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The hydrothermal system in Yellowstone National Park has created altered rock and hot-spring deposits that were mapped using AVIRIS (airborne visible and infrared imaging spectrometer) data. The mapped minerals are indicative of the geologic processes and environments that controlled their formation. Ongoing volcanic activity is expressed at the surface by geothermal features, earthquakes, and geologically recent caldera formation. Precipitated minerals such as siliceous sinter and travertine on the surface are derived from chloride-rich alkaline solutions that are leaching silica and calcite from the underlying country rock. Siliceous sinter and montmorillonite are associated with hydrothermal systems abundant in hot water, whereas kaolinite and alunite are associated with acidic-vapor-dominated hydrothermal systems.