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  • Yang Li1, John F. Slack2, and David L. Kelley3
  • 1 Peking University
  • 2 U.S. Geological Survey (Emeritus)
  • 3 Chakana Copper Corp

Yellowstone Field Trip Summary (Co-sponsored by SGA and SEG)

Led by Jeff Hedenquist (University of Ottawa) and Stuart Simmons (University of Utah)

 

Fig. 1. Stuart Simmons, using a simple field sketch, describes why variations exist among Yellowstone’s hydrothermal features.

We recently had the privilege of participating in the SGA–SEG Yellowstone field trip, an exceptional learning experience led by two world experts in active geothermal systems and ancient epithermal environments, Jeffrey Hedenquist and Stuart Simmons. This trip brought together 25 participants representing 10 countries, with backgrounds from students to professionals in academia, industry, and government, creating a truly international and interdisciplinary atmosphere.

For many visitors, Yellowstone is mainly a holiday destination—a place to see a variety of wildlife, admire dynamic geysers, vivid colors, and dramatic landscapes, but often without an understanding of the geological processes generating them. This time, guided by two leading experts, the familiar sites of Yellowstone were transformed into a natural laboratory, where each feature becomes a window to understanding fluid flow, permeability evolution, mineral deposition, and the dynamics of magmatic-hydrothermal interactions, as expressed in the acid sulfate, alkaline chloride, and carbonate systems of the park. 

Fig. 2. Clepsydra Geyser forms a striking, bowl-shaped pool where hot water constantly churns and overflows. The vivid colors around the vent reflect silica sinter, sulfur, and iron-rich deposits created by continuous mineral precipitation. Steam drifts across the surface, hinting at the vigorous hydrothermal activity occurring just below. This feature is a classic example of how water chemistry, heat flow, and permeability interact to shape Yellowstone’s dynamic geyser landscape.

9 August 2025, Upper Geyser Basin

We began by learning from Stuart Simmons how water compositions control the minerals that form in hot springs. Water compositions in turn vary due to differences in temperature, pH, and fluid pathways (Fig. 1-4).

Mixed in with detailed descriptions of the geochemical processes throughout the park were some lighter moments and hands-on demonstrations (Fig. 5).

Fig. 3. This active mud volcano showcases one of Yellowstone’s most fascinating hydrothermal expressions. Thick, clay-rich mud continuously bubbles as CO2-rich gases rise through the shallow subsurface, creating small, rhythmic bursts across the pool. The pale, mineral-rich slurry reflects intense acid-sulfate alteration of surrounding rocks, while the cracked margins record repeated wetting and drying cycles.
Fig. 4. Grand Prismatic Spring, a large alkaline-chloride hot spring characterized by blue water, a slightly basic pH, thermophilic bacteria and archaea, and colorful bacterial mats.
Fig. 5. Trevor Burr (left) and Justin Milliard (right) showed how to professionally forecast the next burst of hot waters as human seismometers at the Black Sand Pool. They lie flat on the ground, sensing low-frequency seismic waves produced by the hydrothermal system underneath. We stand by, watching and feeling the subtle movement below our feet
Fig. 6. As Jeff Hedenquist (right) walked us through the origins of the hydrothermal eruption deposits in outcrop near Yellowstone Lake, George Steele stood by, diligently taking notes.
Fig. 7. While the group examines textures and contacts up close at Yellowstone Lake, Jeff is walking towards the camera, while dedicated “students” take notes on the driftwood log.

10 August 2025, Isa Lake and Grand Canyon of the Yellowstone

Near Yellowstone Lake, we examined a roadside outcrop that beautifully exposes a sequence of volcanic eruption materials, hydrothermal breccias, and zones of intense alteration (Fig. 6). On the shore of Yellowstone Lake, we paused at another outcrop that records both eruptive and alteration processes shaping the caldera’s history. Blocks of eruption-derived breccia, hydrothermally altered sediments, and lake-reworked debris scatter the shoreline, offering a compact summary of Yellowstone’s past (Fig. 7).

