Earth's crust drips 'like honey' into its interior beneath the Andes

Earth’s crust drips “like honey” into our planet’s hot interior beneath the Andes mountains, scientists have found.

By setting up a simple experiment in a sandbox and comparing the results with actual geological data, researchers have found solid evidence that earth Earth’s crust has “bullied” across hundreds of miles in the Andes after being swallowed up by the thick mantle.

The process, called lithospheric drip, has occurred for millions of years and in various locations around the world – including Turkey’s central Anatolian Highlands and the Great Basin of the western United States – but scientists have only studied it in recent years. The researchers published their findings on Andean drops on June 28 in the journal Nature: Earth & Environment Communication (opens in a new tab).

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“We have confirmed that the deformation on the surface of the Andes Mountains area has a large part of the lithosphere. [Earth’s crust and upper mantle] under the avalanche,” Julia Andersen, a researcher and doctoral candidate in earth sciences at the University of Toronto, said in a statement. “Because of its high density, it drips like cold syrup or honey deeper into the planet’s interior and is likely responsible for two major tectonic events in the Middle Andes – shifting the region’s surface topography by hundreds of kilometers and both rattling and stretching the surface crust itself.”

The geological outer region of the Earth can be divided into two parts: the crust and upper mantle which form the rigid solid rock slab, the lithosphere; and hotter, more pressurized rocks such as the plastics of the lower mantle. The lithospheric (or tectonic) plates float in this lower mantle, and their magmatic convection currents can pull them together to form oceans; rub against each other to trigger earthquakes; and collide, sliding one under the other, or exposing fissures in the plate to the fierce heat of the mantle to form mountains. However, as scientists are beginning to observe, this is not the only way mountains can form.

Lithospheric droplets occur when two lithospheric plates collide and squeeze to such a degree that they thicken, creating long, heavy droplets that seep into the bottom of the planet’s mantle. As the droplets continue to seep downwards, their increased weight pulls on the crust above, forming hollows on the surface. Eventually, the droplet weight becomes too large to remain intact; its long lifeline was broken, and the crust above it poked upwards across hundreds of miles — making mountains. In fact, researchers have long suspected that such subsurface stretching might have contributed to the formation of the Andes.

The Central Andean Plateau consists of the highlands of Puna and the Altiplano — a 1,120-mile (1,800-kilometer) long, 250-mile (400 km) wide stretch that stretches from northern Peru through Bolivia, southwest Chile, and northwestern Argentina. It was created by the subduction, or slipping under, of the heavier Nazca tectonic plate under the South American tectonic plate. This process breaks 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 shows features in the Central Andean Plateau that cannot be explained by the slow and steady upward thrust of the subduction process. In contrast, parts of the Andes look like they emerged from a sudden upward pulse 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 has volcanic centers and large basins such as the Arizaro and Atacama.

These are all signs of a lithospheric drop. But to be sure, scientists need to test that hypothesis by modeling upland soils. They filled plexiglass tanks with a material that simulates the Earth’s crust and mantle, using polydimethylsiloxane (PDMS), a silicone polymer about 1,000 times thicker than table syrup, for the undercoat; a mixture of PDMS and modeling clay for the upper mantle; and a sand-like layer of tiny ceramic balls and silica balls for the crust.

“It’s like creating and destroying a tectonic mountain belt in a sandbox, floating in a simulated magma pool — all in very precise sub-millimeter measured conditions,” Andersen said.

To simulate how droplets 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 the droplet slowly formed and then plummeted into long, distended droplets. “Drips happen for hours on end, so you’re not going to see much happening from one minute to the next,” Andersen said. “But if you check every few hours, you’ll clearly see the changes – it just takes patience.”

By comparing their model surface images with aerial images of Andean geological features, the researchers noticed a marked similarity between the two, strongly suggesting that the features in the Andes were indeed formed by lithospheric droplets.

“We also observed crustal shortening with folds in the model as well as depression-like depressions on the surface, so we believe that droplets are very likely to be the cause of the deformation observed in the Andes,” Andersen said.

The researchers say their new method not only provides strong evidence of how some of the main Andean features formed, but also highlights the important role of geological processes other than subduction in the formation of Earth’s landscape. It has also proven effective for viewing the effects of other types of subsurface drips elsewhere in the world.

Originally published in Live Science.

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