Geyser

Seal the lid of a home pressure cooker tight, and the water stays hot well past 100 degrees Celsius without boiling. A geyser's underground plumbing works on the very same principle.

The deeper you go underground, the more the weight of the water above piles up, and the pressure gets enormous. Groundwater squeezed by that pressure can't boil even far above 100 degrees, so it keeps storing heat. This is called superheated water.

Then the heat hits its limit. A big bubble of steam forms at the deepest point and pushes up through the narrow channel, and everything changes in an instant. As soon as a little of the water above gets pushed out, the pressure holding down the water below drops all at once. Like a soda when you pop the cap, the groundwater whose pressure was just released flashes into steam, and it blasts a huge column of water out above the surface.

What really separates an ordinary hot spring from a geyser is the shape of the underground channel. In an ordinary hot spring, the channel is wide, so hot and cold water mix naturally by convection and flow gently out at the surface.

A geyser's channel, on the other hand, is very narrow and winding, so the water at the top and bottom can't mix. On top of that, silica dissolved from the surrounding rock sticks to the channel walls and forms a hard, waterproof coating. Thanks to this sturdy plumbing, the channel can withstand the huge steam pressure underground without bursting.

After an eruption, cold groundwater seeps back into the empty channel and fills it. It needs recharging time until that water is heated by the magma and boils up again. So instead of flowing nonstop, a geyser rests, then erupts in bursts. Still, geysers with fairly regular gaps, like Old Faithful, are actually rare. Most geysers erupt at uneven intervals.

For a geyser to form on Earth, three conditions have to line up perfectly: plenty of groundwater, a powerful heat source like magma, and a narrow rock channel that can handle high pressure. These conditions are so picky that, even among the world's volcanic regions, geysers show up in only a few special places.

Surprisingly, geysers aren't found only on Earth. On Enceladus, a cold icy moon of Saturn in the outer solar system, ice geysers (ice volcanoes) erupt with force. This is solid fact: the Cassini probe flew right through the plumes and even measured what they are made of. Observations suggesting similar eruptions on Jupiter's moon Europa have also been reported, but those are still being confirmed.

On these icy moons, instead of magma, the pull of the planet's gravity (tidal friction heating) warms a huge ocean underground. Water vapor and ice grains are shot out into space through cracks in the icy shell, and salt and organic matter have been detected in those grains. From this, scientists are looking for key clues about whether an alien ocean could support life.