The information for this guide was taken from the Norris Geyser Basin Trail Guide, which is available at the trailhead. Norris is outside the Yellowstone Caldera, but inside the first and largest caldera. Norris is one of the most active earthquake areas in the park. This is one of the most acidic hydrothermal areas in Yellowstone. Many acidic geysers, which are rare in the rest of the world, are here. Steamboat Geyser is the tallest active geyser in the world. Norris Geyser Basin is one of the hottest and most dynamic of Yellowstone's hydrothermal areas. Many hot springs and fumaroles have temperatures above the boiling point (200°F) here. Water fluctuations and seismic activity often change features. It's hard to imagine a setting more volatile than Norris. It is part of one of the world's largest active volcanoes. And it sits on the intersection of three major faults. One runs from the north; another runs from the west. These two faults intersect with a ring fracture from the Yellowstone Caldera eruption 640,000 years ago. These conditions helped to create this dynamic geyser basin. Each year at Norris new hot springs and geysers appear; others become dormant. Geologic events cause many of these changes. Even small earthquakes can trigger changes in hydrothermal behavior. Some changes are brief; others last longer. Geysers and hot springs may also create changes in themselves. Some Norris hot springs, like Cistern, rapidly dissolve underground rock. As hot water moves toward the surface, the dissolved minerals deposit along subterranean passages and around the surface vents. Eventually, these deposits can choke off the flow of water. New features may be born as hot, pressurized water seeks a route to the surface. Some features in Norris Geyser Basin can undergo dramatic behavioral changes simultaneously. Clear pools become muddy and boil violently, and some temporarily become geysers. Geysers cease erupting or have altered cycles. New features appear. This sudden activity is known as a "thermal disturbance" and can last a few days or more than a week. Gradually, most features return to "normal." Why this happens is not fully understood. Norris has the greatest water chemistry diversity among Yellowstone's hydrothermal areas. Multiple underground hot water reservoirs exist here and as their water levels fluctuate, concentrations of chloride, sulfate, iron, and arsenic change. Although Norris is known for its acid features, it also has alkaline hot springs and geysers. As underground waters and chemistry shift, they could contribute to sudden dramatic changes in minerals and pH. Further study will help unravel the mystery of this phenomenon. The Colors of Norris Many of the colors you see here are evidence of thermophiles (heat-loving microorganisms) and their activity. Yellow deposits here typically contain sulfur. They form when hydrogen sulfide gas (the rotten egg odor you may have noticed) is converted to sulfur. Some thermophiles live in these areas because they use chemicals like sulfur for energy. They form communities of mats and streamers (formations that look like waving clumps of hair) in the hottest acidic runoff, which measure between 140°F and 181°F. Dark brown, rust, and red colors abound in Norris and contain varying amounts of iron. Red-brown mats may also contain bacteria and archaea that help build the mats by metabolizing and depositing iron. These iron-oxide deposits often contain high levels of arsenic. These communities form in water below 140°F. Emerald-green mats color many of the runoff channels of hot springs and geysers here. Algae are the dominant life forms in these mats and contain chlorophyll, a green pigment that helps convert sunlight to energy. Some bacteria and archaea grow in these mats, which form below 133°F. Dark blackish-green mats form in even cooler water. An alga called Zygogonium forms these communities of mats and streamers. Color placement within thermal water changes, in part, because temperatures and chemistry change. In a hot spring, for example, the hottest water is closest to a hot spring's vent. As the water flows outward, it gradually cools. This range of water temperature, called a thermal gradient, supports various thermophilic habitats. Chemical composition also changes as water flows from thermal features, mixes with other water sources, and is diluted or concentrated. As temperatures or chemical compositions change, microbial populations—and the colors they create—shift to a location they favor. Norris Geyser Basin supports an astounding diversity of life. The majority of species here are microscopic thermophiles—heat-loving microorganisms. They survive in conditions of high heat and acidity or alkalinity that would instantly kill most other life forms. This domain includes bacteria that can cause disease, fertilize soil, recycle material, and renew oxygen.