Yellowstone’s dramatic geysers and rainbow-hued hot springs have long astounded visitors and inspired generations of painters and photographers. But their cousins, the mudpots, hold their own unique allure for those who take the time to observe them in all their gooey glory.

In fact, Ferdinand Hayden, during his 1871 expedition to Yellowstone, described Mud Volcano—the park’s largest group of mudpots—as “the greatest marvel we have met with.” Today, Yellowstone National Park geologist Dr. Jefferson Hungerford shares Hayden’s fascination with these bubbling, splattering, gurgling, and—let’s face it— smelly thermal features.

“What makes them so fun to watch is the bubbling that occurs in the mud,” says Hungerford. “Watching bubbles form in slow motion then fracture and burst is mesmerizing, and actually very calming, like watching ocean waves.”

Formed by standing surface water acidic enough to dissolve surrounding rock into clay, the mudpots have a similar structure to hot springs, but with a lower water supply. “Mudpots are typical to areas with highly acidic pH levels combined with low water conditions,” says Hungerford. “Gas or water vapor thermally heated by underlying volcanic activity pushes through the mud, creating the bubbles we see on the surface.”

When you visit a thermal area in Yellowstone, you’re likely to smell the mudpots before you see them. So what’s up with that odor? “In acidic systems we often have hydrogen sulfide gas that emanates through the mud, giving mudpots that familiar sulfur, or ‘rotten egg,’ smell,” says Hungerford.

Mudpots are sometimes called paint pots due to their palette of earthy colors. While the mud starts out white, iron oxides tint it varying shades of pink, brown, orange, and gray. But the mudpots won’t necessarily maintain a static appearance. Hungerford says that repeat visitors may notice the features can change over time or across seasons.

“All these thermal features are really dynamic. For instance, some mudpots can emerge and be active for a month or so, then become fumaroles, or steam vents. As conditions change, the mudpots can change. Plus, every bubble is a unique feature in itself,” he adds, “so when watching mudpots you will never have the same experience twice.”

MUST-SEE MUDPOTS

Artists Paintpots

A 1.1-mile boardwalk and gravel trail south of Norris Junction circles around a colorful hydrothermal area with hot springs, two large mudpots, and fantastic views of Mount Holmes.

Fountain Paint Pot

This half-mile boardwalk loop, part of the Lower Geyser Basin north of Old Faithful, provides access to many types of thermal features in one spot, including mudpots and several active geysers.

Mud Volcano

A short boardwalk segment leads to this thermal area just off the road at the southern end of Hayden Valley. Mud Volcano was once covered by a volcano-shaped mud deposit, before an 1870s thermal explosion blew the top away to reveal the mudpots we see today.

West Thumb Paint Pots

A scenic, 0.6-mile boardwalk loops around West Thumb Geyser Basin, passing by mudpots, colorful hot springs, and dormant geysers on the shore of Yellowstone Lake. Here, a major volcanic explosion left a caldera, forming a large indent in the shoreline around 150,000 years ago.

 

This article was originally published in the Fall 2019 issue of Yellowstone Quarterly.