View over Spouter Geyser and the Iron Spring Creek in the Black Sand Geyser Basin of Yellowstone National Park, Wyoming, USA

Opalescent pool in the Black Sand Basin of the Upper Geyser Basin in Yellowstone National Park, Wyoming, USA

Opalescent pool in the Black Sand Basin of the Upper Geyser Basin in Yellowstone National Park, Wyomin, USA. 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.