Lava flows are a threat, but are usually slow enough so that people can get out of the way.
In a large eruption, vast areas can be covered by ash [cooled fragments of rock], presenting a much greater hazard than lava flows.
Pyroclastic flows are extremely hazardous because
they can cascade down a volcano's sides at high speeds.
Debris flows [lahars] are another serious volcano-eruption hazard.
A lahar can travel great distances, often burying everything in its path.
