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towering mountain spews clouds of ash and gas. Huge boulders shoot into the sky, while a river of red-hot lava stampedes down the slopes. It's an eruption! Volcanoes like this exist. But most of the world's largest volcanoes are actually wide and smooth, with lava slowly flowing for months at a time. Why? Like everything about volcanoes, these differences begin underground. MOLTEN ROCK The Earth's core is very hot—in places, it's as hot as the surface of the Sun. All that heat melts underground rock into magma, or molten lava. Magma is lighter than the surrounding rock, so it's always trying to rise. This build-up of heat and pressure is what leads to all eruptions. But what kind of eruption will it be? Though there are many types, they all depend on the thickness of the magma. THROUGH THICK AND THIN Fluid magma is thin and flows easily. It escapes through vents in the Earth's crust as an even, predictable stream. But viscous magma is another story. The thick, heavy liquid has a harder time escaping through vents, and also traps gases underneath it, making the pressure build even more. When it finally does break through, the release causes an explosion. l d Majestic, frightening, and fuelled by hidden fury, volcanoes offer much more than meets the eye. 18 OWL September 2020 Incredible! TEXT: JOHN CROSSINGHAM PHOTOS: GETTYISTOCK, DREAMSTIME ILLUSTRATION: DREAMSTIME