Subduction zones, where one tectonic plate dives beneath another, transport large amounts of oceanic crust deep into the Earth. Along this boundary, intense forces deform the crust, transforming it into a type of metamorphic rock called blueschist. One key mineral in blueschist, glaucophane, appears to play a major role in how these rocks deform, but exactly how it flows under pressure and temperature isn't well understood.

To investigate this, we ran lab experiments that simulate the high pressures and temperatures found deep in subduction zones. We deformed glaucophane samples and used the results to develop a mathematical description (a 'flow law') for how it deforms. We also compared these results with observations from naturally deformed blueschist rocks from ancient subduction zones now exposed at the Earth’s surface.

Our findings show that glaucophane-rich rocks likely deform by a mechanism called dislocation creep at lower temperatures (below about 500 °C), and shift to a different, diffusion-based mechanism at higher temperatures. This change in deformation behavior helps explain how rocks behave as they are buried deeper during subduction and gives us better insight into the mechanical properties of subduction zones.