Hot-Water and Steam Phenomena
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.