Geoscientists have shown how atomic-scale crystal flaws can unravel in a domino-sequence that causes rocks to shift from creeping smoothly and slowly to jolting dramatically.
It’s the first time that scientists have been able to analyse this so-called ‘mild-to-wild’ transition in rocks from Earth’s mantle, and they say it might contribute to causing earthquakes at depths where the hot, ductile rocks are otherwise less likely to rupture.
“These small crystal movements have been overlooked, yet they could play a role in driving rapid motion on larger scales – much like what happens during deep earthquakes,” said David Wallis from Cambridge Earth Sciences, who led the study.
In a set of experiments, Wallis and team used a nano-sized diamond probe to make indentations in olivine – the most common component of Earth’s upper mantle, where deep earthquakes occur – and watched how microscopic flaws moved through the crystal.
They observed that as one crystal defect shifted it triggered its neighbours to move – causing a jerky movement within the crystal. Taken together, thousands of these avalanching crystal jolts could add up to larger-scale movements, potentially offering a trigger mechanism for deep-focus earthquakes that scientists struggle to explain.
Wild deformation
Nearly 25 percent of earthquakes occur more than 50 kilometres below the Earth's surface. Unlike shallow earthquakes, the cause of intermediate and deep earthquakes is poorly understood – one problem being that the high temperature and pressure conditions at depth require a very different driving mechanism.
Shallow earthquakes occur when stresses build up in cold, brittle rocks at the surface, causing a fault to break. But rocks beneath 50 kilometres don’t break so easily because heat and pressure make them ductile, much like how a blacksmith can work hot metal on an anvil.
Wallis’ findings suggest that heat at depth might kick-start a transition within the rock, causing it to behave ‘wildly’ so it deforms intermittently and jerkily rather than flows continuously. Materials such as ice and some metals are known to deform wildly, but it’s the first time that scientists have shown that mantle rocks can do the same.
After experimenting on olivine in the lab, Wallis and his co-authors used calculations to predict how olivine crystals may behave under a range temperatures. When simulating the high temperatures seen in the upper mantle, they found that the crystals deform wildly – behaviour that was most dramatic when they zoomed in to the finest scale.
The findings show that heat might be key in promoting wild plasticity, but this doesn’t fully explain why deep earthquakes happen, said Wallis. “It’s likely wild deformation is a contributing factor, defining how the rocks behave and providing a mechanism for the jerky bursts of movement needed for earthquakes.”
One existing explanation for deep earthquakes is that the flow of upper mantle rocks generates heat that makes them weaker, resulting in a feedback loop that can culminate in them slipping in an earthquake. This theory compliments Wallis’s work, because that heat may also kick-start the unzipping of crystal defects and the transition to wild behaviour.
A question of scale
According to Wallis, this behaviour has previously been missed in rocks because scientists haven’t looked closely enough. “We tend to think of flow down in the mantle as being very slow and continuous. If you put a GPS at the surface, it will show the same gradual movement manifested above-ground,” said Wallis.
Rocks might appear to behave mildly when viewed from Earth’s surface and at a large-scale, but zooming out averages the miniscule crystal movements, he explained, “we’ve been missing the fine details, giving an incorrect interpretation of how the material is behaving. Really, we need to be thinking about how individual grains move in bursts of deformation.”
Wallis now plans to expand his experiments by listening in on the crystals as he deforms them. As the crystal defects shuffle about they send out soundwaves, he said, “we expect the colder crystals will be quieter, but hotter conditions should trigger an avalanche of movement, coupled with noise, from the crystal dislocations.”
Reference: Wallis, D., Kumamoto, K. M., & Breithaupt, T. (2026). Mild-to-wild plasticity of Earth’s upper mantle. Nature Geoscience, 1-6.
Feature images: (left) photo of olivine minerals, (right) cross section through an olivine crystal showing cross-hatched defects after indentation. Credit: David Wallis.