Why does a geyser spout?

The geyser called Strokkur is a very popular fenomen in the geothermal area Geysir.
Geyser Strokkur shooting hot water and steam high up in Iceland.

The geysers in Iceland are located in high-temperature areas with hot water (more than 100°C or 230°F) in the ground. There are cracks in the ground and inside those cracks there is water. In the Geysir geothermal area the cracks reach hundreds of meters down into the ground. When the hot soil heats up the water it is boiling hot, or even super hot. When the water is overheating some place deep down in the crack the water flashes into steam and the steam needs a lot more space than the hot water. The only way to get that space is by shooting upwards. Above the super hot water there is more hot water so when the super hot water flashes into steam the steam shoots the hot water above high up into the air. That’s why geysers spout.

If you want a private tour to Geyser and the Golden Circle then you can look into that here