What drives these fountains of superheated water, and why aren’t there more of them?
Geysers form when water is superheated by volcanic activity underground, but can’t move freely as it circulates towards the surface. Instead, pressure builds up until the water explodes upwards in a giant gush.
Since water needs to encounter hot rock, some geyser fields are found above upwellings of hot rock from deep within the Earth. Others are found near crustal plate boundaries where there is volcanic activity and broken, fractured rock. Rivers, snow or rainwater trickling through the Earth can provide a constant source of water.
Most geysers form where there’s a silica-rich rock known as rhyolite. Rising hot water dissolves the silica in the rhyolite and carries it upwards through natural pipes in the rock where it’s then deposited as a rock called geyserite. The silica seals the pipe against water pressure and narrows its walls.
Every geyser has a different plumbing and reservoir system, but there are two main types. “Cone’ or ‘column’ geysers like Old Faithful erupt in a steady column from a beehive-shaped nozzle of geyserite.
They tend to have one reservoir of water with a single pipe leading from it to the surface. ‘Fountain’ or ‘pool’ geysers erupt from a large pool of water in a series of powerful bursts. They are thought to have a reservoir fed by two water sources – descending shallow, cold water and hot water rising from below.
As geysers need a rare combination of geological conditions to form, they’re found in just a handful of places. There are around 50 geyser fields worldwide and most have just a few geysers. The biggest – Yellowstone, USA – has almost half the world’s geysers.