File:Thermal image of Sulphide Spring (morning, 11 June 2016) 2.jpg

English: The temperature scale is in degrees Fahrenheit.

Hot springs are sites where groundwater emerges at the Earth’s surface (or on the seafloor). Hot spring water has to be higher in temperature than the human body (an admittedly arbitrary definition): over 98° Fahrenheit or over 37° Celsius. Geysers are hot springs that episodically erupt columns of water. The highest concentration of geysers and hot springs anywhere is in Yellowstone’s Upper Geyser Basin (northwestern Wyoming, USA).

Sulphide Spring is located in the southeastern part of the Geyser Hill Group, 110 meters southeast of Giantess Geyser. It is east-west elongated, narrowing westward, is about 2 to 2.5 meters in length, and ranges from a little over 1 meter to less than 1 meter in width. Its runoff channel drains to the southeast and south, heading toward the Firehole River. The geyserite border is moderately to poorly developed, probably indicative of the spring being a relatively young feature. Geyserite, also called siliceous sinter, is a friable to solid chemical sedimentary rock composed of opal (hydrous silica, a.k.a. opaline silica: SiO2•nH2O). Geyserite forms by precipitation of hydrous silica from the issuing hot spring water. The silica is ultimately derived by superheated groundwater leaching of subsurface, late Cenozoic-aged rhyolites, a common volcanic rock at Yellowstone. Sulphide Spring has not been observed erupting, so it is not a geyser.