It would be reasonable to hear the term “volcanic bomb” and presuppose that such an object tends to explode. But a specific type of volcanic bomb rarely lives up to the second half of its name: These objects get blasted into the air, crash into the ground and disappointingly fail to detonate.
These volcanic bombs — plasticky, partly molten blisters of magma no smaller than a peach — are shot out of a volcano submerged by a shallow body of water, like a lake or the sea close to shore. In the process, the bombs acquire plenty of water. That trapped water encounters the bomb’s scorching-hot innards and gets vigorously boiled into steam.
The sudden accumulation of steam within the projectile should blast the bomb apart in midair. “Rocks cannot survive in the face of that pressure,” said Mark McGuinness, a mathematician at Victoria University of Wellington in New Zealand. And yet so many of these bombs become duds, hitting the ground with an anticlimactic thud.
Solving this riddle would do more than scratch a longstanding scientific itch. Volcanic bombs, a fundamental part of many explosive eruptions, are also a lethal hazard. If more of them blew up midflight, that would be preferable to their clonking someone on the head.
Wishing to crack the case, Ian Schipper, a volcanologist at Victoria, joined forces with Dr. McGuinness and Emma Greenbank, also a mathematician at the university. They built a mathematical model that simulated the launch of a bomb from a virtual volcano and replicated the changing pressures and temperatures of the orb’s insides.
Reporting their results this Wednesday in The Proceedings of the Royal Society A, the team concludes that water both makes and defuses these doughy volcanic bombs.
Volcanic bombs are a common feature of an array of explosive eruptions. This includes Surtseyan eruptions, named after Surtsey, a volcano off the Icelandic coast that explosively grew above the waves in the 1960s until it formed a new island.
