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The outer part of the Earth is constantly subjected to major changes as a result of plate tectonics. One example of this is when heavier oceanic plates "dive" under lighter continental plates during a process called subduction. But in several mountain ranges the opposite is observed, namely that pieces of isolated oceanic plates (ophiolites), are found on top of a continental plate and at a great distance from their oceanic origin. Until now, geologists had no clear physical explanation for this puzzling phenomenon, but researchers from Utrecht and Rennes have now found an answer. Their research has been published in the leading journal Nature Communications.

In most cases it is the heavier oceanic plate that descents into the Earth’s interior during plate convergence. However, sometimes the continental part of a plate can also be dragged down with the subducting oceanic plate, resulting in placement of the continental plate below an oceanic plate.

Like a balloon held under water, the natural buoyancy of continental crust means it is lighter than its surroundings and wants to rise. Buoyed up by the ascending continental crust, the overlying oceanic plate is forced upwards and breaks apart as the continental crust returns to the Earth’s surface. Consequently, the broken-off piece of oceanic plate becomes separated from the main plate and is preserved as a distant ophiolite sheet. These novel concepts have been tested with state-of-the-art computer simulations and are validated against geologic observations from well-known ophiolite sheets including Oman and New Caledonia.

Article

Kristóf Porkoláb, Thibault Duretz, Philippe Yamato, Antoine Auzemery & Ernst Willingshofer, ‘Extrusion of subducted crust explains the emplacement of far-travelled ophiolites’, Nature Communications volume 12, Article number: 1499 (2021),  .