Although Yellowstone is geologically outstanding in many ways, the great
abundance, diversity, and spectacular nature of its thermal (hot-water
and steam) features were undoubtedly the primary reasons for its being
set aside as our first National Park (fig. 43). The unusual
concentration of geysers, hot springs, mudpots, and fumaroles provides
that special drawing card which has, for the past century, made the Park
one of the world’s foremost natural attractions.
To count all the individual thermal features in Yellowstone would be
virtually impossible. Various estimates range from 2,500 to 10,000,
depending on how many of the smaller features are included. They are
scattered through many regions of the Park, but most are clustered in a
few areas called geyser basins, where there are continuous displays of
intense thermal activity. (See frontispiece.) The “steam” that can be
seen in thermal areas is actually fog or water droplets condensed from
steam; so the appearance of individual geyser basins depends largely on
air temperature and humidity. On a warm, dry summer day, for example,
the activity may seem very weak (fig. 44), except where individual
geysers are erupting. On cold or very humid days, however, “steam”
plumes are seen rising from every quarter.