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.

Scientists have uncovered a long-missing piece of the volcanic puzzle: rising magma doesn’t just form explosive gas bubbles when pressure drops—it can do so simply by being sheared and “kneaded” ...