A deep channel reaches from the surface far into the Earth. Cold water seeps down the channel on to very hot rocks. Then it fills the channel. The water at the bottom gets hot from the rocks. Gradually the heat at the bottom of the column rises far above the boiling point for water.
Steam starts to form.
The rising bubbles lift the water column a little, pushing some of the water in the channel over the opening onto the surface of the Earth. That makes the water lighter, and more water is able to turn into steam. Which lifts the column more, until suddenly all the water near the bottom of the channel expands into steam and forces out the rest into a steam explosion. After the water and steam settle onto the Earth, some water seeps back into the Earth and fills up the channel again. It's intervals range from 35-120 min.