Geyser quick guide

Photo of Grand Prismatic Spring from the "Attractions within 3-4 hours drive of Idaho Falls" page.

Yellowstone National Park is home to some 10,000 thermal features, over 500 hundred of which are geysers. In fact, Yellowstone contains the majority of the worlds geysers. Within Yellowstone's thermal features can be seen the product of millions of years of geology at work. Much of Yellowstone sits inside an ancient volcanic caldera (the exploded crater of a volcano). The last major caldera forming eruption occurred 600,000 years ago. For hundreds of thousands of years following that, subsequent lava flows slowly filled in most of the caldera. Even now, in some places, nearly molten rock resides as little as 2-5 miles below the surface. Heat from the volcanic activity makes its presence known by heating ground water and creating the thermal features we now see. The four basic types of thermal features present in the Park are geysers, hot springs, fumaroles, and mudpots. Many of these are concentrated in Yellowstone's major geyser basins: Upper, Midway, Lower, Norris, West Thumb, Shoshone and Heart Lake.

Geysers are hot springs that erupt periodically. The eruptions is the result of super-heated water below-ground becoming trapped in channels leading to the surface. The hottest temperatures are at the bottom of these channels (nearer the hot rock that heats the water) but the deep water cannot vaporize because of the weight of the water above. Instead, steam is sent upwards in bubbles, collecting in the channel's tight spots until they essentially become clogged, leading to a point where the confined bubbles actually lift the water above, causing the geyser to overflow. This causes the pressure to decrease until suddenly violent boiling occurs throughout much the length of the column, producing a tremendous volume of steam which forces the water out of the vent in a superheated mass. This is an eruption. As the eruption continues, the heat and pressure gradually decrease, and the eruption stops when the water reservoir is depleted or the steam runs out. The two types of geysers are fountain geysers (which shoot water out in various directions through a pool) and cone geysers (which shoot water out in a fairly narrow jet, usually from a cone-like formation).

Figure of the workings of a geyser

Hot Springs are similar to geysers, but their underground channels are large enough to allow rapid circulation of water. Rising hot water releases heat energy by evaporation or hot water runoff, while convection currents return the cooler water to the underground system, thus maintaining equilibrium. The microorganisms which live in and around the hot springs often make the pools very colorful.

Fumaroles are holes or vents from which steam rushes into the air. It is like a hot spring, but lacks liquid water. Either there isn't enough water or the underground rock is too hat and boils off all of the water so a pool can't form. The small amount of water that does seep into the area is converted to steam and expelled from the vent, oftentimes creating a hissing noise.

Mudpots are thermal areas where water-saturated sediment (similar to clay) is affected by super-heated steam below. Rising steam forces its way upwards through the mud and ground water, bursting upwards sending showers of mud into the air, as if in a small explosion.

The Upper Geyser Basin has the largest concentration of geysers in the world. Learn more about the basin by following this link.

Located in the Lower Geyser Basin are some very interesting and easy to see thermal features including some very nice mud pots. Learn more about the basin by following this link.

Norris Geyser Basin is the hottest and most changeable of Yellowstone's geyser basins. Learn more about the basin by following this link.

The formations at Mammoth Hot Springs are formed differently than the formations in the geyser basins. The sinter formations of the geyser basins are formed when silica dissolved from rock deep underground, by extremely hot water, is deposited at the surface when the water cools. The travertine terraces at Mammoth are formed from limestone deposited when carbon dioxide gas is released from the warm limestone impregnated water that reaches the surface at Mammoth.

The carbon dioxide makes the water slightly acidic allowing it to dissolve and carry large amounts of limestone in solution as it flows underground to Mammoth. Once the water reaches the surface, the carbon dioxide is released and the limestone falls out of solution and is deposited as travertine terraces. This deposition can be quite rapid, sometimes being measured in feet per year. By comparison, sinter deposits in the Upper Geyser Basin are measured in inches per hundred years.

As might be expect from such rapid deposition, water flows at Mammoth are constantly being redirected. Water is the life blood of the terraces. When the water stops, they dry up and disintegrate. Thus, the activity at Mammoth changes quickly. There are few places that geologic changes occur so quickly.

As of July 1997, the best activity at Mammoth was at Minerva Terrace and at Canary Spring. Both of these must-see attractions are located on the Main Terrace. It is easiest to explore the Main Terrace in two parts. First, park your car at the bottom, preferably at the eastern most parking area. (It is the highest in elevation of the parking areas at the bottom and the one most distant from the town site). From there walk the loop trail of the lower portion of the Main Terrace. Be sure to pass by Minerva Terrace. Once you've done this, then, return to your car and drive to the one way, Upper Terrace Drive. In this way you will avoid the steep stairs to the top of the Main terrace. On the Upper Terrace Drive be sure to stop at the Main Terrace Overlook and take the trail on the far right to Canary Spring. For the past few years, Canary Spring has been truly spectacular.

The activity at the Mud Volcano area is interesting and worth exploring. The temperatures here are lower than at the geyser basins. In most cases, what looks like boiling is actually just the evolution of gasses, especially carbon dioxide.

Midway Geyser Basin:: Midway is home to two of the largest thermal features in Yellowstone. Grand Prismatic Spring, at over 300 feet across is the largest hot spring in Yellowstone (see the picture at the top of this page.) The various colors of the cyano-bacteria around the pool are quite pretty. Excelsior Geyser was the largest geyser in the world in the 1880s. Now it is just a very large crater that emits a copious amount of water. After nearly 100 years of inactivity, it had some minor eruptions in 1984. there are a few other pools and geysers at Midway.
West Thumb Geyser Basin:: West Thumb is a small collection of geysers and hot springs located on the shore of Lake Yellowstone. This backdrop can be very pretty. Two of Yellowstone's prettiest pools, Black Pool and Abyss Pool are found here.
Black Sand Basin:: Black Sand Basin is actually part of the Upper Geyser basin. There are a couple of frequent small geysers and a couple of large very infrequent geysers here but the stars of the area are the very colorful hot springs.
Biscuit Basin:: Biscuit Basin is also part of the Upper Geyser Basin. There are a number of small geysers here. The most spectacular is probably Jewel Geyser which erupts about every 10 minutes to 10-25 feet. Biscuit basin was named for biscuit shaped sinter formations formerly found around Sapphire Pool. For a few years following the 1959 Hebgen Lake earthquake, Sapphire became a major geyser. In the process, it greatly enlarged its crater and washed all the biscuit formations away. Sapphire is now a quiet green colored pool.
Lone Star Geyser Basin:: This area is reached by an easy and enjoyable three mile trail that follows the Firehole River. This is one of the few trails in Yellowstone open to bicycles. The small basin is dominated by Lone Star Geyser. Lone Star erupts at three hour intervals and is very predictable. About 1/2 hour prior to the major eruption it has a minor eruption which lasts about 5 minutes. The major eruption lasts about 30 minutes and is concluded with a loud steam phase. The eruption reaches about 45 feet. Rarely, the major eruption is preceded by two minor eruptions. Lone Star is a nice destination for a day hike and picnic, assuming you take bug repellent.
Shoshone Geyser Basin:: Shoshone Geyser Basin is in the backcountry, only accessible by a 9 mile trail. It contains at least 40 geysers. Many of which are quite active. The main performer is Minute Man Geyser which can erupt to 40 feet.
Heart Lake Geyser Basin:: Heart Lake Geyser Basin is in the backcountry, only accessible by trail. the first geysers are encountered about five miles in. There are close to 40 geysers in the basin which stretches for about 2 1/2 miles along Witch Creek. Two of the major geysers are 50' Rustic and 60' Glade.