How Math Solved the Case of the Volcanic Bombs That Didn’t Explode

Scientists have long been puzzled by flying blobs of magma that remained intact, but didn’t have a good way to study them.

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.