Yellowstone National Park - Subterranean Pressure Cooker

As water trickles down through the cracks and nears molten rock, it becomes extremely hot, much hotter than it ever could on the surface. This is because it is under pressure, and, as anyone who has ever used a pressure cooker probably knows, pressure raises the boiling point of water. At Yellowstone's elevations, water normally boils at about 199ºF, but, trapped in the deep passageways beneath geysers, it can reach temperatures of 400ºF and more without boiling. Yet although it cannot boil, it can circulate. The hottest water rises to a point where the pressure is less intense, and then begins to bubble.

As the bubbles move up through passages in the earth, they push water before them, and it leaps out of the ground in small spurts. Sometimes this is all there is to a geyser: it spurts continuously. The big geysers, such as Old Faithful, are another story. When water spurts out of them, it is as though the lid were taken off the pressure cooker. Instantly, the deep mass of superheated water boils and roars from the earth.

Spectacular as they are, geysers are but a small part of Yellowstone's incredible range of thermal features. In all, there are about 10,000 such features in the park, ranging from fumaroles, roaring hillside vents or small whistling holes from which no water emerges, only live steam, to beautifully tinted, placid-looking hot lakes. There are bubbling mud pots, caused when fumaroles emerge into subterranean basins, sending acid groundwater up to melt rock into clay. In an area known as the Fountain Paint Pot, hot gases belch up through such mud-filled pits, staining the mud with colored minerals to create a rainbowed moonscape of foot-high cones and plate-sized craters that plop and pop and reshape themselves like the work of some demented medieval alchemist. Elsewhere are ominous acid lakes and cauldrons aboil with a sulfurous black broth.

Hot pools, into which water does not erupt but flows steadily, are often delicately colored, both by minerals and by tiny living creatures that thrive in an unlikely world of water that is too cool to boil yet too hot to be touched. Most of these algae and bacteria form thick green clumps in water that ranges from 122º to 140º F. As the water temperature rises, however, different kinds of algae are able to thrive. Their colors are a guide to the heat of the water. First the green life is replaced by orange, and then, at 163º to 167º F, the orange gives way to yellow. There is even life in the water that approaches the boiling point, but it is tiny, colorless, invisible to our eyes.

In combination with the minerals in the water and in the geyserite beneath it, the living organisms turn hot pools into amazingly patterned palettes of color. In the depths of large springs the water shades to a deep, cobalt blue; but if the geyserite on the bottom contains with the blue of the water to create luminous and variable shades of green. Around the edges, depending on the heat of the water and its acidity, algae add tones of yellow, bright green, orange, brown, and even purple.