The explosiveness of a volcanic eruption depends on how many gas bubbles form in the magma—and when. Until now, it was thought that gas bubbles were formed primarily when the ambient pressure dropped ...

One of the most explosive volcanoes in U.S. history began its eruption with a trickle, not a blast. Mount St. Helens' gas-laden magma oozed into the cone before the mountain finally erupted in 1980.