File:Bronze Spring (early afternoon, 1 June 2013) 5.jpg

English: 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).

Bronze Spring is in the southeastern part of the Geyser Hill Group, located 106 meters south-southeast of Giantess Geyser. It has a rounded subtrapezoidal outline and is about 3 to 3.5 meters across. The vent is located on the western side of the pool. Runoff channels head southeast and south from the spring. This feature is usually a quiet pool with some rising steam bubbles that disturb its surface. Eruptions are rare and usually moderately low. In recent years, small eruptions from Bronze Spring have been reported in September 2001, July 2008, August 2009, August 2013, and July 2014.

Bronze Spring’s border rocks consist of slightly scalloped, light gray subaerial geyserite and light brownish subaqueous, moderately pustulose geyserite. 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.