Science Puzzle
Why Do Some Volcanoes Explode While Others Ooze?
On Hawaii, lava pours out of Kilauea in glowing rivers, and people can watch from a safe distance. In 1980 Mount St Helens in the USA blew away the top of the mountain in a single blast, flattening forests for kilometres around.
Both are volcanoes erupting molten rock. Why do some explode while others just ooze?
The Answer
It depends on how sticky the magma is. Runny magma lets its gas bubble out easily, so the lava just flows. Thick, sticky magma holds its gas in until the pressure becomes so great that it bursts out explosively.
Deep underground, magma holds a lot of dissolved gas, mostly steam and carbon dioxide, kept in by the pressure. As the magma rises the pressure drops and the gas comes out as bubbles, just as bubbles appear when you open a bottle of fizzy drink. In runny magma, like the basalt of Hawaii, the bubbles rise and escape, throwing up fountains of lava but no great blast.
Magma rich in silica, the main ingredient of sand and glass, is far stiffer. The bubbles can't escape, so they swell and the pressure builds and builds. When something gives, the magma is torn into countless tiny shreds that cool into ash, and the whole lot erupts in a column that can rise more than 20 kilometres. Blowing bubbles through water is easy; blowing them through honey isn't, and they burst out with a splutter.
The surprise is that the explosive ones are the cooler ones. Hawaiian basalt erupts at about 1,150 °C. Sticky, silica-rich magma is often only 700 to 900 °C.
The principle: Viscosity controls eruptions. Sticky, silica-rich magma traps gas until it bursts out explosively, while runny magma lets gas escape gently and flows away as lava.