Yellowstone  is, without a doubt, the most geologically fascinating place on this planet. Here, the forces that elsewhere lie deep within the earth seem close enough to touch. They’re palpable not only in the geysers and hot springs but also in the lake that fills part of an enormous caldera, the earthquakes that shake this land, the evidence of massive glaciations, and the deeply eroded Grand Canyon of the Yellowstone .
When geologists began to study the Yellowstone area in depth, they found a surprising pattern. Extending far to the southwest was a chain of volcanic fields, the most ancient of which—16 million years old—lay in northern Nevada. In addition, geologic fault lines created a 100-mile-wide semicircle around the Yellowstone area, as if some deep-seated force were pushing the land outward and upward the way a ripple moves across a pond.
Both the volcanic activity and the wavelike pattern of faults are caused by the same source: a plume of superheated material moving up from the core of the planet through a narrow tube, creating a hot spot beneath the earth’s crust. As the North American continent slid to the southwest through the eons, this plume traced a line of volcanoes across the West, and as the land continues to move, the plume causes massive deformations in the crust that show up as mountain ranges, fault lines, and earthquakes. It’s a little like somebody tugging a piece of cloth across a candle. Approximately 40 such hot spots are known to exist around the globe, but most are beneath the seas. Other than Yellowstone, the best-known example is the chain of Hawaiian Islands. The earth has been moving over the Yellowstone hot spot at the rate of 15 miles per million years for the last 10 million years, and on the present track, the spot might eventually end up in Hudson Bay in 100 million years or so.
The volcanic activity revealed by Yellowstone’s geysers, hot springs, and fumaroles is not always benign. Within the last two million years there have been three stupendous volcanic eruptions in the area, the most recent taking place 640,000 years ago. The largest of these eruptions occurred two million years ago and created an event beyond the realm of imagination: 600 cubic miles of ash were blasted into the atmosphere! This eruption—probably one of the largest to ever occur on earth—ejected 17 times more material than the massive Tambora eruption of 1815, an explosion that was heard 1,600 miles away. The first Yellowstone event was 2,400 times as large as the 1980 eruption of Mount St. Helens, and it carried ash east to Iowa, north to Saskatchewan, south to the Gulf of Mexico, and west to California. This titanic infusion of ash into the atmosphere undoubtedly affected the global climate for years to follow.
The process that creates these explosions starts when molten rock pushes up from the center of the earth, causing the land to bulge upward into an enormous dome. Eventually the pressure becomes too great and fractures develop around the dome’s margins, sending hot gases, ash, and rock blasting into the atmosphere. After the most recent eruption (640,000 years ago), the magma chamber collapsed into a gigantic, smoldering pit reaching 28 by 47 miles in surface area and perhaps several thousand feet deep. Through time, additional molten rock pushed up from underneath and flowed as thick lava over the land. The most recent of these lava flows was 70,000 years ago.
Two resurgent domes—one near Old Faithful  and the other just north of Yellowstone Lake  near LeHardys Rapids—have been discovered by geologists. Measurements at LeHardys Rapids showed that the land rose almost three feet from 1923 to 1985, before subsiding until 2004, when the land began to rise rapidly—as much as 17 cm (6.7 inches) in just three years. This upsurge is raising the outlet of Yellowstone Lake, creating new beaches on the north shore while flooding forests along the lake’s southern margins. Obviously, Yellowstone’s volcanism is far from dead, and scientists believe another eruption is possible or even likely, although nobody knows when it might occur.
The Yellowstone Volcano Observatory (http://volcanoes.usgs.gov/yvo )—a partnership of the USGS, Park Service, and University of Utah—monitors Yellowstone’s ever-changing volcanic system and produces an excellent small publication about the park’s geologic origins; find it in visitor centers.
Not everything in Yellowstone is the result of volcanic activity. The entire Yellowstone region has undergone a series of at least eight major glaciations during the last million years, the last of which—the Pinedale Glaciation—began about 70,000 years ago. At its peak, the Pinedale Glaciation covered almost all of Yellowstone and reached southward into Jackson Hole  and north much of the way to Livingston, Montana. Over Yellowstone Lake, the ice field was 4,000 feet thick and covered 10,000-foot mountains. This period of glaciation ended about 15,000 years ago, but trees did not appear in the Yellowstone area until 11,500 years ago, and it wasn’t until about 5,000 years ago that the landscape began to appear as it does today.
Yellowstone is famous for geysers, and geyser-gazers will not be disappointed. At least 60 percent of the world’s geysers are in the park, making this easily the largest and most diverse collection in existence. Yellowstone’s more than 300 geysers are spread throughout nine different basins, with half of these in Upper Geyser Basin , the home of Old Faithful .
Geysers need three essentials to exist: water, heat, and fractured rock. The water comes from snow and rain falling on this high plateau, while the heat comes from molten rock close to the earth’s surface. Massive pressures from below have created a ring of fractures around the edge of Yellowstone’s caldera. This is one of the hottest places on the planet, with heat flows more than 60 times the global average.
