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.

Volcanoes are often framed as nature’s most violent spectacles, yet some of the largest on Earth ooze lava quietly for years without a single dramatic blast. Scientists have now pulled together a ...