Certain rocks that sink far down into Earth's interior together with plates are transformed into different types under the enormous pressure that prevails there. During this UHP metamorphosis (UHP: ultra-high pressure), silica (SiO₂) in the rock, for example, becomes coesite, which is also referred to as the UHP polymorph of SiO₂. Although it is chemically still silica, the crystal lattices are more tightly packed and therefore denser.

When the plates move upwards again from the depths, the UHP rocks also come to the surface and can be found in certain places in the mountains. Their mineral composition provides information about the pressures to which they were exposed during their vertical journey through Earth's interior. Using lithostatic pressure as a unit of measurement, it is possible to correlate pressure and depth: the higher the pressure, the deeper the rocks once lay.

Until now, research had assumed that UHP rocks were buried at a depth of 120 kilometers. From there, they returned to the surface together with the plates. In the process, ambient pressure decreased at a stable rate, i.e., statically. However, a new study by Goethe University Frankfurt and the universities of Heidelberg and Rennes (France) published in Nature Communications calls this assumption of a long, continuous ascent into question.

Among those involved in the study on the part of Goethe University Frankfurt were first author Cindy Luisier, who came to the university on a Humboldt Research Fellowship, and Thibault Duretz, head of the Geodynamic Modeling Working Group at the Department of Geosciences.