Grand Prismatic Spring is the largest hot spring in the United States and the third largest in the world, located in the Midway Geyser Basin of Yellowstone National Park. Measuring approximately 370 feet (113 metres) in diameter and over 120 feet (37 metres) deep, it discharges an estimated 560 gallons (2,120 litres) of water per minute at a temperature of around 160°F (70°C). The spring is fed by groundwater heated by the park’s underlying volcanic system, which drives geothermal activity throughout the region.
The feature is best known for its vivid, concentric rings of colour, ranging from deep blue at the centre to greens, yellows, oranges, and reds toward the edges. These colours are produced by microbial mats composed of thermophilic bacteria and archaea, which thrive in specific temperature ranges. In the hottest water near the centre, only heat-tolerant microbes survive, giving the water its clear blue appearance. Cooler zones at the edges support different microbial communities, producing the yellow and orange tones visible year-round. Seasonal variations in microbial composition can subtly alter the colour patterns.
Visitors can view Grand Prismatic Spring from two primary locations. The boardwalks of Midway Geyser Basin provide close-up access to the spring’s edge, offering a detailed view of the microbial mats and mineral deposits. For a broader perspective, the Grand Prismatic Spring Overlook Trail, accessed from the Fairy Falls parking area, provides an elevated vantage point from which the entire spring and its colours can be seen.
Geologically, the spring is part of Yellowstone’s extensive hydrothermal system, driven by heat from a shallow magma chamber beneath the park. Rain and snowmelt seep into the ground, become superheated, and rise back to the surface through fractures, emerging as hot springs, geysers, and fumaroles.
Grand Prismatic Spring is a key feature for understanding the intersection of geology, microbiology, and chemistry in Yellowstone. It is also one of the park’s most photographed sites, attracting large numbers of visitors in summer. Due to its geothermal nature, leaving the designated boardwalks is prohibited, both for visitor safety and to protect the delicate microbial ecosystem that gives the spring its distinctive appearance.
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