Volcanoes and Volcanic Hazards

Volcanoes are openings in the Earth's crust through which molten rock, gas, and fragmented material reach the surface. Their form and behavior depend largely on the chemistry of the magma feeding them, which in turn reflects their tectonic setting. Magma that is low in silica is runny and flows easily, allowing gas to escape gently; magma that is high in silica is thick and viscous, trapping gas until pressure builds to an explosive release. This single variable, viscosity, explains much of the difference between the two great volcano families and the hazards each poses.

Shield volcanoes form from low-silica basaltic lava, typically at divergent boundaries and hotspots. Because the lava is fluid, it spreads over wide areas before solidifying, building broad, gently sloping cones that resem