Media release

From: Macquarie University

Researchers at Macquarie University have produced new models showing where rocks melt in the deep Earth, helping us to understand where to expect giant earthquakes to occur.

The biggest and most destructive earthquakes occur where two tectonic plates collide. At these collision zones, one plate slides beneath the other, sinking into the deep Earth. In a paper published today in Nature Communications, researchers Dr Michael Förster and Dr Kate Selway found that there is molten rock and fluids at shallower depths in these collision zones than generally thought.

The fluids and molten rock lubricate the faults where the two plates slide past each other so extremely large earthquakes do not occur at these depths.

To make these calculations, the researchers considered how different rocks will melt in the collision zones between tectonic plates. As rocks get carried down with the sinking tectonic plate into the deep Earth, they heat up and many will eventually melt. Dr Förster carried out laboratory experiments on samples of mud and sediment from the ocean floor. By heating the samples to temperatures greater than 1000 °C, he found that they melt at shallower depths than most rocks in collision zones, producing molten rock, the mineral phlogopite, and saline fluids.

Dr Selway compared these experiments with measurements of the physical properties of the deep Earth. She found that there are zones of high electrical conductivity that match the predicted locations and conductivities of the molten rock, phlogopite and saline fluids. These high conductivity zones will enable researchers to map out the distribution of molten rock and fluids in collision zones.

As these results are tested against locations of historic earthquakes and are applied to known collision zones between tectonic plates, it is hoped that they will improve the accuracy of earthquake risk assessments.

Dr Selway, who is an honorary senior lecturer in Macquarie University’s Department of Earth and Environmental Sciences, says “These subduction zones, where two tectonic plates collide, are among the most geologically important places on Earth because they control the locations of volcanoes and earthquakes. It's exciting to have a new model that explains these measurements of electrical conductivity at subduction zones."

The research paper “Melting of subducted sediments reconciles geophysical images of subduction zones” was published in Nature Communications today.