The Earth’s crust drips “like honey” into the planet’s warm interior beneath the Andes, scientists have discovered.
By setting up a simple experiment in a sandbox and comparing the results with actual geological data, researchers have found compelling evidence that Earth’s the crust has been “cut away” over hundreds of miles in the Andes after being swallowed up by the viscous mantle.
The process, called lithospheric dripping, has been going on for millions of years and in several places 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 on the Andes drip on June 28 in the journal Nature: Communication 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 has a large part of the lithosphere [Earth’s crust and upper mantle] during the landslide,” Julia Andersen, a researcher and doctoral candidate in geosciences at the University of Toronto, said in a statement. “Because of its high density, it dripped like cold syrup or honey deeper into the planet’s interior and is probably responsible for two major tectonic events in the Central Andes – shifting the surface topography of the region by hundreds of kilometers and both crushing and stretching the surface crust itself .”
The outer regions of Earth’s geology can be broken down into two parts: a crust and upper mantle that form rigid sheets of solid rock, the lithosphere; and the hotter, more pressurized plastic-like rocks in 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 collide them, slide one under the other, or expose a gap in the plate to the violent 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 takes place when two colliding and collapsed lithospheric plates are heated to such a point that they thicken, creating a long, heavy drop that seeps into the lower part of the planet’s mantle. As the drop continues to seep down, its growing weight pulls on the crust above, forming a pool on the surface. Eventually the drop’s weight becomes too great for it to remain intact; its long lifeline snaps, and the crust above it leaps upwards for hundreds of kilometers – forming mountains. In fact, scientists have long suspected that such underground stretching may have contributed to the formation of the Andes.
The central Andean plateau consists of the Puna and Altiplano plateaus—an approximately 1,120-mile-long (1,800-kilometer), 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 sliding beneath, of the heavier Nazca tectonic plate beneath 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 the story. Previous studies also point to features on the central Andean plateau that cannot be explained by the slow and steady upward movement of the subduction process. Instead, parts of the Andes look like they sprung up from sudden upward pulses in the Earth’s crust throughout the Cenozoic Era – Earth’s current geological period, which began about 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 lithospheric dripping. But to be sure, the researchers needed to test that hypothesis by modeling the plateau’s ground. They filled a Plexiglas tank with materials that simulated the 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 and silica spheres for the crust.
“It was like creating and destroying tectonic mountain belts in a sandbox, floating on a simulated magma basin – all under incredibly precise sub-millimeter measured conditions,” Andersen said.
To simulate how a drop might form in Earth’s lithosphere, the team created a small, high-density instability just above the lower mantle layer on their model, and recorded with three high-resolution cameras as a drop slowly formed and then sank into a long, “The drip happens over hours, so you won’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 the images of the model’s surface with 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 are 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 were formed, but also highlights the significant role of geological processes other than subduction in shaping Earth’s landscape. It may also prove effective in detecting the effects of other types of underground seepage elsewhere in the world.
Originally published on Live Science.