What is the primary type of water that feeds geysers?

Geysers are primarily fed by groundwater. This groundwater is heated by geothermal energy from the earth's interior, typically found in areas with volcanic activity. As the groundwater seeps down through fractures and porous materials in the Earth's crust, it can encounter hot rocks or magma, raising its temperature significantly. When this superheated water is unable to escape due to pressure or a blockage, it can reach a critical point where it suddenly erupts through the surface in the form of steam and water.

While rainwater can contribute to the groundwater supply, it is the groundwater itself that undergoes the heating process essential for geyser activity. Surface runoff and lake water are not significant contributors to geyser formation since they do not undergo the necessary geothermal heating nor do they often interact with the underground geothermal systems that generate geysers.