Exploring USGS volcano observatories—Part 4: Yellowstone
Yellowstone National Park is the site of Earth's largest concentration of geysers, as well as steam vents, hot springs, and mudpots. These hydrothermal features attest to the region's volcanic history, which spans over two million years and is the reason that the Yellowstone Volcano Observatory was established. Old Faithful is visible in the upper right corner of this aerial view of the Park's Upper Geyser Basin. INSET: Heart Spring, also in Upper Geyser Basin, is a pool of near-boiling, blue water about 10 feet across and 15 feet deep surrounded by mats of white sinter and orange-brown algae. Castle Geyser can be seen in the background. National Park Service photos.