The eruption may have been triggered when new, "Dacite 1" magma entered into a system containing "Dacite 2" magma and pressurized the system, causing magma to begin ascending to the surface. In the prelude to the eruption, magma moving upwards to the future vents caused earthquakes beginning at a shallow reservoir at a depth of ; according to the accounts of priests people in Arequipa fled their houses out of fear that they would collapse. The rising magma appears to have intercepted an older hydrothermal system that existed as much as below the vents,parts of which were expelled during the eruption. Once the magma reached the surface, the eruption quickly became intense.