How Volcanoes Erupt Quiz
Magma rises
Deep under the ground, in places, rock gets hot enough to melt into magma. This happens where tectonic plates pull apart, where one plate sinks under another, and over hotspots, like the one under Hawaii.
Where a plate sinks, it carries water down with it. The water lowers the melting point of the hot rock above, so it melts more easily.
Magma is lighter than the solid rock around it, so it rises, a bit like a blob in a lava lamp. It often collects in a magma chamber a few kilometers below the surface, where it can sit for years before an eruption.
Gas makes the bang
Magma holds dissolved gases, mostly water vapor and carbon dioxide. As it rises, the pressure on it drops and the gas forms bubbles, just like when you open a shaken bottle of soda.
What happens next depends on how runny the magma is. Thin, runny magma lets the bubbles escape easily, so the volcano pours out rivers of lava, as in Hawaii and Iceland.
Thick, sticky magma, rich in silica, traps the gas until the pressure blasts it apart. That makes explosive eruptions, with towering columns of ash and pyroclastic flows: fast, scorching avalanches of hot gas, ash and rock. Mount St. Helens in 1980 was an eruption of this kind.
Kinds of volcanoes
Runny lava builds shield volcanoes, which are broad with gentle slopes, like Mauna Loa in Hawaii. Alternating layers of lava and ash build stratovolcanoes, the steep, classic cones like Mount Fuji. Small cinder cones form from bits of lava thrown into the air.
Most of the world's active volcanoes sit in the Ring of Fire, a horseshoe of plate edges around the Pacific Ocean.
Scientists can't predict eruptions exactly, but they watch for warning signs: swarms of small earthquakes, the ground swelling, and changes in the gases a volcano gives off.