Old Faithful Geyser, Yellowstone National Park, Wyoming.
Fumaroles, which emit mixtures of steam and other gases, are fed by conduits that pass through the water table before reaching the surface of the ground. Hydrogen sulfide (H2S), one of the typical gases issuing from fumaroles, readily oxidizes to sulfuric acid and native sulfur. This accounts for the intense chemical activity and brightly colored rocks in many thermal areas.
Hot springs occur in many thermal areas where the surface of the Earth intersects the water table. The temperature and rate of discharge of hot springs depend on factors such as the rate at which water circulates through the system of underground channelways, the amount of heat supplied at depth, and the extent of dilution of the heated water by cool ground water near the surface.
Black Growler steam vents (fumaroles), Norris Basin, Yellowstone National Park, Wyoming.
Mammoth Hot Springs, Yellowstone National Park, Wyoming.
Volcano Environments
There are more than 500 active volcanoes (those that have erupted at least once within recorded history) in the world—50 of which are in the United States (Hawaii, Alaska, Washington, Oregon, and California)—although many more are hidden under the seas. Most active volcanoes are strung like beads along, or near, the margins of the continents, and more than half encircle the Pacific Ocean as a “Ring of Fire.”