Parts of the lowest crust and lithospheric mantle are missing beneath many mountain ranges worldwide. Numerical modelling shows that the deep crustal root of a mountain belt can become so dense that it breaks away and sinks into the underlying mantle.
Neil Krystopowicz and Claire Currie at the University of Alberta, Canada, simulated the formation of a mountain range as two tectonic plates converge using a thermal-mechanical numerical model. During the simulated plate collision, the crust thickens to create a high mountain range at the surface, while a deep crustal root forms below. Because the pressures at depth are higher, the root undergoes metamorphism and is transformed into eclogite, an unusually dense rock type. The eclogitic root is denser than the underlying mantle, so it sinks further into the mantle. This, in turn, causes localized deformation at the surface above the root, and promotes further crustal thickening and further eclogitization.
In the simulations, the root eventually becomes gravitationally unstable, peels away from the remainder of the crust and sinks. Hot mantle wells upwards in the wake of the sinking crustal root, causing magmatism and abrupt uplift at the surface.
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Whitchurch, A. Dense mountain roots. Nature Geosci 6, 85 (2013). https://doi.org/10.1038/ngeo1724