Yellowstone is a country teeming with an estimated 10,000 thermal features. Of these only three percent are geysers. The rest are steaming pools, hissing fumaroles, bubbling mud pots or warm seeps. Most of Yellowstone's geysers are small, and sputter and splash, barely reaching ten feet in height. Only six grand geysers, those which erupt 100 feet or higher on a predictable daily basis, exist. Old Faithful, the most famous of these, erupts once approximately every 45 to 90 minutes.

Beneath the thermal basins lie the mechanisms which control these features. The most essential element is magma-underground molten rock. No one knows exactly how close to the surface this body of magma lies. Geologists believe the earth's crust is less than 40 miles thick in Yellowstone compared to 90 miles under most other land areas. Intruded into this thinner crust. like bubbles, are magma intrusions-also known as plutons. These intrusions are typically at a depth of four to ten miles. In most regions of the earth's crust the temperature gradient averages about 1.4°F per 100 feet. A test well drilled at Norris Geyser Basin during the late 1920s revealed a temperature of 401 °F at a depth of 265 feet. Thus the ground temperature gradient at this point is an astonishing 123°F per 100 feet.

"The Wonders of Yellowstone" - 98 Minutes - ~Telly Award Winner for Nature and Wildlife~ Two years in the making and just released, "The Wonders of Yellowstone" video has been highly requested, produced in DVD format and is now available. Take a complete tour of Yellowstone National Park as our Narrator Cathy Coan guides you to all the wonders of the park including all the geyser basins, wildlife, waterfalls and much more. We previously sold travel packets but these packets, maps and trail guides are all available at the park for free or minimal charge. More Info or Order Online

A transition zone exists between the hot molten magma and the crust. This pliable layer of partially molten crystalline rock is close to its melting point. This zone heats water which has seeped down from the surface. Water filters through fissures, cracks, and porous rock and eventually circulates to a depth of about two miles. There it becomes heated by molten crystalline rock to a temperature above its surface boiling point. But it does not boil because of the pressure in the underground network. As the superheated waterworks its way up through the subterranean chambers and conduits, pressure on it is relieved. The naturally formed plumbing system traps superheated water before it reaches the surface and cools. Near the surface, when pressure is suddenly released, boiling explosions occur forming steam. The sudden expansion in volume then triggers a series of reactions leading to a geyser eruption. The frequency or interval of an eruption depends on how much energy is spent and how quickly a geyser recovers. Thus Geysers vary in their interval, duration, and volume of discharge. These variables also determine the opening or vent surrounding a geyser. Two types of mineral deposits form around geyser vents. Cone-type formations have formed around Old Faithful, Castle, Beehive, and Lone Star geysers. Fountain-type formations, which contain a pool of water over their vents, have formed around Great Fountain and Grand geysers.

Yellowstone's thermal features could not exist without
the presence of three essential elements. First there
must be a heat source. This heat source, a magma intrusion, typically at a depth of four to ten miles, heats the surrounding crystalline rock by conduction. The second essential element is water, which falls as rain and snow and percolates to a depth of two to three miles following cracks and fissures. The water collects in a porous reservoir where it becomes heated by the magma intrusion.The superheated water wants to expand. Because of the depth and pressure the water begins to rise and the heated water moves through the third essential element, rhyolite, asilica-bearing rock, hard and strong enough to withstand heat and pressure. As the thermal water reaches the surface pressure is relieved and the water emerges as either a geyser, hot spring, fumarole or mud pot.

Mineral-laden water, rich in silica, deposits on the lining of subterranean chambers and conduits and forms a natural plumbing system which is located within 200 feet of the surface. A geyser must have a nearly vertical underground tube that connects with side chambers or porous rock, where water can accumulate and act as a reservoir. A geyser must also have a constriction or narrowing near the surface in the natural plumbing system. This constriction acts as a check valve, like the workings of a pressure cooker. In some twisted networks, water can cool substantially; lacking a constriction, water oozes from the vent as a hot spring. Water does not reach the surface in a fumarole. Only steam and gases echo up its throat, causing it to hiss and roar.

Geysers maintain a delicate balance between water and steam. A slight change can upset an eruption. A pre-eruptive splash may trigger a major eruption or cause a delay. Man can also have this same effect. Coins, sticks, stones, handkerchiefs, or soap thrown into a Yellowstone thermal feature can cause it to erupt prematurely, or more likely, cause it to clog, wither, and die. Earthquakes also play a major role in upsetting the delicate balance of geysers. Near midnight on August 17, 1959 an earthquake, epicentered twelve miles north of West Yellowstone near Hebgen Lake, shook eight surrounding states. It measured 7.1 on the Richter scale, formed a twenty foot displacement, and carved a slab from a mountain side which dammed the Madison River. In Yellowstone thermal activity increased. Geysers began to erupt, some with new vigor. Dormant geysers awoke and hot pools surged with excess water. The earthquake also caused some geysers to decrease in activity and shut others off completely.

