In this study, we analyzed surface cracks and faults formed during a volcanic eruption in 2011-2012 at the Cordón Caulle volcano in Chile. As magma rose and formed a shallow underground dome, it bent and fractured the surface above, creating distinctive patterns of faults. By carefully examining layers of volcanic ash deposited at the same time, we found that faults around the edges of the dome recorded progressively smaller movements over time. This means that as the eruption went on, the underground pressure decreased, and the dome gradually spread out sideways, relaxing stress.

Our findings provide a rare geological 'time-log”, showing how magma bodies evolve during eruptions. This may help scientists better recognize similar hidden magma intrusions at other volcanoes, improving our ability to understand the dynamics of recent and ancient eruptions, and anticipate changes in volcanic activity.