Castle Geyser, a cone-type geyser located in the Upper Geyser Basin, is likely the oldest geyser in the basin. Its major water eruptions typically last 20 minutes and reach up to 90 feet (30m) in height, and are followed by a lengthy noisy steam phase. Castle's eruptions are classified as major, in which a steam phase follows the water eruptions, and minor, with no steam phase. Intervals following a major eruption are relatively predictable, typically about 12 hours, although the interval following a minor eruption is unpredictable in length. Castle Geyser's sinter cone is likely at least 5,000 years old, with one source suggesting it is perhaps 15,000 years old. It sits upon a much broader, deeper sinter foundation, created by a previous hot spring and estimated to be over 200,000 years old! Castle Geyser's cone, 12 feet (4m) high, is the largest among geysers in the Upper Geyser Basin in Yellowstone National Park. The deeply colored grounds adjacent to Castle Geyser are caused by thermophilic cyanobacteria and algae in Tortoise Shell Spring.
Castle Geyser erupting. Upper Geyser Basin. Yellowstone National Park, Wyoming, USA