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Emerald Pool is the most colorful and famous of Black Sand Basin's features. It is a deep emerald green color with yellow and orange fringes forming the outer ring of the pool. The color is the result of a lower temperature, 154.6 degrees Fahrenheit, 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 asborbs the other hues of the color spectrum, and the combination produces its green color. The outer ring is a fringe of yellow and orange, where the temperatures are cooler due to a change in bacteria and algae growth.
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Sunset Lake is a shallow thermal pool with a soft sinter bottom and with yellow and orange bacteria and algae edges. In 1959, an earthquake triggered an eruption at Sunset Lake, and the surge of hot water killed the bacteria and algae in the run-off channels. Sunset Lake has erupted only occasionally since 1959. Eruptions surge heated waters of 180 degrees Fahrenheit into the air three feet high but may reach eight to ten feet. The pool discharges into Iron Creek, and overflows into Rainbow Pool creating a large microbial mat between the two thermal features.
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Rainbow Pool displays the colors of a rainbow along its edges. Algae and cyanobacteria are responsible for the varied colors. Rainbow Pool has only erupted a few times in the past. In a rare eruption in 1948 it reached a height of 25 feet. The last known eruption was in 1973.
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Spouter Geyser is believed to have been a continuous geyser, and today it continues to erupt almost regularly with only a short 1 to 2 hour interval. The geyser erupts from a splashing, undulating, blue pool of water, and is catergorized as a fountain type geyser. Eruptions shoot heated water of temperatures near 200 degrees Fahrenheit to a height of five to seven feet. The crater, formed of sinter with intricate scalloped edges and rosette beads, drains after an eruption. There is no existing subsurface connection with any other spring in the basin, so the overflow from the geyser discharges into Opalescent Pool.
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Opalescent Pool is another colorful pool, and 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. By the early 1950s it was nearly a dry pool. The Spouter Geyser began erupting and its run-off flowed into Opalescent Pool. 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 skeleton of trees. This silica, a non-crystalline compound, slowly impregnates the wood and over time, with the absence of oxygen, could eventually petrify the wood. Opalescent Pool offers a magnificent contrast in its natural creation of white skeletons amidst a rainbow colored pool.
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Cliff Geyser is named for its cliff-like wall of geyserite formed around the crater and for its location on the edge of Iron Creek. The geyser is irregular in its eruptions, but 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. The eruption is followed by a tremendous amount of steam. In the first half hour, the eruption reaches its highest point, and gradually subsides until the crater empties. The interval of eruptions is irregular, lasting between a half hour to 18 hours, When the geyser is active there are usually one or two eruptions a day. There may also be periods of dormancy lasting weeks or possible even years.
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The river running through this area is Iron Spring Creek. It is as colorful as the hot springs.