Midway Geyser Basin Trailhead

Managing agency

National Park Service

Midway Geyser Basin is small but spectacular. Excelsior Geyser is a 200 x 300 foot crater that constantly discharges more than 4,000 gallons of water per minute into the Firehole River. Grand Prismatic Spring, Yellowstone’s largest hot spring, is 200–330 feet in diameter and more than 121 feet deep. Hot Springs Hot springs are the most common hydrothermal features in Yellowstone. Their plumbing has no constrictions. Superheated water cools as it reaches the surface, sinks, and is replaced by hotter water from below. This circulation prevents water from reaching the temperature needed to set off an eruption. Color and Heat Lovers Hydrothermal features are also habitats in which microscopic organisms survive and thrive. They are called thermophiles: "thermo" for heat and "phile" for lover. Although they are too small to be seen with the naked eye, trillions are grouped together and appear as masses of color. They are nourished by energy and chemical building blocks. Colorless and yellow thermophiles grow in the hottest water. Orange, brown, and green thermophiles grow in cooler waters. Imagine living in near-boiling temperatures, in hydrothermal features with the alkalinity of baking soda, or in water so acidic that it can burn holes in clothing. Microorganisms in Yellowstone need these extremes to survive. Geysers Geysers have constrictions in their plumbing systems that prevent water from moving freely to the surface where heat would escape. Water beneath the constrictions creates a buildup of steam. Eventually the steam pushes water past the constrictions and the geyser erupts. Accessibility Wheelchair-accessible parking, boardwalk, and restrooms. Use Caution in Hydrothermal Areas Stay on boardwalks and designated trails. Hydrothermal water can severely burn you. Never run, push, or shove. Supervise children at all times. Do not scratch hydrothermal mats. You are responsible for your safety. Think safety, act safely. Yellowstone is a dangerous place.