Mammoth Hot Springs are a must-see feature of Yellowstone National Park in part because they’re so different from other thermal areas in the area. This is largely because limestone is a relatively soft type of rock, allowing the travertine formations to grow much faster than other sinter formations. It has been described as looking like a cave turned inside out.
How Mammoth Hot Springs Was Formed
At Yellowstone each year, the rain and melted snow seeps into the earth. Cold to begin with, the water is quickly warmed by heat radiating from a partially molten magma chamber deep underground, the remnant of a cataclysmic volcanic explosion that occurred 600,000 years ago.
After moving throughout this underwater “plumbing” system, the now hot water rises up through a system of small fissures. Here it also interacts with hot gases charged with carbon dioxide rising up from the magma chamber. As some of the carbon dioxide is dissolved in the hot water, a weak, carbonic acid solution is formed.
In the Mammoth area, the hot, acidic solution dissolves large quantities of limestone on its way up through the rock layers to the hot springs on the surface. Above ground and exposed to the air, some of the carbon dioxide escapes from the solution. Without it, the dissolved limestone can’t remain in the solution, so it reforms into a solid mineral. This white, chalky mineral is deposited as the travertine that forms the terraces.
Mammoth Hot Springs Terraces
Mammoth Hot Springs has two terrace boardwalks, the upper and lower. Approximately 50 hot springs lie within the area.
Lower Terrace Boardwalk
You can access the lower boardwalk from the parking lot or the Grand Loop Road.
Liberty Cap is among the best known of the Lower Terrace's features. Rising 37 feet in the air, this hot spring cone was named in 1871 for its resemblance to the peaked caps worn during the French Revolution. The cone shape formed when the hot spring’s plumbing system had a continuous flow for perhaps hundreds of years. Over that time period, the internal pressure was high enough to push the water to a great height, allowing the mineral deposits to build up.
Minerva Spring is another favorite because of its wide range of colors and intricate travertine formations. Its activity though has ebbed and flowed since records were first kept about it in the 1890s. The feature was completely dry in the early 1900s, but started flowing again by 1951.
You can access the upper boardwalk from the one-way Upper Terrace Drive and parking lot. The road winds among springs for 1.5 miles before it loops back for a half mile.
The Upper Terraces include Prospect Terrace, New Highland Terrace, Orange Spring Mound, Bath Lake, White Elephant Back Terrace and Angel Terrace.
Orange Spring Mound is so named for its color, which is created by bacteria and algae, and shape, which is a result of very slow water flow and mineral deposition. Highly unpredictable, Angel Terrace is known for its pure white formations and the colorful microorganisms seen in its active periods.
Where to See Mammoth Hot Springs
Mammoth Hot Springs is just south of the North Entrance to Yellowstone (near Gardiner, Montana). The area is accessible by car year round. Boardwalks cover about 1.75 miles around the Upper and Lower Terraces. Expect the easy hike (there are only about 300 feet of elevation gain) to take about an hour.