Eclogitization
Eclogitization is the tectonic process in which the high-pressure, metamorphic facies, eclogite (a very dense rock), is formed. This leads to an increase in the density of regions of Earth's crust, which leads to changes in plate motion at convergent boundaries (where rock sinks beneath other rock).
There is the argument that collision between two continents should slow down because of continental buoyancy, and that for convergence to continue, it should do so at a new subduction zone where oceanic crust can be consumed. Certain areas such as the Alps, Zagros, and Himalayas (where continental collisions have continued for tens of millions of years, in the middle of land, creating mountain ranges) contradict this argument, and have led geologists to propose a continental undertow that continues subduction. This continental undertow is explained by the slab pull concept. Slab pull is the concept that plate motion is driven by the weight of cool, dense plates and that heavier plates will begin to subduct. Once a descending slab is disconnected there must be a force that continues subduction. Eclogitization is the mechanism for continuing subduction after slab detachment in a subduction zone.
Eclogitization typically occurs at two locations in a collisional fold mountain (fig 2), in the subduction of crust and at the base of the crustal root of the overriding crust. At these zones high pressures are reached, as well as medium to high temperatures, and eclogitization commences. Metamorphic re-crystallization during burial can lead to a significant density increase (up to 10% in the case of eclogitization), meaning approximately 300–600 kg/m3 of crustal rocks and continental lower crust and oceanic crust reach higher density than the mantle.
This density increase acts as the main driver in the convection of Earth's mantle. It also explains the disconnection of a tectonic unit from the descending lithosphere, subsequent continuation of subduction, and the exhumation following subduction.
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