Montana
The Lone Peak Tram was designed to move with the rippled rock glacier underneath it. The tram base has moved more than ten feet since 1995–96.
A skier hikes along the Headwaters, one of several lateral arms that make up Lone Peak’s Christmas tree laccolith.
If you were to design the ideal mountain for a world-class ski resort, you might come up with something like Lone Peak, the centerpiece of Big Sky Resort in Big Sky, Montana. Big Sky’s tagline is “The Biggest Skiing in America,” and even if that’s no longer true in terms of acreage, the hair-raising double black diamond runs off Lone Peak’s 11,166-foot summit certainly set the bar for sheer grandeur. From the air, look for a solitary conical peak in the midst of several mountain ranges, with the village of Big Sky on the east side.
Lone Peak dominates the Big Sky horizon but it isn’t entirely alone. The pyramidal peak sits at the nexus of three mountain ranges: the Spanish Peaks, Gallatin Range, and Madison Range, which have very different formation stories. The Spanish Peaks to the north are composed of 2-billion-year-old metamorphic gneiss that was uplifted into mountains around 60 million years ago during the Laramide orogeny, which built the Rocky Mountains. The Gallatin Range, stretching northeast toward the Bozeman Valley, is made up of mostly volcanic rocks that erupted between 70 and 50 million years ago, related to the subduction of the Farallon Plate under the North American Plate. The Madison Range to the southwest is made up of fossil-rich sedimentary rocks that in some places were baked into metamorphic rock by underlying volcanism.
Lone Peak lies at the edge of the Madison Range and is actually a failed volcano that never erupted at the surface. Instead, magma rose up through a vertical conduit but then spread out sideways underground, flowing between layers of sedimentary rock, forming lateral arms of dacite and andesite. This type of eruption produces a Christmas tree laccolith: a structure with a central trunk and radiating lateral arms that intrude into existing sedimentary rock.
Lone Peak sits at the junction of the Spanish Peaks and the Gallatin and Madison Ranges. Cedar Mountain, behind Lone Peak to the left in this image, is part of the Madison Range.
The underground eruption that built Lone Peak took place around 68 million years ago, but the pyramid we see today is the result of more recent carving by extensive glaciers during the Pleistocene Epoch. The bowl-shaped cirques on each side of the peak were scooped out by glaciers, some of which survive today.
The rocks in the bottom of these cirques are curiously rippled because they sit on top of moving rivers of ice hidden beneath the rocks. These rock glaciers move downhill over time; the base of the Lone Peak Tram, which whisks skiers up to the summit, was built on top of a mile-long rock glacier and designed to move to accommodate the slow downhill creep. Sliding at a rate of around seven inches per year, the tram base has moved backward more than ten feet since it was built in the winter of 1995–96.
You might fly over Lone Peak en route to Bozeman, Montana.
