Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Jefferson Hungerford, Park Geologist, and Kiernan Folz-Donahue, Field Geologist, both with Yellowstone National Park.

Geologically speaking, things are always changing at Yellowstone—especially in the hydrothermal areas. It’s simply the nature of the complex hydrothermal system that overlies one of the planet’s largest magma reservoirs. 

The summer of 2024 was a busy time, with July’s hydrothermal explosion at Biscuit Basin and the first hydrothermal explosion ever recorded by geophysical monitoring data in Norris Geyser Basin. In addition to these higher energy events, a new hydrothermal feature popped up right in front of our eyes—literally! 

While driving south from Mammoth Hot Springs towards Norris Geyser Basin early on August 5 last summer, a park scientist noticed a billowing steam column through the trees and across a marshy expanse. The eagle-eyed scientist notified the park geology team to verify if this was indeed new activity.

Looking south from near a pullout along the Mammoth to Norris road just north of the Nymph Lake overlook. On the other side of the marsh is a tree-covered rhyolite lava flow, and at the base of the flow is a new thermal feature marked by a plume of steam and that formed in early August 2024/USGS, Mike Poland, September 1, 2024.

This new hydrothermal feature is within a region called the Roadside Springs thermal area, which is a collection of spatially distinct areas of altered rock and hydrothermal features (data for Yellowstone thermal areas is at https://data.usgs.gov/datacatalog/data/USGS:661d5eb7d34e7eb9eb7e3a41). This new feature is at the foot of a rhyolite lava flow about 3 meters above the marsh below, and it lies within a swath of warm, hydrothermally altered ground that is approximately 60 meters (about 200 feet) long. 

Map showing the Roadside Springs thermal area, located just north of Nymph Lake along the Norris-Mammoth highway. Hydrothermal ground is shaded purple. New hydrothermal features formed in 2003 on the north side of Nymph Lake, and also in 2024 a bit further north from the lake/Figure by Jefferson Hungerford, Yellowstone National Park.

Soon after it was identified, park geologists trudged through the marshy ground to get a closer look that the feature, which had a temperature of 77 °C (171 °F). A very thin veneer of grey silicious clay barely covered the surrounding surface, indicating its very young nature.

Although this hydrothermal activity may seem new to us, it may also just be just the latest manifestation of activity a short distance away that kicked into existence more than two decades ago. On March 10, 2003, a similar type of hydrothermal activity was first observed on the other side of the same rhyolite lava flow where the new feature is located, just west of Nymph Lake. This hydrothermal activity persists through this day but is much less energetic than when it first formed.

Are the new feature and the activity that started in 2003 hydrologically connected? Probably. One could run a line along the axis of the older active area and it would intersect the new feature. This line also follows the trend of faults that run from Norris Geyser Basin northward to Mammoth Hot Springs and beyond.

The new feature remained prominent into the fall of 2024, with a strong steam plume that was especially visible during chilly mornings. But as fall began to turn into winter, the steam plume gradually disappeared. The feature remains active, but there is some water in the vent, decreasing the amount of steam that is released. Whether or not the strong plume returns in the summer of 2025 remains to be seen.

Geologists have mapped more than 100 major hydrothermal areas in Yellowstone National Park, and there are many more than 10,000 hydrothermal features within its boundaries. The activity from these features waxes and wanes with time—you might even say that some of them pick up steam! Sorry…we couldn’t resist.