Eurasia and India are half the size they once were, according to new research, which may also help solve a long-standing mystery about where all that missing land ended up.
A team of geoscientists from the University of Chicago who have been studying the tectonic plates the continents sit on have discovered that half the mass that was there 60 million years ago gradually slipped under the Earth’s surface.
The way this happened was long thought to be impossible.
When the Eurasian and Indian plates started smashing together all those millennia ago, the Himalayan mountains were created in one of the most dramatic visualisations ever of the impact plate tectonic forces can have. The research team calculated the amount of land mass before and after this collision to better understand its effect.
Their results showed that the difference in mass was huge, and this could only be explained by the missing chunk disappearing down into the Earth’s mantle. This wasn’t how these types of tectonic plates were thought to behave.
“We’re taught in Geology 101 that continental crust is buoyant and can’t descend into the mantle,” said Miquela Ingalls, the graduate student who led the project.
Whereas oceanic crust can slide down into the mantle where it melts and mixes together, so-called continental crust is less dense and was thought to get pushed back up instead, like a football in a pool.
“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, geophysical sciences professor and collaborator on the project. “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.”
This theory actually helps answer some puzzling questions about the Earth’s mantle.
Elements like lead and uranium sometimes crop up in volcanic lava, and it has been a bit of a mystery about where they came from. They are abundant in the continental crust, but are not known to originate in the mantle, where lava comes from.
This suggests that the mantle has somehow been contaminated by the crust, and this was surprising when the belief was that the two never came into contact. This mystery has been cleared up now that researchers know that continental crust can sink too.
“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,” said Rowley. “And they can then be re-extracted and seen in some of those volcanic materials that come out of the mantle today.”