From the bookcover flaps:
Grand Prismatic Hot Spring
Named for the prism of reflected colors in the steam it produces, Grand Prismatic Hot Spring is one of the most spectacular sights in North America.
Grand Prismatic is ringed by a stunning variety of colors, which are actually created by a number of species of thermophiles (heat-loving bacteria) that flourish in the various narrow temperature ranges. The blue water in the center is too hot to support any bacterial life, while the outer orange rings represent the coolest water.
Thermophilic bacteria populations range in color from green to red, depending upon the temperatures in which each specie thrives.
The colors seen in these photographs are a function of each bacteria specie and of the changing light and cloud cover. They have not been created artificially.
Mammoth Hot Springs
The terraces of Mammoth Hot Springs are formed when heated water ascends through the ancient limestone deposits of the region. This hot water carries dissolved calcium and bicarbonate to the surface of the terraces, where the carbonate combines with calcium to precipitate as travertine.
Freshly-deposited travertine limestone is bright white in color and easily can be mistaken for snow and ice in these photographs. While initially formed as snow-white rock, microorganisms living on the surfaces of the travertine gradually create beautiful shades of oranges, pinks, yellows, greens, and browns.