Geysers operate because cold water is dense and sinks, while hot water is less dense and rises. The periodic eruption of geysers is caused by constrictions in the underground channels that prevent an adequate heat exchange with the surface. Precipitation slowly moves into the earth, eventually contacting the molten rock. Because of high pressures at these depths, water can reach extreme temperatures without vaporizing (as in a pressure cooker). As this superheated water rises back toward the surface, it emerges in hot springs, fumaroles, mud pots, and geysers. The most spectacular of these phenomena are geysers. Two general types of geysers exist in Yellowstone. Fountain geysers (such as Great Fountain Geyser) explode from pools of water and tend to spray water more widely, whereas cone-type geysers (such as Old Faithful) jet out of nozzlelike formations.
In a geyser, steam bubbles upward from the superheated source of water and expands as it rises. These bubbles block the plumbing system, keeping hot water from reaching the surface. Eventually, however, pressure from the bubbles begins to force the cooler water above out of the vent. This initial release triggers a more violent reaction as the sudden lessening of pressure allows the entire column to begin boiling, explosively expelling steam and water to produce a geyser. Once the eruption has emptied the plumbing system, water gradually seeps back into the chambers to begin the process anew. Some of Yellowstone’s geysers have enormous underground caverns that fill with water; in its rare eruptions, massive Steamboat Geyser can blast a million gallons of water into the air!
As an aside, geyser is one of the few Icelandic terms in the English language; it means “to gush forth.” There are probably 30 active geysers in Iceland, many more on Russia’s Kamchatka Peninsula, and a few in New Zealand, where geothermal development has greatly lessened geyser activity.
A fine online source with details about Yellowstone’s geysers—and others around the globe—is maintained by the nonprofit Geyser Observation and Study Association (www.geyserstudy.org ). The GOSA website has frequent postings of geyser activity, and volunteer observers are often present in the park during the summer, particularly at Grand Geyser in Upper Geyser Basin .
Unfortunately, some of Yellowstone’s geysers have been lost because of human stupidity or vandalism. At one time, it was considered great sport to stuff logs, rocks, and even chairs into the geysers for a little added show. Chemicals were also poured into them to make them play. As a result of such actions, some geysers have been severely damaged or destroyed, and some hot springs have become collection points for coins, rocks, sticks, and trash. Such actions ruin these thermal areas for everyone and destroy something that may have been going on for hundreds of years.
Although geysers are Yellowstone’s best-known features, they make up only a tiny fraction of perhaps 10,000 thermal features in the park. Hot springs appear where water can reach the ground surface relatively easily, allowing the heat that builds up in the chambers of geysers to dissipate. When less groundwater is present, you may find fumaroles, vents that shoot steam, carbon dioxide, and even hydrogen sulfide gas. Mud pots (also called paint pots) are essentially wet fumaroles. Hydrogen sulfide gas combines with water to produce hydrosulfuric acid. This acid breaks down surrounding rocks to form clay, and the clay combines with water to create mud. As gas passes through the mud, it creates the bubbling mud pots. Probably the best examples of these different forms are at Fountain Paint Pot in Lower Geyser Basin , where geysers and hot springs are found in the wetter areas below, while mud pots and fumaroles sit atop a small hill. Hydrogen sulfide is, of course, poisonous, and toxic fumes sometimes kill bison and elk that cluster around the geothermal areas in the winter. Scientists joke that Norris Geyser Basin  would be considered an EPA Superfund site if it weren’t within a national park!
The colors in Yellowstone’s hot springs come from a variety of sources, including algae and bacteria as well as various minerals, particularly sulfur, iron oxides, and arsenic sulfide. The algae and bacteria are highly temperature-specific and help create the distinct bands of colors around many hot springs. Interestingly, many of these algal species are found only in hot springs, although they exist around the world. The bacteria have proven of considerable interest to science because of their ability to survive such high temperatures. One such organism, Thermus aquaticus, was discovered in a Yellowstone hot spring in 1967, and scientists extracted an enzyme that was later used to develop the increasingly important technique of DNA fingerprinting. Above 194°F, even these hot-water bacteria, archaea (a single-celled microorganism), and algae cannot survive, so the hottest springs may appear a deep blue because of the water’s ability to absorb all wavelengths of light except blue, which is reflected back into our eyes.
A couple of other terms are worth learning before heading out to see the sights of Yellowstone. Sinter (also called “geyserite”) is a deposit composed primarily of silica. The silica is dissolved by hot water deep underground and brought to the surface in geysers or hot springs. At the surface the water evaporates, leaving behind the light gray sinter, which can create large mounds (up to 30 feet high) around the older geysers. The rate of accumulation is very slow, and some of the park’s geysers obviously have been active for many thousands of years. The other precipitate that is sometimes deposited around Yellowstone’s hot springs and geysers is travertine, consisting of calcium carbonate that has been dissolved underground.
The surface around many of the hot springs and geysers is surprisingly thin, and people have been killed or seriously injured by falling through into the boiling water. Stay on the boardwalks in developed areas, and use extreme caution around backcountry thermal features. If in doubt, stay away!