Black Sand Basin, an isolated group of the Upper Geyser Basin, was originally named the Emerald Group by A.C. Peale in 1878. But turn of the century tourists began calling it Black Sand Basin because of the small fragments of black obsidian sand which cover portions of the basin.

Black Sand Basin contains a small collection of jewel-like geysers, and colorful hot springs. Emerald Pool is the most colorful and famous of these springs. It is a deep emerald green fringed by an outer ring of yellow and orange. Another colorful pool is Opalescent Pool. This recently formed pool inundated a stand of lodgepole pine, creating a stand of white skeletons amidst a rainbow-colored pool. An unusual geyser formed on the bank of Iron Creek. Cliff Geyser formed a rim or wall-like ridge of sinter around its crater from which it erupts 30 to 40 feet high.

The famous Handkerchief Pool was once the drawing attraction to Black Sand Basin. Turn-of-the-century tourists dropped their handkerchiefs into this small spring. Convection currents then whisked their laundry away where it would reappear again at the surface, freshly laundered.

OPALESCENT POOL
Temperature 144°F Dimensions 28x55 feet. Depth 6 feet. Opalescent pool has a cooler temperature than other thermal features at Black Sand Basin. Early in its history Opalescent was a boiling spring, surrounded by smaller springs. In the early 1950s it was a small dry pool, then the run-off from Spouter Geyser flowed into it. The increased water flow flooded the surrounding area, killing the lodgepole pine. Since then silica has precipitated upon the dead tree trunks creating the white "bobby sock" trees. This silica, a non-crystalline compound, slowly impregnates the wood and over time, with the absence of oxygen, could eventually petrify the wood.

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SPOUTER GEYSER
Temperature 199.9°F Interval 1-2 hours. Duration 10-11 hours. Height 5-7 feet. Spouter Geyser, once believed to be a continuous geyser, does erupt almost constantly with only a short 1 to 2 hour interval. It is a fountain-type geyser and erupts from a splashing, undulating, blue pool of water. The jets shoot through the pool to a height of five to seven feet. The crater, formed of sinter with intricate scalloped edges and rosette beads, drains after an eruption. No subsurface connection is known to exist with any other spring in the basin. Overflow from Spouter discharges into Opalescent Pool.

CLIFF GEYSER
Temperature 191.8°F Interval irregular. Duration 30 minutes to 3 hours. Height 40 feet. A. C. Peale, geologist for the 1872 Hayden Expedition, named this feature for its cliff-like wall of geyserite formed around the crater and for its location on the edge of Iron Creek. An indication of a pending eruption is that the crater nearly fills with boiling water. As the eruption begins, jets of water explode through the pool 15 to 40 feet high. This is accompanied by a tremendous amount of steam. The eruption reaches its highest point the first half hour and gradually subsides until the crater empties. The interval is irregular, lasting between a half hour to 18 hours, and there may be weeks or even years of dormancy. When it is active there are usually one or two eruptions a day.

EMERALD POOL
Temperature 154.6°F Dimensions 27x38 feet. Depth 25 feet. Named for its emerald green color, it is one of the main attractions at Black Sand Basin. The color is the result of lower temperatures which have allowed yellow bacteria and algae to grow on the lining of the pool. The clear water of the pool reflects the blues but absorbs the other hues of the color spectrum. The combination of blue and yellow then produces green. Objects thrown into the pool and natural debris have caused a further decrease in temperature, resulting in a change of bacteria and algae growth and thus a change of color. The edge of the pool is now orange and brown. If the temperature continues to decrease, the pool may lose its emerald color.

RAINBOW POOL
Temperature 161°F Dimensions 100x130 feet. Depth 27 feet. The edges of this pool display the color of the rainbow, hence the name. Algae and cyanobacteria are responsible for the varied colors. This pool has only erupted a few times in the past. During one eruption in 1948 it reached a height of 25 feet. The last known eruption was in 1973. The famous Handkerchief Pool, located along the southern edge of Rainbow, was a popular pool at the turn-of-the-century. Tourists dropped handkerchiefs at one end of the pool and convection currents would pull them under. A moment later they would reappear in another vent, freshly laundered. The pool has not functioned since 1929 when it became plugged by human vandalism. It is now a small spouter and inaccessible because of microbial mats. Underground connections exist between Rainbow Pool, Green Spring and Handkerchief Pool.

SUNSET LAKE
Temperature 180°F Dimensions 145x191 feet. Depth 23.5 feet. Sunset Lake is a shallow thermal pool with a soft sinter bottom and yellow and orange bacteria and algae edges. The pool discharges into Iron Creek, and overflows into Rainbow Pool creating a large microbial mat between the two thermal features. The 1959 earthquake triggered an eruption of Sunset Lake and the surge of hot water killed the bacteria and algae in the run-off channels. It has erupted only occasionally since 1959; during eruption it surges three feet high but may reach eight to ten feet. No known underground connection exists between Rainbow Pool or other Black Sand Basin thermal features.