When groundwater enters underground fractured or caverns in hot igneous rocks where it is heated to boiling temperatures, it can form a geyser.

Here's how it works:

* Heat Source: The hot igneous rocks provide the heat source for the water.

* Confined Space: The fractured or cavernous space acts as a container for the water.

* Pressure Build-up: As the water is heated, it expands and increases pressure within the confined space.

* Boiling Point: When the pressure becomes high enough, the water reaches its boiling point and turns into steam.

* Eruption: The steam, which occupies a much larger volume than water, forces the water out of the ground in a violent eruption, creating a geyser.

So, the combination of heat, pressure, and a confined space allows for the formation of geysers.