Beneath the Andes Mountains, parts of the Earth’s outermost crust have sunk into the Play-Doh-like layer of the mantle below, slowly dripping into the planet’s interior like honey.
As reported in the journal Communications Earth & Environment, scientists at the University of Toronto studied the wrinkle-like features in the topography of several regions of the Puna Plateau among the central Andes Mountains in South America.
Below these peculiar features, seismic imaging and the sedimentary rock record revealed significant amounts of the lithosphere (the rocky outer layer of the Earth) had sunk into the planet’s next layer, the upper mantle, over the span of millions of years.
“We have confirmed that a deformation on the surface of an area of the Andes Mountains has a large portion of the lithosphere below avalanched away,” Julia Andersen, lead study author and a PhD candidate in the department of 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 kilometers and both crunching and stretching the surface crust itself,” Andersen explained.
This is a process known as lithospheric dripping. Back in 2009, scientists discovered a similar example of lithospheric dripping beneath the Great Basin in the western US. It occurs when the lowest layer of Earth’s outer shell thickens after being warmed, eventually causing it to slowly “drip” into the semi-viscous mantle below.
“The discoveries show that the lithosphere can be more volatile or fluid-like than we believed,” added Professor Russell Pysklywec, co-author of the study and Andersen’s PhD supervisor.
To gain a better understanding of this phenomenon, the scientists set up a small experiment using a bunch of supplies you could pick up from a hardware store.
It consisted of a tank filled with polydimethylsiloxane (PDMS) – a silicone polymer fluid approximately 1,000 times thicker than table syrup – to act as Earth’s lower mantle. To serve as the upper-most solid section of the mantle, they then added a layer of PDMS and modeling clay. Finally, a layer of sand was used to top it off and recreate the Earth’s crust.
Just as they hoped, the tank showed the harder outer material slowly but surely dripping into the more viscous lower level, just like how the lithosphere must sink into the mantle. Crucially, it also showed that it created patterns on the surface that closely resemble the topographical features of the Andes.
“We compared our model results to geophysical and geological studies conducted in the Central Andes, particularly in the Arizaro Basin, and found that the changes in elevation of the crust caused by the drip in our models track very well with changes in elevation of the Arizaro Basin,” Andersen says.
“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.”