– Scientists at the University of Chicago have discovered a very large part of our planet’s continental crust that was there 60 million years ago is missing from the Earth’s surface. What happened do it? How do you make half the mass of two continents disappear?
Did the birth of the Himalayas destroy an entire continent?
Three geoscientists examined the collision of Eurasia and India, which began about 40 to 60 million years ago. During the collisions the Himalayas were created and the process is still in slow progress.
Using cutting cutting-edge computational software, the scientists computed with unprecedented precision the amount of landmass, or “continental crust,” before and after the collision. The results were truly surprising.
“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 in geophysical sciences who led the project as part of her doctoral work.
The loss of so much mass could be possible if the missing chunk had gone back down into the Earth’s mantle. However, from a scientific point of view there are problems with this theory and the researchers considered it more or less impossible on such a scale.
When tectonic plates come together, something has to give. 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. But 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’t 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 who is one of Ingalls’ advisors and a 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.”
So what happened to the missing continental crust?
In their study published by Nature Geoscience the researchers explain there were only a few places for the displaced crust to go after the collision.
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.
Some of the crust was thrust upward, forming the Himalayas, some was eroded and deposited as enormous sedimentary deposits in the oceans, and some was squeezed out the sides of the colliding plates, forming Southeast Asia.
“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,” said David Rowley, a professor in geophysical sciences. “
And they can then be re-extracted and seen in some of those volcanic materials that come out of the mantle today.”