Hot springs are created as water seeps into the earth's surface and is heated geothermally. The earth's pressure forces the heated water to ascend to the surface as a hot spring.
Hot springs are usually heated by geothermal heat from the earth's mantle. The temperature of rocks beneath the earth generally increases with depth. If water percolates deeply enough, it is heated by hot rocks before emerging through the surface again as a hot spring. In volcanic areas, hot springs are heated after water comes into contact with underground deposits of magma. Sometimes, after coming into contact with magma, the water's temperature may rise to such a degree that a geyser or fumarole is created.
Hot springs in volcanic areas generally approach the boiling point. As such, these magma-heated springs can be extremely dangerous and sometimes deadly. In mountainous areas, like the American Rockies, rain falls on the peaks and then percolates in porous sedimentary rocks. As the water passes through the rocks, it collects a variety of mineral components. Since heated water can hold more dissolved solids, the mineral-rich water of hot springs is often touted as therapeutic. However, hot springs can also be a breeding place for dangerous or infectious organisms called thermophiles.