How do emergency agencies decide when to evacuate people before a volcanic eruption?

Volcanologists monitor multiple warning signs before recommending evacuation. Earthquake swarms, rapid ground deformation, increased gas release, and changes in heat or water chemistry can all suggest magma is moving upward. Authorities then weigh the volcano's history, nearby population, road access, and likely hazards such as ashfall, lava flows, or pyroclastic flows. In 1991, Pinatubo in the Philippines was successfully evacuated after strong warnings, saving many lives. Good decisions depend on both science and clear communication with the public.