[Illustration: NORRIS GEYSER BASIN, as viewed northward from the Norris Museum. This is one of the most active thermal areas in Yellowstone, but the photograph was taken on a warm dry summer day when little hot-water and steam activity was visible from a distance. Clouds of water droplets (the visible “steam” in thermal areas) normally form only when the air is cool and (or) moist. The floor of the basin is covered by a nearly solid layer of hot-spring deposits. (Fig. 44)] [Illustration: HEAT FLOW AND SURFACE WATER. Diagram showing a thermal system, according to the explanation that water of surface origin circulates and is heated at great depths. (Based on information supplied by D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H. Truesdell.) (Fig. 45)] Water enters at ground surface and sinks in conduit formed by fault or fracture Surface (meteoric) water sinks to levels perhaps as much as 10,000 feet below ground. Heated far above its normal boiling point, it begins to rise toward the surface Descending cool surface water Permeable zone allows water to flow through it Cooling magma chamber Water begins to boil near ground surface because of greatly reduced pressures Rising hot water Hot spring or geyser [Illustration: 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).