Six classes of alkaline hot springs and geysers are here distinguished in Yellowstone National Park based on activity and eruptive style: (1) non-surging, non-boiling springs, (2) gently surging, non-boiling springs, (3) non-surging to gently surging, boiling springs; (4) strongly surging springs, (5) fountain geysers, and (6) cone geysers. Each displays morphologically distinctive siliceous sinter deposits in the high-temperature (> 73°C) parts of the vent and outflow systems. Non-surging and gently surging, non-boiling springs that do not overflow their rims develop thin digitate to lilypad-like sinter rims that grow horizontally over the pool surfaces. Similar springs with rim overflow have rims that accrete vertically. Boiling springs often have nearly circular, steep-sided pools and rims dominated by splash and rim-overflow and composed mainly of spicular and columnar sinter. Fountain geysers, characterized by broad fountain-like eruption bursts, have a central vent surrounded by a zone of spicular sinter beyond which is a broad, flat-bottomed moat, but lack steep sinter cones. Cone geysers have narrow vents, jet-like discharges that cool rapidly and fall close to the vent, and steep-sided spicule-dominated sinter cones.
Sinter deposition takes place within three main zones: (1) fully subaqueous sides and bottoms of vent pools, outflow basins, and outflow channels; (2) periodically wetted splash zones surrounding hot springs and geysers; and (3) largely subaerial parts of the rim and outflow areas wetted mainly by overflow and surging and that dry quickly. Fully subaqueous surfaces are covered by fine earthy particulate siliceous sediment (PSS) composed largely of amorphous silica spherules and spherule aggregates that appear to have precipitated around heterogeneous nuclei, including microbes. Splash zones of hot springs and geysers are characterized by spicular sinter. Largely subaerial zones, wetted mainly by rim overflow and surging with little or no splash, show complex varieties of terraced, columnar, and knobby sinter. The highest silica deposition rates occur where surfaces are wetted frequently and evaporate to dryness, such as the tops of spicules, columns, and knobs. Associated lower areas and crevices that remain wet or moist show slow accumulation of PSS. Most continuously wet surfaces in high-temperature (> 73°C) hot spring and geyser vent and outflow systems are probably covered by microbial biofilms. Although microbes may locally influence rates of silica deposition and sinter microstructure, the results of this study suggest that the depositional sites, morphology, and macrostructuring of high-temperature sinter are controlled largely by the processes of surface wetting, evaporation, and drying.