Yellowstone's Geology

Within Yellowstone's borders are vast lakes, raging rivers, and mountains seemingly made out of glass. What created this intriguing and varied landscape? Fire and ice. Fire in the form of molten rock that today fuels the park's geothermal basement, and ice in the form of incredibly thick and massive glaciers that once covered the region.

Down through the past 2 million years Yellowstone's climate and geology have changed drastically. At one time it was much warmer than it is now. That can be seen in the variety of species represented in the park's petrified trees. Along with pines turned to stone are sago palms, figs, and magnolia. It also was much colder, as indicated by glacial evidence.

Driving Yellowstone's thermal features is a hot spot in the Earth's crust. Here in northwestern Wyoming the crust is so thin that heat generated by the Earth's molten core reaches the surface.

Old Faithful Geyser, Yellowstone
Old Faithful Geyser

Almost 17 million years ago, the plume of hot and partly molten rock known as the "Yellowstone hotspot" first erupted near what is now the Oregon-Idaho-Nevada border. As North America drifted slowly southwest over the hotspot, there were more than 140 gargantuan caldera eruptions – the largest kind of eruption known on Earth – along a northeast-trending path that is now Idaho’s Snake River Plain.

The hotspot reached Yellowstone about 2 million years ago, yielding three huge caldera eruptions about 2 million, 1.3 million and 642,000 years ago. Two of the eruptions blanketed half of North America with volcanic ash, producing 2,500 times and 1,000 times more ash, respectively, than the 1980 eruption of Mount St. Helens in Washington state, university researchers say. Smaller eruptions occurred at Yellowstone in between the big blasts and as recently as 70,000 years ago.

Yellowstone continues to be an area scientists describe as being geologically active. Earthquakes occur frequently in Yellowstone and its landscape is still being pushed up ever-so-slightly by underground pressures.

Volcanoes were common in Yellowstone's prehistoric past. In fact, Mount Washburn is an extinct volcano and Yellowstone Lake site in the middle of a huge volcanic crater that was created by a volcanic eruption about 600,000 years ago. This oval-shaped crater mesures 28 miles by 47 miles. When the last of North America's big ice ages melted away about 8,500 years ago, water from the retreating glaciers filled the caldera and created Yellowstone Lake. The lake covers 84,000 acres and is more than 300 feet deep in places.

Volcanoes also created Obsidian Cliff near Mammoth Hot Springs by turning molten rock into glass. Native Americans used chips of this mountainside to make arrowheads and for trade.

Roaring Mountain, Yellowstone NP
Roaring Mountain in winter, Yellowstone.

To understand how Yellowstone's geothermal features work, imagine a large underground plumbing system. When rain falls and snow melts, the water sinks into the ground through cracks in the Earth's crust. Some of the water travels downward for miles. There it is trapped like water in a pressure cooker and heated far above boiling by hot rock.

When the pressure gets too great, the water heads back to the surface. In the case of geysers, when the water reaches the surface through narrow passages in the rock it erupts in a towering spray of water and steam. If the underground cracks that funnel the water wide as they near the surface, the pressure eases and small pools of water -- hot springs -- are formed.

Depending on the path the groundwater takes to reach the surface, some of these pools can be extremely hot and rapidly boiling. Some, though, are the temperature of bath water.

Mud pots -- pools of steaming and bubbling mud -- are created when the hot water mixes with clay and minerals. Blustering fumaroles result when steam, not water, reaches the surface.

Of Yellowstone's 10,000 or so geothermal features, between 200 and 250 are active geysers. Old Faithful, which spouts water more than 100 feet into the air, is the most famous. But it's neither the tallest nor the most regular of the park's geysers. In recent years the intervals between Old Faithful's eruptions have ranged from 66 minutes to 80 minutes. The changes are tied to earthquakes that effect the underground movement of water that feeds the geyser.

The park's, and the world's, tallest geyser is Steamboat, whcih has rocketed columns of hot water 400 feet into the sky. Unfortunately, Steamboat's eruptions are not predictable, so consider yourself extremely fortunate if you happen to see one.

You can find geothermal features throughout most of Yellowstone. The greatest concentrations of geysers, hot springs and mud pots are found in seven areas: the Upper, Midway, and Lower geyser basins; the Norris Geyser Basin; Mammoth Hot Springs; West Thumb, and the Mud Volcano area north of Lake. Old Faithful is located in the Upper Geyser Basin, where you'll also find Morning Glory Pool.

Hot spring runoff, Norris Geyser Basin, Yellowstone
Colorful hot spring runoff in Norris Geyser Basin.

The Steamboat Geyser is in the Norris Geyser Basin, which is the oldest, hotest, and most colorful of Yellowstone's geyser basins. The blues, reds, oranges, and yellows that bloom in many of this basin's hot springs and geyser runoff channels are created by minerals, water temperature, tiny plants called algae, and microscopic life forms that live in the hot water. The hottest areas usually are colored white and yellow.

Because Yellowstone is geologically active, its thermal features change from year to year. Some new geysers and hot springs appear, others vanish. The thermal areas grow in size, or shrink.

The terraces at Mammoth Hot Springs are created when dissolved minerals carried to the surface by hot springs precipitate. These deposits are called geyserite, or travertine. Whitish-colored travertine deposits are formed when limestone has been dissolved by water water turns solid. Most of Yellowstone's travertine deposits are found at Mammoth Hot Springs in its terraces.

Geyserite is grey colored and much harder than travertine. These deposits are found throughout the rest of the park's thermal areas.

Some deposits grow very big. The Lone Star Geyser south of the Old Faithful complex has a cone more than 9 feet tall. And at Mammoth Hot Springs, it has been estimated that the minerals brought to the surface build an 8-inch layer of rock on the terraces each year.