How rain becomes a hot spring, and sometimes a geyser
Nobody agrees how warm a spring must be to count as hot: definitions range from anything warmer than its surroundings to above 50 degrees Celsius. What they share is the heat source, groundwater warmed by magma or by deep rock heated partly through the decay of uranium, thorium and potassium.
Groundwater gets hot in two ways. Near volcanoes, magma sits close enough to the surface to warm it directly. Elsewhere, rock simply grows hotter with depth, a rise called the geothermal gradient, and water that seeps down far enough, usually along shattered fault zones, comes back up warm. Between 45 and 90 percent of the heat leaking out of Earth is thought to come from radioactive isotopes in the mantle. Warm Springs, Georgia, is a non-volcanic example: rain and snow on nearby mountains sink about 910 metres through the Hollis Quartzite before resurfacing. Franklin D. Roosevelt valued its waters and built his Little White House there.
A geyser needs a buried cistern beside shallow magma, as at Yellowstone. The water there is heated beyond its normal boiling point, but the weight of the water above keeps it from boiling. Once a little spills out at the surface, pressure falls, some water flashes to steam and drives out more, and the cistern empties in a runaway burst before cooler water refills it. With too little water, the spring only vents steam as a fumarole; mixed with clay, it becomes a mud pot.
Hot water dissolves more minerals than cold, so spring chemistry varies widely. Alkaline chloride springs are saturated with silica, which cools into geyserite, a type of opal that builds broad platforms. In acid sulfate springs, hydrogen sulfide oxidizes to sulfuric acid, driving pH as low as 0.8 and rotting rock into clay. Bicarbonate springs lose carbon dioxide at the surface and pile up tall travertine terraces, while iron-rich springs host microbes that form clumps of oxidized iron. Heat-loving thermophiles thrive at 45 to 80 degrees Celsius, and gentler communities take over as the water cools farther from the vent, and some researchers suggest life itself may have begun in such places.
Flows range from seeps to rivers. Japan's Tamagawa Hot Spring in Akita pours out 150 litres a second, feeding a stream 3 metres wide at 98 degrees. Evans Plunge in South Dakota, built in 1890, is called the world's largest natural warm-water indoor pool. People have soaked in such springs for thousands of years, though some are hot enough to scald.
Source: Hot spring
