…excerpt from Gillian Richardson’s book Kaboom!, from the section about Geysers.
Not all explosive eruptions from the earth spew lava or hot molten rock. Not all of them are destructive or deadly. Some are actually entertaining, attracting crowds of people who line up for the show. You’ve likely heard of Old Faithful, in Wyoming, in Yellowstone National Park. This geyser regularly sends up hot water and steam in sudden spouts 30–60 meters (100–200 feet) high. That could be about as high as a 20-story building.
Geysers can’t just pop up anywhere. They need exactly the right ingredients: a constant source of underground heat, a water supply, and a place for that water to collect so it can be heated. A river flowing into an underground pool and hot, solid rocks or magma of an ancient volcanic site to heat the water are perfect. Most of Earth’s geysers are in Yellowstone, an area that was formed by many volcanoes long ago. Russia, Chile, New Zealand, and Iceland also have geysers.
A geyser’s eruption is actually an explosion of steam created when a large amount of stored water reaches boiling temperature and turns to steam. Because steam needs 1600 times more space than water, you can imagine what must happen. To escape from its underground chamber, the steam forces its way through the closest vent it can find and up to the surface. It appears suddenly as a column or fountain of frothy water and superheated steam—superheated because the boiling point of water is higher the deeper the water is below Earth’s surface. Water may reach 230 degrees C (450 degrees F) before it becomes steam. When it suddenly finds a way to the surface…look out!