Where rainwater or groundwater comes into contact with hot rocks below the surface and is heated. The water rises to the surface through fractures and cracks. Hot springs,geysers and fumaroles (small outlets of steam and gas). The water may have a high mineral content and be acidic due to dissolved gases such as sulphur dioxide. Rocks can be corroded and crumble in hydrothermal areas. On the plus side natural Spas are often in hydrothermal areas.
Famous geysers are the origial Geyser in Island and Old Faithfull In Yellowstone.