Mammoth Hot Springs

The National Park Service explains that most of Yellowstone's thermal formations are siliceous sinter and geyserite, while the Mammoth terraces are travertine. Hot water carrying dissolved carbon dioxide forms a weak acid that dissolves limestone underground. When the water reaches the surface, carbon dioxide escapes and calcium carbonate settles out as chalky white rock. Wikipedia adds that the water travels along a fault from the Norris Geyser Basin area and cools to about 170 degrees Fahrenheit (80 degrees Celsius) before emerging; the NPS notes that scientists have suggested either the main Yellowstone magma chamber or a smaller nearer heat source.

The deposits grow quickly. A 2010 article in the NPS publication Yellowstone Science reported travertine forming at about 5 millimetres per day, with one monitored site gaining 0.3 to 0.6 metres in ten months, and described the terraces as up to 73 metres thick and covering more than 4 square kilometres. Wikipedia describes Terrace Mountain as considered the largest carbonate-depositing spring in the world, a superlative worth treating with caution.

Individual springs here switch on and off, and their flow can wander from one part of the hillside to another. The NPS notes that Mound Terrace was inactive for decades before it began flowing again. Dry terraces turn grey as they weather while active ones stay bright white, so the hillside looks different from one trip to the next. Colour in the warm runoff comes from algae and bacteria that tint the travertine brown, orange, red and green.

The NPS reports that for hundreds of years Shoshone and Bannock people collected minerals from the terraces to make white paint. Fort Yellowstone and the Mammoth Hotel stand on Hotel Terrace, an older terrace formation, and sinkholes on the Parade Ground are a reminder that the ground beneath is still thermally active. Wikipedia lists around 70 named springs and terraces, among them Liberty Cap, Cleopatra Terrace and Orange Mound Spring.

Boardwalks cover both the Lower and Upper Terraces. The NPS describes the Upper Terrace trailhead as giving close views of Dryad, Canary, Grassy and Cupid springs. The NPS advises that most of these boardwalks are not maintained during winter, so check its pages for current access, trail conditions and any closures. Stay on the boardwalks: the travertine is fragile and the ground can be hot and unstable.

For a private tour, a Voyzo vehicle and driver with door-to-door pickup can make the northern part of the park straightforward to combine with other stops, and your group can walk the terraces at its own pace.