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Figure 1. Map of the global distribution of geysers (Steingisser 2006: adapted from Bryan 1995). the exact number of geysers at these locations is uncertain because geysers can disappear and reappear over time.

underground passage through which the heated water passes on its way toward the surface. If the water can flow freely toward the surface, it bubbles out as a hot spring. Alternatively, if there is enough pressure to prevent the water from rising easily to the surface, it may eventually burst to the surface in an eruption.

Because the geologic and hydraulic components respon- sible for geysers are so precisely balanced, there may be sig- nificant natural variability in the temperature, discharge, peri- odicity, or (non)eruptive characteristics of individual features within a single basin. In many cases, the behavior or charac- teristics of a feature will change with no identifiable cause. In other examples, natural events such as earthquakes, volcanic eruptions, and landslides have created changes in multiple fea- tures simultaneously. For example, the 7.1 magnitude Hebgen Lake earthquake outside of Yellowstone in 1959 caused major changes in geothermal features throughout the park. The dis- charge of many hot springs greatly increased while others were nearly drained. Geysers erupted that had never before been recorded. In many thermal features throughout the park, the temperature increased and their waters became murky with sediment (Marler 1964).

of those locations. Figure 1 shows these locations as reported by Bryan in 1995. Even in the few sites where geysers have been carefully documented, enumeration at any scale is problem- atic if not impossible because geysers are inherently unstable both temporally and spatially. Historical and contemporary accounts often disagree about the number of features. The definition of what constitutes a geyser rather than a boiling spring may change from one observer to the next, the length of field observations may vary, and access can be difficult. There is, however, general agreement in the contemporary lit- erature regarding those areas that historically have contained significant numbers of geysers. Figure 2 shows geyser counts for the eight historically largest geyser basins, for which rea- sonably high quality data are available on geyser distributions and associated human impacts. Although precise counts are elusive because of the nature of geysers, all sources agree that Yellowstone has more geysers than any other thermal area in the world.

Human Impacts to Geysers

Thermal features such as hot springs, geysers, fumaroles, and mud pots can assume characteristics of one another on a monthly, yearly, or decadal scale. Geysers are the most rare of these forms and occur in close proximity to other geothermal features, although the degree of inter-connectedness between geothermal features is still poorly understood.

Global Distribution of Geysers

Globally, there are at least 40 locations where geyser activ- ity has been documented, but geysers are now extinct in many

Historical Use of Geothermal Resources

It is believed that human use of geothermal resources may date as far back as the Paleolithic period, but concrete evidence only dates to 8,000 to 10,000 years ago, as exemplified by archeological finds near hot springs in North America. The first written evidence of human use of hydrothermal basins dates to the twelfth century BC, when the Etruscans estab- lished urban centers such as Bolsena, Populonia, and Saturnia near geothermal sites to extract hydrothermal minerals such as sulfur, kaolin, and travertine for export to foreign markets. Bathing in thermal springs was also highly valued for spiritual

ellowstone Science

17(1) • 2009

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