Heat and volcanic gases from slowly cooling magma rise and warm the dense salty water that occupies fractured rocks above the Yellowstone magma chamber. That brine, in turn, transfers its heat to overlying fresh groundwater, which is recharged by rainfall and snowmelt from the surface. Water boiling at depth below the surface is hotter than the temperature of boiling at the surface. If it rises quickly, this superheated water can flash to steam, propelling both steam and hot water to the surface as a geyser. More commonly, hot water rises and loses its heat at a steady rate, flowing to the surface as a hot spring.