Mammoth Hot Springs: Built While You Watch

The terraces at Mammoth are not old in the way that most geological features are old. They are not the accumulation of millions of years. What you are looking at is being built right now, this season, at rates measurable in weeks.

Hot water rises from underground, carrying dissolved calcium carbonate leached from the limestone layers far below. When the water reaches the surface and cools, it can no longer hold the mineral in solution. The calcium carbonate precipitates out. It settles. Over days, over weeks, it builds into the stepped travertine formations, pale cream and rust-brown and white, that terrace the hillside at Mammoth. The process is biological as well as geological: thermophilic bacteria in the water tint it blue-green and gold and orange, different species thriving at different temperatures in the flow.

The practical consequence is that Mammoth Hot Springs looks different every season. Features that were active in nineteen seventy-two have gone dormant. New vents have opened elsewhere on the hill. The thermal plumbing beneath Mammoth shifts as the magma chamber below the park breathes. Terraces that appear in historical photographs have disappeared. A new formation might emerge this year that didn't exist last year.

The Army stationed its Yellowstone headquarters here from eighteen eighty-six for a practical reason: Mammoth was the park's most accessible entrance from the north, and the Gardiner gateway was already a small settlement. Fort Yellowstone's stone buildings still stand around the parade ground. The Albright Visitor Center, where the Army officers lived and worked, is the best place in the park to understand the military administration that preceded the Park Service. The terrace boardwalk begins just up the hill.