INFRARED IMAGE of a part of Upper Geyser Basin. Infrared instruments, sensitive to heat, are able to detect “hot” spots in the landscape. Note especially the sharp “image” of Old Faithful. (Image courtesy of National Aeronautics and Space Administration.) (Fig. 46)
A second, equally essential ingredient for thermal activity is water. Many thousands of gallons are discharged by the hot springs and geysers in Yellowstone every minute—where does all this water come from? Studies show that nearly all the water originates above ground as rain or snow (meteoric water; [fig. 45]), and that very little comes from the underlying magma (magmatic water).
The mechanism for heating the water, on the other hand, is a matter of some uncertainty. Until a few years ago the heating was assumed to occur near the ground surface and to be caused by hot magmatic gases (mostly steam) rising from the underlying magma chamber. Deep wells drilled recently in many thermal areas throughout the world (including research drill holes in Yellowstone), however, suggest a better explanation. According to this explanation, the surface water enters underground passages (fractures and faults) and circulates to great depths—as much as 5,000-10,000 feet in some areas ([fig. 45])—there to become heated far above its surface boiling point. Research drill holes in Yellowstone, for example, have demonstrated that water of surface origin exists at all depths at least to the maximum drilled (1,088 feet), and that the water reaches temperatures up to at least 465°F. The increase in temperature with depth causes a corresponding decrease in the weight (density) of the water. Because of this, the hot, “lighter,” water begins to rise again toward the ground surface, pushed upward by the colder, “heavier,” near-surface water which sinks to keep the water channels filled. Thus is set into motion a giant convection current which operates continuously to supply very hot water to the thermal areas ([fig. 45]). Just how deep the waters circulate in Yellowstone no one really knows; as a guess, the depth probably is at least 1 or 2 miles.
The effect of pressure on the boiling temperature of water also plays a vital role in thermal activity. In a body of water, the pressure at the surface is that exerted by the weight of air above it (atmospheric pressure). Water under these conditions boils at 212°F at sea level and at about 199°F at the elevation of most of the geyser basins in Yellowstone. However, water at depth not only is subjected to atmospheric pressure but also bears the added weight of the overlying water. Under such additional pressures, water boils only when the temperature is raised above its surface boiling point. In a well 100 feet deep at sea level, for example, the water at the bottom would have to be heated to 288°F before it will boil. Thus it follows that in the underground “workings” of hot springs or geysers, (1) The deepest water is subjected to the greatest pressures, and (2) these deeper waters (in Yellowstone) must be heated well above 199°F before they can actually begin to boil. By this same reasoning but in reverse, if the pressure is released, which happens as the water rises toward the ground surface, the “hotter-than-boiling” water will then begin to boil. The boiling will be rather quiet if the pressure is released gradually, as in most hot springs. But if the pressure is released suddenly, boiling may become so violent that much of the water flashes explosively into steam, expanding to several hundred times its normal volume. This expansion provides the necessary energy for geyser eruptions.
Hot-spring deposits and algae
MOUND OF SINTER at Castle Geyser, Upper Geyser Basin. Lower part of mound has well-defined layers probably deposited by normal hot springs. The upper, irregular part resulted from the vigorous eruptions characteristic of geysers and marks a change in the local hot-spring activity. (Fig. 47)
Nearly all geysers and many hot springs build mounds or terraces of mineral deposits; some are so unusual in form that descriptive names have been given to them, such as Castle Geyser ([fig. 47]). These deposits are generally made up of many very thin layers of rock. Each layer represents a crust or film of rock-forming mineral which was originally dissolved in hot water as it flowed through the underground rocks, and which was then precipitated as the water spread out over the surrounding ground surface.
TERRACES OF TRAVERTINE at Opal Springs, Mammoth Hot Springs area. (Fig. 48)