What lies beneath us is decidedly mysterious. Plate tectonics are responsible for many things, including the creation and destruction of the continents we stand on, but there’s a lot about this epic process that scientists are only just beginning to understand.
As a particularly bizarre example of this, a trio of researchers from the University of Chicago have highlighted that a lot of this continental crust has gone missing. About an entire continents’ worth, in fact.
During the collision between India and Eurasia, which took place between 60 and 40 million years ago, the Himalayas were created – in fact, they’re still being built to a degree. Using cutting-edge computational software, this team calculated with remarkable precision how much landmass there was in the region before, during, and after the titanic meeting of tectonic plates.
“What we found is that half of the mass that was there 60 million years ago is missing from the Earth's surface today,” graduate geophysicist Miquela Ingalls said in a statement.
As described in their Nature Geoscience study, that’s 450 trillion tonnes (496 trillion US tons) unaccounted for, a huge amount of missing matter. This is roughly 45 times the mass of comet 67P.
The only way the crust could have gone is down, but there’s a huge problem with this idea.
When two plates collide, they do one of two things. If one is a continental plate and one is an oceanic plate, the former slides over the latter because oceanic crust is denser. This oceanic crust then collapses into the broiling mantle, either slowly disintegrating and becoming assimilated into its surroundings, or crashing into the boundary with the liquid outer core, remaining in this graveyard for millions of years.
On the other hand, if two pieces of continental crust crash into each other, both with roughly the same density, then they bend, warp, and rise upwards into the sky, creating enormous mountain ranges.
It’s not thought that continental crust could sink (subduct) into the mantle as oceanic crust continually does. “We're taught in Geology 101 that continental crust is buoyant and can't descend into the mantle,” Ingalls noted.
Nevertheless, using 20 years’ worth of geological data, this team’s remarkable simulations demonstrated that often the simplest explanation is the most likely one. After ruling every single other possibility out, they concluded that half of the continental crust involved in this colossal crash must have sunk down into the hellish depths and recycled.
This continental crust would have been like a plastic bottle floating on a lake. Its density would have been so low that unfathomably powerful tectonic forces must have pushed it down into the thick mantle over millions of years. Nowadays, it’s being erupted through volcanoes all over the world in a gargantuan funeral pyre.