What do we call heated groundwater that expands and causes a water eruption?

The phenomenon of heated groundwater that expands and causes a water eruption is known as a geyser. Geysers form when underground water is heated by magma or hot rocks in the Earth's crust. As the water is heated, it turns into steam, which builds up pressure until it eventually forces the water and steam to erupt through the surface, creating a spectacular fountain of boiling water.

The unique venting and eruption process of geysers is a distinguishing characteristic, setting them apart from other geothermal features. For instance, springs are typically outlets where groundwater flows naturally without the explosive force of geysers, and drainage basins refer to the areas where water collects and drains into a common outlet, which is not indicative of an eruption. The term mesosphere simply refers to a layer of the Earth's atmosphere, unrelated to the process of geothermal water eruptions. Therefore, the definition and characteristics of geysers make them the correct answer to the question.