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When the glaciers retreated from Yellowstone, the hydrothermal system lit up

If you could travel to Yellowstone 20,000 years ago, what would you see? Erupting geysers? Scenic thermal springs? Herds of bison and elk? No! In fact, you would see an ice sheet that was thousands of feet thick!

Map showing ice cover in the Yellowstone region
Map showing ice cover in the Yellowstone region. Light shaded areas bounded by black and red lines indicate areas covered during the Pinedale (about 20,000-15,000 years ago) and Bull Lake (about 150,000 years ago) glaciations, respectively. Blue lines are contours in thousands of feet on the maximum reconstructed Pinedale glacier surface. The circled numbers schematically depict the southwest migration of the center of mass of the greater Yellowstone glacial system through time (in thousands of years).

USGS scientist Emeritus Ken Pierce dedicated much of his distinguished research career studying how glaciers advanced and retreated around Yellowstone during the Pleistocene, often referred to as the Ice Age (the period from about 2.6 million to about 11,700 years ago). Pierce and his colleagues used radiocarbon dating and a method called "surface exposure dating" to develop a detailed chronology of glacier advance and retreat in Yellowstone. The surface exposure dating method relies on measurements of rare isotopes (in this case beryllium-10) to estimate how long a rock surface has been exposed to cosmic rays (energetic, subatomic particles that arrive from outside the Earth's atmosphere).

Using surface exposure dating of boulders on moraines (piles of rock deposited by glaciers), Pierce and his colleague Joe Licciardi from the University of New Hampshire showed that the last glacial episode in the Yellowstone region (called the Pinedale Glaciation) reached its maximum sometime between 21,000 years ago in the eastern part of Yellowstone to about 16,000 years ago in the south. During that time, nearly all of Yellowstone was covered by an ice cap up to 4,000 feet thick (about 1,200 meters). By 14,000 to 13,000 years ago, however, most of Yellowstone was ice-free as the Pinedale Glaciation came to a close.

During glaciation, fluid pressure in Yellowstone's hydrothermal system increased and decreased in proportion to the thickness of the ice cover. Following the retreat of the most recent ice cap, pressure in the hydrothermal system decreased substantially, leading to extensive boiling of groundwater that had some dramatic consequences.

In a seminal study published in 1971, USGS Scientists Patrick Muffler, Donald White and Alfred Truesdell showed that several hydrothermal explosions in Yellowstone took place during the waning stages of Pinedale Glaciation. These explosions were produced when groundwater contained in near-surface rocks at high temperatures flashed to steam, violently disrupting the confining rock.

More recently, USGS Scientist Emerita Lisa Morgan and her colleagues examined in detail the distribution, composition, and textures of the deposits from about 30 large postglacial hydrothermal explosions. These explosions were marked by oval-shaped craters ranging in size between 0.04 and 2 square miles (0.1-5.0 km2). Mary Bay, in northern Yellowstone Lake, is the largest hydrothermal explosion crater in Yellowstone (and possibly in the world) and formed about 13,000 years ago.

There are several ongoing studies that aim to unravel the details of Yellowstone's post-glacial hydrothermal activity. These include studies of sediment cores from Yellowstone Lake (part of the HD-YLAKE project) and silica sinter deposits in the Upper Geyser Basin.

This is an aerial photo of Indian Pond hydrothermal explosion crater
Aerial photo of the 0.06 square mile (0.16 square kilometer) Indian Pond hydrothermal explosion crater north of Yellowstone Lake. The deposits from the explosion that formed the crater were dated with radiocarbon to 2,900 years ago. Photo from Yellowstone National Park Photo Collection taken by Jim Peaco in July 2001.

Tourists visiting Yellowstone National Park are fascinated by the eruptions of Old Faithful and Steamboat geysers, which are prime manifestations of Yellowstone's restless hydrothermal system. If tourists could have visited Yellowstone shortly after the last glaciers retreated (about 14,000 years ago), however, they would have witnessed much more vigorous and explosive hydrothermal systems.

You can watch interviews with Ken Pierce here and here, with Patrick Muffler here, and with Lisa Morgan here, which provide perspectives from decades of USGS research at Yellowstone.