Towards better volcanic eruption predictions: Water content determines storage depth of volcanic arc magmas

Scientists may be one step closer to accurate and reliable predictions of volcanic eruptions; water largely initiates and fuels volcanic eruptions, and their new research connects the depth at which magma below active arc volcanos is stored to water, and specifically to the amount of it the magmas contain. These findings rule out previously held assumptions that magma’s storage depth is largely controlled by neutral buoyancy in the surrounding rock, with magma rising through cracks in Earth’s crust because the molten rock is more buoyant than the surrounding crust. Understanding the physical constraints over magma depth is crucial to refining physics-based eruption-forecasting models for active arc volcanos. Magma is stored deep within the Earth’s crust, often until something triggers its eruption to the surface. However, what controls the depth at which magma is stored remains poorly understood. The depths of volcanic arc magmas – those formed in tectonic subduction zones – vary considerably, laying roughly between 0 and 20 kilometers below the surface. Its widely assumed that this depth is determined by their buoyancy. Although this prevailing notion has been rigoruosly evaluated for magma below mid-ocean ridge volcanos, it remains largely untested for magma below highly active arc volcanos. By comparing new and existing observations of magmatic water content with geophysical observations of magma storage depth below active arc volcanos in the Aleutian Islands of Alaska, Daniel Rasmussen and colleagues discovered that the amount of water magmas contain determines the magmas’ depths. Magmas that contained more water tended to be stored deeper in the Earth’s crust. According to Rasmussen et al., degassing of water from these magmas changes their viscosity, allowing ascending magmas to stall at depths where they’re still buoyant. The results suggest that magmas buoyant at their storage depth could provide an additional driving force for ascent during an eruption.