Eruptions are driven by the rapid expansion of bubbles formed from water and other volatile substances trapped in molten rock as it rises beneath a volcano, and the scale of the eruption is determined during the first 10 seconds of bubble growth in the rocks, scientists at McGill University in Montreal reported.
It's like shaking a bottle of a carbonated soda and then opening the lid, they said, and whether the drink -- or a volcano -- erupts dramatically of slowly is a function of bubble growth and gas loss.
The McGill researchers examined the growth of volcanic bubbles in real time by heating water-bearing molten rock with a laser.
Initially thousands of small bubbles per cubic centimeter formed, trapping gas inside them, but they swiftly coalesced into a foam of larger bubbles whose strength rapidly decreased while the rate of gas loss increased.
While in most cases gas escapes rapidly enough to outpace bubble growth, resulting in smaller eruptions, under exceptional rates of bubble expansion large eruptions may result, the study found.
Even molten rock with small amounts of water has the potential to create devastating, large eruptions, the scientists said.