When heated groundwater is trapped in narrow spaces, the pressure builds up and causes water to actually rocket upward.
A geyser is the result of such a pressurized spring.
Most geysers do not erupt constantly, but rather in periodic spurts, because pressure decreases during an eruption and then increases again after an eruption.
Old Faithful, probably the most famous geyser in the world, got its name for erupting in regular cycles lasting 90 minutes.
Its eruptions last for a couple of minutes and discharge 15,000 to 30,000 liters of water during each eruption