Why do volcanoes erupt?
Buried magma is buoyant and full of dissolved gas, when pressure drops, that gas explodes free.
It starts with melted rock
The mantle beneath us is solid but searing. Where pressure drops or water sneaks in, at mid-ocean ridges, subduction zones, and hotspots, pockets of that rock melt into magma.
magma is hotter and lighter than the rock around itit rises toward the surfacebecause buoyancy pushes less-dense material up
rising magma collects in a chamberpressure builds undergroundbecause more magma arrives than can escape
pressure exceeds the strength of the rock abovea path to the surface opensbecause something has to give
Dissolved gas is the trigger
Deep down, magma holds water vapor, carbon dioxide, and sulfur gases dissolved under crushing pressure, the same way a sealed soda holds its fizz. As magma climbs, the pressure falls and those gases come out of solution as bubbles.
deep magma, gas dissolved and quiet
shallow magma, foaming with expanding bubbles
Runny or sticky decides the violence
high-silica magma, sticky, gas trapped, explosive blast
low-silica magma, runny, gas escapes, gentle lava flows
Thin basaltic magma lets gas slip out as it rises, so Hawaiian volcanoes ooze rather than blast. Thick, silica-rich magma traps its gas until the pressure detonates, the difference between a lava fountain and Mount St. Helens.