Some 60 million years ago, what's thought to be the largest continental collision in Earth's history occurred, when the landmass of India ran into the Eurasian tectonic plate.
That epic clash – which is still ongoing millions of years later – gave birth to the mountainous landscape of the Himalayas. But according to a new study, the collision didn't just send landmasses upwards – it also drove an enormous amount of the two continents underground.
A team of geophysicists led by the University of Chicago used new modelling techniques to estimate the amount of landmass that existed before the continental collision occurred.
Based on the total continental crust that we're left with today, they say a gigantic amount of the two plates has gone missing.
"What we found is that half of the mass that was there 60 million years ago is missing from Earth's surface today," says one of the team, Miquela Ingalls.
The researchers were surprised by just how much of the continental crust appears to have gone AWOL, but even more unexpected is where the team thinks it all went.
Ordinarily, when tectonic plates run into one another, continental crust is expected to rise up due to its low density and buoyancy.
"We're taught in Geology 101 that continental crust is buoyant and can't descend into the mantle," Ingalls explains.
But according to the researchers' calculations, the only way to account for the reduced, post-collision continental mass of the Indian and Eurasian plates is if all that crust headed down below.
"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," says researcher David Rowley.
"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."
According to the researchers, previous attempts to calculate the amount of continental mass before the collision didn't allow for the possibility that such significant amounts of crustal matter could have been forced underground.
But by analysing 20 years' worth of geological data on tectonic plates, and using new estimates on how these plates can move, the team thinks a "large-scale subduction of continental crust" is the only possible explanation.
That's because the only other places for the crust to have gone can't account for half the former mass of the Indian and Eurasian continents. Some of this landmass was thrust upwards – like the Himalayas.
The rest was squeezed to the side during the collision – forming Southeast Asia – or eroded into sediment that fell into the sea. But none of these landmasses are large enough in themselves to represent the former pre-collision crust.
"Accounting for all of these different types of mass loss, we still find that half of the continental crust involved in this collision is missing today," says Ingalls.
"If we've accounted for all possible solutions at the surface, it means the remaining mass must have been recycled wholesale into the mantle."
While the findings may cause some controversy in the geophysics community, the team thinks its hypothesis can also explain why volcanic activity sometimes spews up elements that are thought to be extremely rare in the mantle, such as lead and uranium.
In other words, what goes down, must come up.
"The implication of our work is that, if we're seeing the India-Asia collision system as an ongoing process over Earth's history, there has been a continuous mixing of the continental crustal elements back into the mantle," says Rowley.
"And they can then be re-extracted and seen in some of those volcanic materials that come out of the mantle today."
The findings are published in Nature Geoscience.