Yellowstone’s climate and geology have drastically altered over the past 2 million years. At one time it was much warmer than it is now, evidenced by the variety of species represented in the park's petrified trees. Along with pines turned to stone are sago palms, figs, and magnolia.
Glacial erratics on the road to Lamar Valley, Yellowstone National Park / Rebecca Latson
The warm weather became cold weather some 150,000 – 160,000 years ago, when the Bull Lake glaciers covered the landscape. You can see evidence of this glaciation parkwide, including thermal kames at Mammoth Hot Springs, glacial moraines and outwash in the Madison Valley (west of Seven Mile Bridge), and glacial erratics strewn over the landscape just off the road between Tower Junction and Lamar Valley.
Today, within Yellowstone’s 2.2 million acres (>890,000 hectares) are more than 10,000 hydrothermal features in Yellowstone National Park, 500 of which are geysers. These geysers, mudpots, hot springs, fumaroles, and travertine terraces are all evidence of the constant activity circulating beneath the park’s landscape. And just two ingredients are required to keep things going: heat and water.
Providing the heat ingredient is a hotspot in the Earth's crust. Almost 17 million years ago, the subterranean 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 via plate tectonics, the caldera-forming eruptions traveled along a northeast-trending path that is now Idaho’s Snake River Plain.
Upon reaching the Yellowstone region, this hotspot yielded three caldera eruptions (2 million, 1.3 million, and 642,000 years ago) that blanketed half of North America. Two of the three eruptions produced 2,500 times and 1,000 times more volcanic ash, respectively, than the 1980 eruption of Mount St. Helens in Washington state. In between those big blasts were smaller eruptions, one of which occurred as recently as 70,000 years ago.
As a result, Yellowstone is filled with volcanic features. Mount Washburn is an extinct volcano, Yellowstone Lake is the result of melting glaciers filling an oval-shaped caldera 28 miles by 47 miles (45 km by 76 km) about 8,500 years ago, and Obsidian Cliff is the result of the rapid cooling of lava into glass. Fun fact: Native Americans used chips of this volcanic glass mountainside to make arrowheads for hunting and to trade.
That’s the heat ingredient. Now, for the water ingredient.
The hydrothermal system of Yellowstone National Park / NPS file
To get a better understanding of Yellowstone's hydrothermal features, visualize a large underground plumbing system. At the very bottom of this system of “pipes” through the rock layers is a chamber of molten rock known as magma. Rain and melting snow sinks through the ground into the cracks, trickling beneath the surface for miles. Ultimately, this water, trapped under immense pressure, undergoes the process of superheating beyond boiling point by the magma. The superheated water, still liquid due to the immense subterranean pressure, is pushed back up toward the surface. Depending upon the route, constrictions along the way, and amount of pressure, the water either explodes topside in a fury of steam and hot water droplets as a geyser, flows into quiet pools of hot water known as hot springs, escapes as steam through vent openings, wets the soil to create mudpots bubbling with escaping gases, or, if traveling up through limestone, dissolves calcium carbonate along the way to deposit it topside as the scalloped travertine terraces you see at Mammoth Hot Springs. All this expelled water ultimately flows back down through those same pipes to repeat the entire process.
There are five types of hydrothermal features found in Yellowstone: geysers, hot springs, fumaroles, mudpots, and travertine terraces. The greatest concentrations of these hydrothermal features are found in seven areas of the park: Upper, Midway, and Lower geyser basins; Norris Geyser Basin; Mammoth Hot Springs; West Thumb, and the Mud Volcano area north of Yellowstone Lake.
Geysers
Watching Old Faithful erupt from Observation Point, Yellowstone National Park / Rebecca Latson
Of the more than 500 geysers found within the park, the most famous is Old Faithful, which spouts water 106 – 185 feet (32 – 56 m) into the air. Old Faithful, however, is 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 35 minutes to 120 minutes, with an average of 92 minutes.
According to the National Park Service:
Some scientists think parking lots and other development have affected the area’s groundwater and led to this increase [in interval time between eruptions]. Others think the increase may be due to changes within the geyser’s underground channels. Following the 1959 Hebgen Lake earthquake, the period of time between Old Faithful’s eruptions became longer. This interval has increased even more following other earthquakes.
The park's, and the world's, tallest geyser is Steamboat geyser, which has rocketed columns of hot water more than 300 feet (91 m) into the sky. Steamboat's eruptions are not predictable, so consider yourself extremely fortunate if you happen to see one. Traveler’s Note: make sure you have a cloth to wipe off the dissolved minerals deposited onto your vehicle from Steamboat’s eruption. These minerals can damage the glass and paint on your vehicle.
Hot Springs
Morning Glory Pool, Upper Geyser Basin, Yellowstone National Park / Rebecca Latson
Two of the park’s most colorful hydrothermal features are Morning Glory Pool and Grand Prismatic Spring. Both hot springs get their color from heat-loving microorganisms and water temperature. Blue colors indicate the hottest water temperature (189oF / 87oC), while oranges and yellows represent the cooler-water spectrum (“Cool” is a relative term) between 149 - 165oF / 65 – 74oC.
Believe it or not, something else that can change hot spring colors is tourist trash! Over the years, people have thrown coins, clothing items, and logs into Morning Glory Pool, causing it’s deep, sapphire blue to disappear and be replaced with green, yellow, and orange colors. Trash clogged the heat vents into the pool, creating cooler water circulation, which in turn invited a different type of microorganism to live within the thermal waters.
Mudpots
Mudpots along Artists' Paintpot Trail, Yellowstone National Park / Rebecca Latson
The three most famous mudpots are seen along the Fountain Paint Pot Nature Trail, Artists Paint Pots Trail, and Mud Volcano. These mudpots are like hot springs, except with less water. The standing surface water is acidic enough to break down the rock into soupy clay, and the heated water and gases below ground push up to the surface, creating slow-motion bubbles. Some mudpots, like Red Spouter in the Fountain Paint Pot Area, are seasonal, depending upon the season and the height of the water table. You might see it bubbling and frothing, and you might merely see steam issuing through the vents. No matter the time of year, however, you will smell it due to the presence of hydrogen sulfide gas.
Fumaroles
Steaming fumaroles on a crisp autumn morning at Roaring Mountain, Yellowstone National Park / Rebecca Latson
Found all around the park, including Roaring Mountain, Norris Basin, and Mud Volcano, fumaroles are the hottest geothermal features in the park. Also called steam vents, fumaroles lack water in their systems, and instead constantly release hot steam which tends to be more acidic than geysers or hot springs.
Travertine Terraces
The colors Palette Hot Spring's travertine terraces, Yellowstone National Park / Rebecca Latson
These hydrothermal features occur when hot springs rise through limestone, dissolving the calcium carbonate of which limestone consists. Once the hot water reaches the surface, it cools and deposits the calcite, in turn which builds up to create the travertine terraces found throughout the park. The most famous of these are the Mammoth Hot Springs travertine terraces.
Fire and Ice have sculpted Yellowstone landscape. The glacial ice is long gone, leaving vestiges on the landscape, but the fire remains alive beneath the surface.
