Earth’s crust is dripping “like honey” into our planet’s warm interior beneath the Andes mountains, scientists have found.
By setting up a simple experiment in a sandbox and comparing the results to real geological data, the researchers found compelling evidence that Earth the crust was “avalanched” for hundreds of kilometers in the Andes after being engulfed by the viscous mantle.
The process, called lithospheric dripping, has been happening for millions of years and in multiple places around the world – including Turkey’s central Anatolian plateau and the western United States’ Great Basin – but scientists don’t know it. have learned that in recent years. The researchers published their Andean drip findings on June 28 in the journal Nature: Communications Earth and Environment (opens in a new tab).
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“We have confirmed that a deformation on the surface of an area of the Andes Mountains has a large part of the lithosphere [Earth’s crust and upper mantle] below avalanche,” Julia Andersen, researcher and PhD candidate in Earth Sciences at the University of Toronto, said in a press release. “Due to its high density, it flowed like cold syrup or honey deeper into the interior of the planet and is likely responsible for two major tectonic events in the central Andes – shifting the region’s surface topography by hundreds miles and both crush and stretch the surface crust itself.”
The outer regions of the Earth’s geology can be broken down into two parts: a crust and an upper mantle which form rigid plates of solid rock, the lithosphere; and the hotter, more pressurized plastic rocks of the lower mantle. Lithospheric (or tectonic) plates float on this lower mantle, and its magmatic convection currents can pull the plates apart to form oceans; rub them against each other to trigger earthquakes; and colliding with them, sliding one under the other, or exposing a space in the plate to the fierce heat of the mantle to form mountains. But, as scientists have begun to observe, these are not the only ways mountains can form.
Lithospheric dripping occurs when two lithospheric plates have collided and crumpled together heat up to such an extent that they thicken, creating a long, heavy droplet that drips into the lower part of the planet’s mantle. As the droplet continues to seep downward, its increasing weight pulls on the crust above, forming a pool on the surface. Eventually, the weight of the droplet becomes too great for it to remain intact; her long lifeline snaps and the crust above her shoots upward for hundreds of miles – forming mountains. In fact, researchers have long suspected that such subterranean stretching may have contributed to the formation of the Andes.
The central Andean plateau includes the Puna and Altiplano plateaus – a stretch approximately 1,120 miles long (1,800 kilometers) and 250 miles wide (400 km) that stretches from northern Peru to Bolivia, southwest Chile and northwest Argentina. It was created by the subduction, or underslipping, of the heavier Nazca tectonic plate beneath the South American tectonic plate. This process warped the crust above it, pushing it thousands of miles into the air to form mountains.
But subduction is only half the story. Previous studies also point to features of the central Andean plateau that cannot be explained by the slow and steady upward thrust of the subduction process. Instead, parts of the Andes seem to come from sudden upward pulses in the crust throughout the Cenozoic Era – Earth’s current geological period, which began around 66 million years ago. The Puna plateau is also higher than the Altiplano and hosts volcanic centers and large basins such as the Arizaro and the Atacama.
These are all signs of lithospheric dripping. But to be sure, the scientists had to test this hypothesis by modeling the floor of the plateau. They filled a Plexiglas tank with materials simulating the Earth’s crust and mantle, using polydimethylsiloxane (PDMS), a silicon polymer about 1,000 times thicker than table syrup, for the lower mantle; a mix of PDMS and modeling clay for the top coat; and a sand-like layer of tiny ceramic spheres and silica spheres for the crust.
“It was like creating and destroying tectonic mountain belts in a sandbox, floating on a simulated magma pool — all under incredibly precise sub-millimeter measured conditions,” Andersen said.
To simulate how a droplet might form in Earth’s lithosphere, the team created a small, high-density instability just above their model’s lower mantle layer, recording with three high-resolution cameras as a droplet slowly formed and then sank into a long, distended blob. “The drip happens for hours, so you wouldn’t see much happening from one minute to the next,” Andersen said. “But if you checked every few hours, you would clearly see the change – it just takes patience.”
By comparing images of the surface in their model to aerial images of geological features in the Andes, the researchers found a marked similarity between the two, strongly suggesting that the features in the Andes had indeed been formed by lithospheric runoff.
“We also observed crustal shortening with folds in the model as well as basin-like depressions on the surface, so we are confident that a drip is most likely the cause of the observed deformations in the Andes,” said Andersen said.
The researchers said their new method not only provides strong evidence for the formation of some key features of the Andes, but also highlights the important role of geological processes other than subduction in molding landscapes. It can also prove effective in spotting the effects of other types of underground drops elsewhere in the world.
Originally posted on Live Science.