Its physical grandeur and charismatic wildlife notwithstanding, Yellowstone can be frustrating to visitors interested in exploring its renowned volcanic features. Geysers, hot springs, and mudpots are plentiful and easy to find, and they’re fascinating in their own right. But what about the BIG picture — the giant caldera, massive lava flows and two expansive resurgent domes? For the untrained eye, those iconic features can be elusive. Partly that’s because they’re almost TOO BIG to see. A short geologic history lesson will help us understand where to look.
“Nature abhors a vacuum” is a common idiom, and it’s true at calderas, including Yellowstone. Yellowstone Caldera formed 631,000 years ago when a prodigious volume of rhyolite magma (about 1,000 cubic kilometers, or 240 cubic miles) erupted explosively from a reservoir 5–10 kilometers (3-6 miles) beneath the surface. Rapid removal of such a large volume caused the roof of the reservoir to collapse, and subsequent landslides into the chasm produced the outline of the caldera we see today — 50 kilometers by 70 kilometers (30 miles by 45 miles) across. Those distances are hard for the human eye and mind to encompass.
But the Yellowstone magmatic system wasn’t finished. Shortly after caldera collapse, magma started refilling the partially drained reservoir. Increasing pressure lifted the caldera floor by hundreds of meters (yards) at two sites that had been foci for the caldera-forming eruption. The resulting bulge in the eastern part of the caldera became the Sour Creek resurgent dome and its sibling in the western part became the Mallard Lake resurgent dome. Both features are elliptical in shape and more than 10 kilometers by 20 kilometers (6 miles by 12 miles) across, with gently sloping flanks. Resurgent domes differ from lava domes in that lava domes form primarily by extrusion of viscous lava onto the surface, whereas resurgent domes form primarily by uplift and deformation of the surface itself, driven by magma accumulating underground.
The Sour Creek dome rose to a height above that of the adjacent caldera rim before uplift there stopped. Its surface, like that of the Mallard Lake dome, is cut by numerous faults that formed in response to intense upwarping. Early postcaldera resurgence probably occurred at the site of the Mallard Lake dome, too, although the evidence is buried beneath younger deposits. Unlike the Sour Creek dome, the Mallard Lake dome experienced another uplift episode about 170,000 years ago. Near the start of the episode, the Mallard Lake rhyolite flow erupted from vents atop the dome, covering most of its surface and adding to its volume. So Yellowstone’s resurgent domes are siblings, but not twins. One formed entirely by structural uplift, the other by a combination of uplift and lava extrusion.
Renewed doming in the western part of the caldera was followed by extrusion of more than a dozen immense rhyolite flows in a series of clustered eruptions from 170,000 to 70,000 years ago. Those flows covered almost the entire floor of the caldera and filled it to its brim in many places. Two exceptions are the high-standing upper flanks of the Mallard Lake and Sour Creek resurgent domes.
So it’s hard to see the caldera and its resurgent domes for two reasons. They’re so big as to be hard to find among Yellowstone’s many smaller attractions, and they’re partly or mostly buried under comparably big lava flows. On a clear day, you can see across the caldera’s entire expanse from accessible vantage points on its rim at Mount Washburn or Lake Butte. Don’t be disappointed by the absence of a deep, steaming cauldron. Remember that nature has mostly filled that void with lava flows. With the help of interpretive signs or a geologic map, you might be able to locate one of these lava flows, the Elephant Back flow, in the vicinity of Le Hardys Rapids. You can see another one along Firehole Canyon Drive, where the flow’s interior has been exposed in cross section by the erosive power of the Firehole River.
And if you’re looking for the resurgent domes, you can see the profile of the Mallard Lake dome by looking west from DeLacy Creek trailhead along the Grand Loop road, about midway between West Thumb and Old Faithful. The Sour Creek dome is visible from several vantage points along the road between Canyon Village and Lake Village. Look to the east across the Yellowstone River for a gently sloping profile on the horizon, reminiscent of the shape of shield volcanoes in Hawaii.
With some geologic insight and a trained eye, you’ll be able to find Yellowstone’s elusive volcanic giants.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Dan Dzurisin, geologist with the U.S. Geological Survey.