Most of the time, volcanoes look like other mountains: solid, serene, capped with snow-a nice place to go camping.
Unless, of course, it happens to explode and pour molten lava all over your campsite! Wouldn't it be nice if we could predict when a volcano was about to erupt? Scientists have recently taken a big step closer toward doing just this.
Let's look inside a volcano. At the center is a reservoir of molten rock, known as magma. This magma is very hot, and under a lot of pressure. When rainwater seeps into cracks in the mountain, the magma turns it into steam and forces it out again. The magma also vents away hot gasses through these same cracks.
This is a volcano's normal state. When gasses and steam vent from a volcano's cracks, it's probably not about to erupt. Sometimes, however, the top layer of magma can cool, and form a hard crust over the rest of the molten rock. When this happens, rainwater is able to collect in the cracks without evaporating, and this water captures a lot of gasses that would ordinarily vent from the magma. Scientific instruments inside the volcano's cracks record a sudden drop in temperature, and much less volcanic gas.
This is a warning sign for an imminent eruption. That cool magma seal allows pressure to build up inside the liquid magma, much like your thumb over the top of a shaken soda bottle will seal in the pressure. When that magma seal gives way it's like taking your thumb off the top of the bottle, and you don't want to be camping on the volcano when that happens!