© news.yale.edu, Credit: Lara WagnerThe revised geometry of the downgoing Nazca plate beneath the Andean mountains in southern Peru and northern Bolivia. Seismic stations are shown as colored cubes. Vertical lines show the location of these stations projected onto the slab.
New work from an international team of researchers improves our understanding of the geological activity that is thought to have formed the Rocky Mountains.
Subduction is a geological process that occurs at the boundary between two of the many plates that make up Earth's crust. An oceanic crustal plate sinks and slides under another plate--either oceanic or continental--and is plunged deep into Earth's mantle. Usually the lower plate slides down into the mantle at a fairly steep angle, sinking rapidly into the warmer, less-dense mantle material. However, in a process called
"flat-slab" subduction, the lower plate moves nearly horizontally underneath the upper plate, sometimes for great distances.
Flat-slab subduction is used to explain volcanism and mountain formation that occurs far from plate boundaries, because the lower, "flat" slab moves inland beneath the surface of a landmass and thereby transmits the friction of the plates sliding against one another far inland. The formation of the Rocky Mountains between 55 and 80 million years ago, according to sedimentary and volcanic records that have been studied in detail since the 1970s, often is attributed to flat-slab subduction as the plate beneath the Pacific Ocean at that time slid beneath the North American continent.
Today, the
largest flat slab is found beneath Peru, where the oceanic Nazca Plate is being subducted under the continental South American Plate. An undersea mountain belt, called the Nazca Ridge, sits on the Nazca Plate, and has been subducted along with the rest of the plate for the past 11 million years, according to previous studies.
Comment: The
Nazca Plate is moving eastwards, towards the South American Plate, at about 79mm per year. The friction between the plates prevents the subducting oceanic plate from sliding smoothly. As it descends, it drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone and triggering nearby volcanic activity. We are in an uptick scenario, especially evident along the Pacific Rim where several plates converge and volcanic reaction is increasing.
Comment: Can't wait to see how this turns out.