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Mud Volcano Dragon's Mouth Spring

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• Sulphur Caldron

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Trail distance - ~ mile (1 km)

Ihou start at i\'lud CllJron Jnd travel clockwise, the ascent is not as steep as it is i f you walk (()Ulltc]"c[ockwisc.

You may scc hear and hisoll here; ohey trail closures, do not approach wildlife, and walk with ntrclTlc caution.

Mud Volcano •

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Grizzly Fumarole

Sour Lake .

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Black Dragon's Caldron •

• Mud Caldron /

. Mud Geyser

Cooking Hillside

Sizzling Basin

Cooking Hillside

o 50 Mete"

Legend

... u"""", Boardwalk. may require assistance

Improved t rail. may be paved; may require assistance

_ "'" Stairs or steep grade

Road ., Stream or runoff channel

• Hydrothermal feature

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200

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Parking

Restrooms

600

200

\ . '. Listen to the sounds, inhale the pungent aroma, and notice the colors of the Mud Volcano area. Much of what you sense comes from sulfur, the dominam mineral. It is present in many forms: hydrogen sulfide creates the area's infamous aroma, microbes convert the hydrogen sulfide into an acid that dissolves rocks and soil, and sulfur minerals paint the features in hues of yellow and shades of gray.

Hydrothermal Area Wildlife

As you wa lk from feature to feature, look for wildlife. You may see marmots, Clark's nutcrackers, mule deer, and elk. In the spring, grizzly bears feed on winter-killed animals. Be especially alert for bison. These magnificent animals appear tame and slow but are wild and quick. During any time of year, do not approach- and never feed- bison, coyotes, birds, or other animals. Wild animals are dangerous and unpredictable.

Dragon's Mouth Spring

This feature has captured imaginations for centuries. The Crow saw the steam as snorts of an angry bull bison. An uknown European American gave it the current name. Its previous names have included Gothic Grotto, Blowing Cavern, and The Belcher. This surging action has decreased since 1994; no one is sure why.

In 1999, more dramatic changes occurred: The water temperature dropped ten degrees and the color changed from green to chalky white.

Mud Volcano

Early explorers to Yellowstone described this feature as a "most repulsive and terrifying sight," a volcano-like cone, 30 feet high and 30 feet wide (9 by 9 m) with mud erupting to cover tall trees. When Nathaniel P. Langford visited it in 1870, however, he saw "a seething, bubbling mass of mud." It 's likely a violent eruption blew out the cone's side, leaving the crater you see today. Rich in iron sulfides and powered by heavy gas discharge, the water constantly undercuts the back wall.

Return to the parking lot and follow the walkway beyond Mud Caldron. This route is less steep than continuing from here.

Mud Caldron

Imagine cracks or a natural "plumbing" system reaching down several thousand feet (several hundred meters) below Mud Caldron to a source of hot water. Steam produced by this boiling rises to the surface and passes through Mud Caldron, heating the water in the process-but not boili ng it. The bubbling is caused by carbon dioxide and other gases rising from below.

Mud Geyser

Mud Geyser was the star attraction of the Mud Volcano Area when first seen by members of the Washburn Expedition of 1870. They described it as "a boiling spring, a placid pond, a deep dry funnel or an active geyser according to the time of one's visit." At that time, muddy water would explode 50 feet (17 m) into the air every few hours. The geyser died in the 20th century, its plumbing clogged with mud and

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gravel. In 1993, soil temperatures skyrocketed for unknown reasons and trees began dying around the geyser's south rim. By January 1995, a new feature on the south bank of Mud Geyser had burst onto the scene. Steam vents and shallow pools sizzled. In 1999, mudpots formed and then exploded, leaving a deep hole with more sizzling features and mud.

"Cooking Hillside"

Small earthquakes frequently shake the ground in Yellowstone. Sometimes they alter hydrothermal features, as happened here in 1978-79 when a swarm of earthquakes struck. Soil temperatures increased to nearly 200°F (94'C). This became a barren landscape of fa llen trees and steam, a "cooking hillside."

Sizzling Basin

In the 1960s, the name Sizzling Basin described the vigorous release of gases here-like that from a hot, grease-filled frying pan. During the 1970s, this pool had cooled and lost enough of its carbon dioxide supply to allow a thick, lush mat of microorganisms to cover most of its surface. They were killed when a swarm of earthquakes struck the area in 1978-79 and superheated the water.

. Churning Caldron

Imagine this froth ing pool as a cooler spring covered with colorful mats of microorganisms. That's how it looked until earthquake activity in 1978-79 superheated the waters and killed some of the mat-forming microbes. In 1996, it began throwing water 3-5 feet (1-1.5 m) high. This once cool pool averages 164°F (73°C).

Black Dragon's Caldron

In 1948, Black Dragon's Caldron (see guide cover) burst onto the landscape along a crack in the earth. It uprooted and coated nearby

trees in thick mud. This fea ture has since shifted south 200 fee t (60 m) along the crack. The spring feeding nearby Sour Lake was much more active before Black Dragon's Caldron formed. Perhaps underground changes diverted heat energy from Sour Lake to form this seething mass of mud, which a park ranger named for its color and the apparent lash ing of the "dragon's tongue."

Sour Lake

Sour Lake, named for its acidic or "sour" water, may look like a pleasant swimming hole, but its water would burn your skin like battery acid. Most of its acid comes from micro­organisms that create sulfuric acid as they consume sulfur. These microorganisms also give the lake its color.

Grizzly Fumarole

Grizzly Fumarole undergoes striking changes, depending on recent precipitation. You might find a large thin-watered mudpot, a series of smaller and thicker mudpots, or a fumarole (steam vent). During dry periods, it may look like an abandoned collection of mud forma­tions with hot water hissing fa r below. Take a moment to listen to these subtle sounds .

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-Across the road from Mud Volcano, you can

..... _ one of the most acidic springs in Yellowstone. - Sulphur Caldron's turbulent waters have a pH of • approximately 1-2, which is about as acidic as car

battery acid or stomach fluids. Its name comes from the large amounts of free sulfur in its waters.

Closer to the river, look for a big depression in the grou nd. Its sides may be coated with gray mud. Watch it for a few minutes; you mighnee .mud flying into the air. - , •

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• of tnjffte as you ero., the l'Qad on foot or parking area. -

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For More Information www.nps.gov/yel/

If you would like to learn more about geology and hydrothermal features, these and other items are sold by the Yellowstone Association in visitor centers: Geysers: What They Are & How They Work, T. Scott Bryan Life at High Temperatures, Dr. Thomas Brock Interpreting the Landscape of Grand Teton and Yellowstone National Parks, John N. Good and Kenneth L. Pierce Seen & Unseen: Discovering the Microbes of Yellowstone, Kathy Sheehan et al. Windows into the Earth: The Geologic Story oj Yellowstone and Grand Teton National Parks, Robert B. Smith and Lee J. Siegel DVDs: Yellowstone: A Symphony of Fire and Water

The Complete Yellowstone

Photos: Cover (Black Dragon's Caldron), Jeff Henry; Mudpot, NPS; Mud Volcano, Jeff Henry; Bison, Tom Cawley; Grizzly Fumarole, NPSlDuckworth; Sulphur Caldron, NPSIj. Suderman