Why Do Volcanoes Erupt?

Magma forms in the mantle and lower crust when conditions allow solid rock to partially melt. Because magma is less dense than surrounding rock, it rises and collects in underground reservoirs called magma chambers. When pressure in a magma chamber exceeds the strength of the rock above it, magma forces its way upward through cracks. As it rises, dissolved gases such as water vapor, carbon dioxide, and sulfur dioxide expand, like bubbles leaving soda when pressure drops. Eruptions are not all alike. High-silica magma is thick and traps gas until pressure releases explosively. Low-silica magma flows more easily and lets gas escape, producing gentler lava flows like those often seen in Hawaii.