New Findings on Rock Movements from the Earth’s Interior

Geologists from Heidelberg and Frankfurt simulate thermo-mechanical behaviour of a white schist from the Alps

Movements of rocks from deep in the Earth to the surface could occur under different circumstances than previously thought, challenging our current understanding of plate tectonics and mountain-building. A research team that includes a geologist from Heidelberg University, Lucie Tajcmanová, has found evidence to support the new hypothesis. Using numerical simulations, researchers from Frankfurt and Heidelberg studied a white schist from the Alps, whose mineral composition makes it especially well-suited to study processes of mountain formation. The analyses call into question previous assumptions on large vertical displacements of rock during such processes.

The continents are largely composed of orogenic belts, elongated and often arch-shaped mountain ranges that arose when tectonic plates collided. They contain rocks from the Earth’s interior that were moved to the surface, explains Prof. Tajcmanová, who works at the Institute of Earth Sciences at Heidelberg University. The geologist and her research group study mineralogical and petrological processes in the Earth’s crust and mantle. If plates descend into the Earth’s interior, rocks travel down with them. The extreme pressure there converts them into other types of rock. These deeply buried rocks formed under ultra-high pressure (UHP) can ascend again during mountain-building processes. Their burial and exhumation history provides important indications about the mechanisms of plate tectonics and mountain-building.

Until now, researchers assumed that these UHP rocks were buried at a depth of 120 kilometres and returned to the Earth’s surface with the plates under continuously decreasing ambient pressure. Studies on a white schist from the Dora-Maira Massif in the western Italian Alps now call this assumption into question. This schist originated during the formation of the Alps from a fluid-altered granite, which has a special chemistry and hence mineralogy. Under the extreme pressure at depth, the silicon dioxide (SiO2) contained in the rock transforms into coesite, a high-pressure polymorph of SiO2, i.e. a mineral with the same composition but with a different crystal structure. The light microscopy studies of the very thin rock samples prepared at Heidelberg University revealed two SiO2 polymorphs embedded in the host material garnet - coesite, formed under high pressure, and its modification quartz, which forms under much lower pressure.

Based on the spatial arrangement of these minerals - the silicon dioxide polymorph quartz forms rings around the coesite crystals at certain points of the rock samples - the researchers assume that the schist was initially exposed to extremely high pressure and then a much lower one. The garnet shows radial, spike-shaped fractures extending in all directions from the SiO2 inclusions - the result of the phase transition from coesite to quartz. Using computer models, the researchers simulated the thermo-mechanical behaviour of the garnet. The models showed that the cracks could have formed much earlier than previously thought possible. Such a fracture pattern can form only if the garnet remains strong in spite of the immense pressure, explains study head Thibault Duretz of Goethe University Frankfurt. This is possible only if the pressure drops very quickly. "We still talk about thousands and even hundreds of thousands of years; but in geological terms, that’s quite short," stresses Lucie Tajcmanová.

The rapid drop in pressure contradicts the assumption that UHP rocks reach a depth of 120 kilometres. "The ascent would take considerably longer than our data suggests," states Prof. Tajcmanová. Based on the current findings, a depth of 60 to 80 kilometres is more likely. At the same time, the researchers suspect that rock units did not continuously move over large distances to the Earth’s surface. They believe it is more likely that fast tectonic processes occurred with rapid changes in pressure and abrupt vertical displacements of plates over small distances. "Our previous assumptions on the processes that move rock from the interior of the Earth to its surface would be invalid, which would also affect our understanding of plate tectonics and mountain-building," adds Lucie Tajcmanová.

The research results were published in the journal "Nature Communications".

C. Luisier, L. Tajcmanová, P. Yamato, T. Duretz: Garnet microstructures suggest ultra-fast decompression of ultrahigh-pressure rocks. Nature Communications (27 September 2023).