The thermal features of Yellowstone could not exist without the rock types found beneath the thermal basins. Hard minerals and rocks are needed to withstand intense heat and pressure. Except for Mammoth Hot Springs, most superheated geyser water passes through rhyolite and volcanic ash and tuff. These rocks consist mainly of silica, a hard mineral found in quartz and glass. When superheated water passes through these rocks it becomes laden with silica and carried to the surface. Some of the silica deposits on the thermal features underground plumbing, thus lining and hardening the conduits. The remainder surfaces and deposits externally. During an eruption a geyser will splash the mineral-laden water around its vent. Given ample time between eruptions, the water will evaporate, and deposit silica. Silica forms sinter, or geyserite, the notable rock formation built around these features. Sinter can form delicate scalloped edges on hot pools and elaborate cones on geysers. The amount of deposit surrounding a feature does not always determine its age. Drilling and core samples made in the Upper Geyser Basin revealed a sinter layer nearly 20 feet thick. Deposition varies and depends on the amount of silica brought to the surface and this is not always consistent. A large sinter mound does not always mean an old thermal feature. But most sinter accumulation is only a minute fraction of an inch annually. Waters flowing from hydrothermal springs contain several dissolved minerals and gases. Waters from these springs are classified into four groups: alkali chloride; acid sulfate; acid sulfate-chloride; and bicarbonate. These minerals in turn determine the water's pH as acidic, neutral or alkaline. Most geyser basins are either acidic or alkaline, and in some, like Norris, acidic and alkaline springs flow side by side. Some of the thermal features emit strong or obnoxious smells and even deadly odoriess gases. These gases, released at the surface after the pressure lowers, include carbon dioxide, hydrogen sulfide, methane, hydrogen, oxygen, nitrogen, ammonia, argon, radon, as well as other noble gases such as helium, neon, krypton, and xenon. Travertine is the mineral formation responsible for the famous Mammoth Terraces. The mineral-calcium carbonate-is carried to the surface like sinter, but dissolves in heated water and precipitates or deposits into rinds or terraces as the water cools and evaporates. The mineral is softer than sinter and does not form the hard concrete-like encrustations and thus cannot withstand the intense heat and pressure needed to form a geyser's natural plumbing system. Calcium carbonate is white when fresh and ages to a dull gray, but in the run-off channels various colors of algae and bacteria add their brilliance, highlighting the delicate travertine draperies.

In Yellowstone, hot springs, pools, and run-off channels exhibit all colors of the rainbow. Bacteria and algae are mainly responsible for brightly colored run-off channels. Different temperatures of water both cause and also permit differences in plant communities and intensities of color. The run-off channel from a hot spring, for example, is white or clear near its source. Only a few single-cell bacteria live in this boiling water, which is 199°F at the average Yellowstone elevation of 7,500 feet. Pure water boils at 212°F at sea level. As the water slightly cools other forms of bacteria develop long hair-like strands which become visible to the naked eye. These thermophilic hot-water-loving species may be remnants of some of the earliest life on earth. As water cools to 167°F farther down stream, the first colorful forms of cyanobacterium (Synechococcus lividus), filamentous green nonsulfur bacterium (Chloroflexus aurantiacus)andpwpte-suHwbactenum(Chromatiun1tepidum) begin to colonize and form laminated mats. Only the first millimeter of a microbial mat actively grows. The top bacteria layer shades the bacteria below and new individuals grow upon the remains of the previous generation, thus forming a laminated mat. In colder, acidiewaterthick, long strands of brown filamentous bacterium (Zygongonium) wave and undulate in the flowing channels. Thus, water chemistry and temperature determine the species present in a run-off channel. Pigments within microorganisms are responsible for their colors. Chlorophyll produces grass-green, carotenoids are yellow, orange or red. And all pigments found in living cells intensify or become muted directly by the amount of light they receive daily or seasonally.

Large hot pools also radiate with brilliant colors ranging from deep blue to emerald green. These waters reflect the blue color rays present in sunlight and absorb the remainder of the color spectrum. A blue hot pool changes its mood from day to day, depending upon the intensity of light or the amount of particulate matter suspended in the water. Fine particles of silica or clay suspended in the water produce a baby blue colored pool, a blue thermal pool lined with yellow sulfur or yellow bacteria produces hues of green, forming emerald-colored pools. Black pools are often lined with orange-colored bacteria which coupled with blue tinted water, results in pools dark-colored in appearance. Other plants and animals thrive among the geyser basins. Small black ephydrid flies live on the microbial mats. They swarm on the shallow run-off channels, feeding upon algae and bacteria, laying their bright orange egg clusters on any small stick or rock projecting above the mat. Their larvae burrow and feed in the microbial mat and can tolerate temperatures up to 116°F. During winter the flies live a precarious life in a warm zone close to the water. If the flies stray away from this warm protective zone, the cold winter air would freeze them in seconds. But they move up and down the run-off channels seeking their proper temperature. In summer other perils take their toll. Large predatory wolf spiders, dragonflies, and killdeers pray upon and eat the small flies and their larvae.

By looking closely at the microbial mats with the naked eye it is possible to find the tiny hot spring mite. No larger than a pin head they are mainly visible because of their bright vermilion color. They are structurally adapted for creeping beneath the surface film at the water's edge and for moving in open water. Temperature controls their distribution. When the temperature is below 113°F they move up stream. When it is above 122°F they move down stream. This behavioral adaptation assures the animals of nearly constant temperatures year round. Adults are predatory upon fly larvae and nymphs are assumed to be parasitic, but their host is not known. The hottest natural waters that fish inhabit does not exceed 104°F, although there may be fish able to tolerate water of 110°F for a few minutes. Fish and aquatic animals (protozoans, rotifers, nematodes, and annelids) confront the problems of extreme heat and also of low oxygen content, for relatively little oxygen is dissolved in hot water. For this reason fish and arthropods do not live in the thermal springs.

The warmth of the thermal basins does stimulate early germination of plant life. During winter, in isolated, protected pockets, mosses, grasses and even flowers can thrive and flower. In early spring when run-off is high and the ground is moist, yellowmonkey flowers line hotspring channels. By early summer, purple fringed gentians cluster near the hot springs in moist bogs or gravelly soil. Later in the summer when the basin soils dry, yellow star-like blossoms of stonecrop appear on the desert-like pavement.

- Carl Schreier - A Field Guide to Yellowstone's  Geyser's
 Hot Springs and Fumaroles