Volcanic hazards include explosions, lava flows, bombs or ballistics, ash or tephra, pyroclastic flows, pyroclastic surges, mudflows or lahars, landslides, earthquakes, ground deformation, tsunami, air shocks, lightning, poisonous gas and glacial outburst flooding known as jökulhlaups. Each hazard has a different consequence, although not all occur in all eruptions or in association with all volcanoes.
Volcanic eruptions are measured using a simple descriptive index known as the Volcano Explosivity Index which ranges from zero to eight. The index combines the volume of material ejected with the height of an eruption column and the duration of the eruption.