Rain and melted snow pass via narrow channels or tubes which descend from an opening on the earth’s surface towards red-hot magma below. Magma makes the rock hot which heats the water at the bottom of the tubes. The water becomes super-heated but does not boil owing to the weight of the water above it. Soon the higher water boils and steam pushes some water out. The pressure below decreases a bit and the lower water boils. More steam forms quickly. Then steam and water shoots out of the ground forming a geyser.