Earth's crust is dripping 'like honey' into its interior under the Andes

Earth's crust is dripping "like honey" into our planet's hot interior beneath the Andes mountains, scientists have discovered. 

By setting up a simple experiment in a sandbox and comparing the results to actual geological data, researchers have found compelling evidence that Earth's crust has been "avalanched away" across hundreds of miles in the Andes after being swallowed up by the viscous mantle.

The process, called lithospheric dripping, has been happening for millions of years and in multiple locations around the world — including Turkey's central Anatolian Plateau and the western United States' Great Basin — but scientists have only learned about it in recent years. The researchers published their findings about the Andean drip June 28 in the journal Nature: Communications Earth & Environment.

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"We have confirmed that a deformation on the surface of an area of the Andes Mountains has a large portion of the lithosphere [Earth's crust and upper mantle] below avalanched away," Julia Andersen, a researcher and doctoral candidate in Earth sciences at the University of Toronto, said in a statement. "Owing to its high density, it dripped like cold syrup or honey deeper into the planetary interior and is likely responsible for two major tectonic events in the Central Andes — shifting the surface topography of the region by hundreds of kilometres and both crunching and stretching the surface crust itself."

The outer regions of the Earth’s geology can be broken down into two parts: a crust and upper mantle that form rigid plates of solid rock, the lithosphere; and the hotter, more pressurized plastic-like 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 one another to trigger earthquakes; and collide them, slide one under the other, or expose a gap in the plate to the mantle’s fierce heat to form mountains. But, as scientists have begun observing, these aren’t the only ways that mountains can be formed.

Lithospheric dripping takes place when two collided and crumpled up llithospheric plates warm to such a point that they thicken, creating a long, heavy droplet that oozes into the lower part of the planet's mantle. As the droplet continues to seep downward, its growing weight tugs on the crust above, forming a basin on the surface. Eventually, the droplet's weight becomes too great for it to remain intact; its long lifeline snaps, and the crust above it springs upward across hundreds of miles — making mountains. In fact, researchers have long suspected that such subsurface stretching may have contributed to the formation of the Andes.

The Central Andean Plateau consists of the Puna and Altiplano plateaus — a roughly 1,120-mile-long (1,800 kilometers), 250-mile-wide (400 km) expanse that stretches from northern Peru through Bolivia, southwestern Chile and northwestern Argentina. It was created by the subduction, or the slipping beneath, of the heavier Nazca tectonic plate under the South American tectonic plate. This process deformed the crust above it, pushing it thousands of miles into the air to form mountains. 

But subduction is only half of the story. Prior studies also point to features on the Central Andean Plateau that can't be explained by the slow and steady upward push of the subduction process. Instead, parts of the Andes look like they sprung from sudden upward pulses in the crust throughout the Cenozoic era — Earth's current geological period, which began roughly 66 million years ago. The Puna plateau is also higher than the Altiplano and holds volcanic centers and big basins such as the Arizaro and Atacama. 

These are all signs of lithospheric dripping. But to be sure, the scientists needed to test that hypothesis by modeling the plateau's ground. They filled a plexiglass tank with materials that simulated Earth's crust and mantle, using polydimethylsiloxane (PDMS), a silicon polymer around 1,000 times thicker than table syrup, for the lower mantle; a mixture of PDMS and modeling clay for the upper mantle; 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 pool of magma — all under incredibly precise sub-millimetre measured conditions," Andersen said.

To simulate how a drip might form in Earth's lithosphere, the team created a small, high-density instability just above the lower mantle layer of their model, recording with three high-resolution cameras as a droplet slowly formed and then sagged into a long, distended drip."The dripping occurs over 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 requires patience."

By comparing the images of their model's surface to aerial images of the Andes’ geological features, the researchers saw a marked similarity between the two, strongly suggesting that the features in the Andes had indeed been formed by lithospheric drip.

"We also observed crustal shortening with folds in the model as well as basin-like depressions on the surface, so we're confident that a drip is very likely the cause of the observed deformations in the Andes," Andersen said.

The researchers said their new method not only provides solid evidence for how some key features of the Andes formed but also highlights the significant role of geological processes other than subduction in the molding of Earth's landscapes. It may also prove effective for spotting the effects of other kinds of subsurface dripping elsewhere in the world.

Originally published on Live Science.