and H2S) are released by the boiling Cl--rich waters of the
deep hydrothermal system and condense just below the
ground surface (Fournier 1989). Subsequent oxidation of H2S
creates sulfuric acid, which reacts with the rhyolitic surface
rocks to form clays and muds, and these materials dominate
the resulting steam-heated (also called acid-sulfate) thermal
areas. Very little thermal water flows away from these steamheated
terrains (Fournier 1989).
Estimated Heat Flow
The water expelled from Yellowstone’s hydrothermal system
carries considerable heat, which in turn reflects the
presence of magma below. Fournier et al. (1976) first estimated
heat flow at Yellowstone by tallying the total Cl- flux
through its rivers using stream gages combined with periodic
chemical analyses. They assumed that the Cl- was
almost entirely derived from a deep parent water of known
composition and heat content. Geochemical characteristics
of Yellowstone hot springs provided corroborating evidence
for the single parent fluid and its initial temperature of
340°C (Fournier 1989 and references therein). The total Clflux
could then be tied to a mass of parent water. The
calculations account for the heat content of the released
Cl--rich waters as well as the boiling they undergo during
ascent and lateral transport. The method can thereby
account for heat flow at distal steam-heated areas as well as