologists have long believed that when it meets the mantle, it is pushed back up like a beach ball in water, never mixing back in.
“We’re taught in Geology 101 that continental crust is buoyant and can not descend into the mantle,” Ingalls said.
The new results throw that idea out the window.
“We really have significant amounts of crust that have disappeared from the crustal reservoir, and the only place that it can go is into the mantle,” said David Rowley, a professor in geophysical sciences.
“It used to be thought that the mantle and the crust interacted only in a relatively minor way. This work suggests that, at least in certain circumstances, that’s not true,” said Rowley.
The scientists’ conclusion arose out of meticulous calculations of the amount of mass there before and after the collision, and a careful accounting of all possible ways it could have been distributed.
Computing the amount of crust “before” is a contentious problem involving careful dating of the ages of strata and reconstructions of past plate positions, Ingalls added.