Driving through the west side of Yellowstone, we were treated to a myriad of hot springs, fumeroles, and other hydrothermal areas.. Hydrothermal features are rarities of nature and Yellowstone has the largest collection of these in the world. Each year, new hot springs and geysers appears and others become dormant. Geologic events, such as small earthquakes, can trigger some of these changes. The geysers and hot springs may also create changes in themselves. Some hot springs cam rapidly dissolve underground rock. As the 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 materialize as hot, pressurized water seeks a route to the surface. Though change takes place naturally in hydrothermal areas, people can disrupt these processes and cause damage. By throwing rocks, sticks, and other objects into a hydrothermal feature people can cause irreparable damage. Foreign objects can be permanently cemented in place, choking off water circulation and ending all activity. Some of the hydrothermal features can undergo dramatic behavioral changes. Clear pools can become muddy and boil violently and some can become temporary geysers. Some geysers can cease eruption or have altered cycles. New features can 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." Water chemistry is diverse among the hydrothermal areas of Yellowstone. The water levels of the many underground hot water reservoirs can fluctuate, thus affecting the concentrations of chloride, sulfate, iron, and arsenic. As the chemistry of the underground waters changes, dramatic changes in minerals and pH can occur, affecting the thermophiles inhabiting these waters which in turn can affect the colors of the waters. Yellow deposits typically contain sulfur which is formed from the conversion of hydrogen sulfide and contributes to the prominent rotten-egg smell. Some thermophiles use the sulfur for energy and form communities in the hottest acidic runoff which measures between 140 and 181 degrees. Dark brown, rust, or red colors contain varying amounts of iron. The bacteria inhabiting these areas metabolize and deposit iron, often containing high levels of arsenic and generally form communities in water below 140 degrees. Green areas of water indicate a high concentration of algae as the dominant life form. The algae contains chlorophyll, a green pigment which converts sunlight to energy. These bacteria form in areas below 133 degrees. The thermophile communities change as the water cools and chemistry changes at the edges of features and runoff channels. Color placement within thermal waters can change due to the changes in temperature and chemistry. For example, in a hot spring, the hottest water is closest to the 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 outward, mixes with other water sources, and becomes concentrated or diluted. As temperatures and chemical compositions change, the microbial populations change - along with the colors they produce - shifting to a location they favor. Yellowstone holds Earth's largest and most diverse collection of geothermal features. This vast collection of thermal features provides a constant reminder of the park's recent volatile volcanic past. Indeed, the caldera itself provides the setting that allows these features to exist. Water falls as snow or rain in the high mountain areas surrounding the Yellowstone Plateau and slowly percolates through layers of porous rock, eventually finding its way through cracks and fissures in the earth's crust created by the ring fracturing and the collapse of the caldera. Sinking to a depth of nearly 10,000 feet, this cold water comes into contact with the hot rocks in the magma chamber beneath the surface. As the water is heated, its temperature rises well above the boiling point. This superheated water, however, remains in a liquid state because of the great pressure and weight pushing down on it from overlying rock and water. Essentially, the result is akin to a giant pressure cooker, with water temperatures exceeding 400 degrees. This highly energized water is not as dense as the colder, heavier water from above, sinking around it. This creates convection currents that allow the lighter, more buoyant, superheated water water to flow back toward the surface through rhyolitic lava flows, following the cracks, fissures, and weak areas of the earth's crust. As the water travels through this natural plumbing system, the high temperatures dissolve some of the silica in the silica-abundant rhyolite, yielding a solution of silica within the water. At the surface, these silica-laden waters form a rock called geyserite or sinter, creating massive geyser cones, the scalloped edges of hot springs, and the expansive, barren landscape of geyser basins. While still underground, some of this silica is deposited on the walls of the plumbing system, locking in the water and creating a system that can withstand the intense pressure needed to form a geyser. As the superheated water rises through this complex plumbing system, the immense pressure exerted over the water drops as it nears the surface. The heat energy, released in a slow steady manner, gives rise to a hot spring.