How do you know when a volcano erupts?

According to the UCSB Geology Department, a volcanic eruption is defined as a volcanic event resulting in the emission of gases, lava, ash or other material from a volcano. These can manifest as a single eruptive pulse, an eruptive phase or an eruptive episode.

Eruptive pulses are a one-time emission, lasting a few seconds to a few minutes. Eruptive phases are eruptions that continue for a few hours or days, switching between ash and lava expulsion. Eruptive episodes are several connected eruptive phases over the course of several days, months or years. In a single eruption, the style, strength and ejected material may change hour by hour.