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.

Think about a mountain that keeps getting taller but never explodes. This might sound impossible, yet it happens more often than most people realize. Volcanoes don't always follow the same playbook of ...