hotspring

A hot spring is a spring that emerges geothermally heated water from the Earth's crust. It forms when groundwater percolates downward, is warmed by contact with hot rocks or magma, and rises back to the surface through fractures or porous rock. The resulting temperature can vary widely from warm to boiling.

Temperatures commonly range from around 25°C to above 70°C; some hot springs are suitable for bathing, while others are too hot or chemically hazardous for direct contact. Volcanic activity and groundwater pathways determine whether a spring is gentle or erupts as a geyser.

Chemistry and minerals in hot springs depend on local geology. Water often contains silica, sulfates, carbonates, or iron compounds, and may be acidic or alkaline. The distinctive colors of many springs come from thermophilic microorganisms that form vivid bacterial mats.

Hot springs host microbial communities, including thermophiles such as cyanobacteria and archaea that tolerate high temperatures. These organisms influence both chemistry and color and can create isolated habitats with limited species diversity.

Hot springs occur worldwide, especially in volcanic or tectonically active regions. They have been used for bathing, healing, and religious purposes for centuries, yielding spa towns in places like Japan and Europe, as well as geothermal tourism in the United States, Iceland, and New Zealand. Some springs are protected to preserve natural features.