One of the main attractions within the park is the steep, hydrothermally altered canyon walls of the Grand Canyon of the Yellowstone, cut deeply by the Yellowstone River. This gorge highlights centuries of acid-sulfate alteration as reflected in the yellow, orange, and pink walls (Fig. 8, 9).

Fig. 8. The Yellowstone River cascades over the Lower Falls, the largest waterfall in Yellowstone National Park. Standing at the overlook, it’s easy to appreciate the immense power of water and volcanic heat working over thousands of years.
Fig. 9. During a brief pause along the trail, Dave Cooke and Trevor Burr take a close look at an altered outcrop, where mineralogical changes and color variations record reactions between hot fluids and rocks.

11 August 2025, Mammoth Hot Springs

Mammoth Hot Springs was the focus of the stops on our third day in the park. These immense fields of travertine deposits are shaped by constantly shifting hot spring waters, providing stark landscapes (Fig. 10-11).

 

Fig. 10. A solitary tree stands frozen in a field of travertine, its trunk engulfed by calcium carbonate deposited from shifting hot spring waters. This silent marker captures the dynamics of Mammoth Hot Springs, where hydrothermal channels migrate rapidly, building terraces in some areas while others dry out.
Fig. 12. A Yellowstone park ranger standing beside Stuart Simmons and Dave Cooke described how our geological observations are connected to broader environmental and management perspectives, reminding us that Yellowstone is not only a natural laboratory but also a living landscape shaped by time, people, and policy.
Fig. 11. Group photo at Mammoth Hot Springs, with the travertine terraces as a backdrop. The cascading carbonate deposits provided one of the most memorable settings of the entire trip.
Fig. 13. Microbial mats flourish in warm water near an influx of hot hydrothermal fluids. The colors and textures reflect subtle changes in temperature, chemistry, and flow conditions. In areas like Yellowstone, such biota play a key role in mediating mineral deposition and recording environmental gradients. A close-up look reveals just how active and vibrant life can be, even in extreme, mineral-rich waters. Width of photo is ~1.2 m.

12 August 2025, Lower Geyser Basin and Grand Teton

On our last day, we were fortunate to meet a Yellowstone park ranger who kindly paused to explain how the park has changed through the years. He described shifts in hydrothermal activity, long-term ecosystem recovery, and the challenges of balancing public access with preservation (Fig. 12).

Before heading back to Jackson, we were able to observe the effect that microbial material plays in the overall dynamics of Yellowstone. Microbial mats and floating patches of “foam” are composed of thermophilic bacteria that mark zones of active hydrothermal input (Fig. 13, 14). 

 

 

Fig. 14. Floating patches of “foam” are accumulations of microbial material produced by thermophilic organisms living in the hot, mineral-rich waters. As warm fluids rise and mix with cooler stream water, microbial communities trap bubbles, organic matter, and fine particles, forming delicate, drifting mats on the surface. Their presence marks zones of active hydrothermal input and subtle chemical gradients that support highly specialized life. Width of photo is ~0.6 m.

Our SGA field trip offered an exceptional opportunity to observe Yellowstone’s hydrothermal architecture and dynamics in real time, as a modern analogue for understanding magmatic-hydrothermal systems and their expression in epithermal and porphyry environments. Across the basin we encountered classic upflow and outflow zones, sinter terraces, hydrothermal breccias, steaming ground, and alteration halos—features that closely parallel those preserved in many epithermal and porphyry districts.

The spatial arrangement of vents, mounds, and geyser basins reflected interplays among topography, permeability contrast, and structural control, illustrating how heat and fluids are channeled from deep magmatic sources to the surface.

For the industry participants, the trip provided an opportunity to observe features of modern hydrothermal systems and compare them to ancient systems targeted in their exploration programs. Of particular importance was understanding the contrasting alteration and geographic positioning between acid-sulfate upflow zones, boiling zones, and steam-heated zones. 

Equally striking was the temporal dimension: episodic venting, overprinting alteration, and temperature-controlled sinter zonation revealed a distinctly pulsed and dynamic hydrothermal system, mirroring the transient behavior inferred from many ancient mineralizing environments. Yellowstone offered a rare modern analogue, showing directly how fluids mix, react, and redistribute heat and metals on short timescales. The trip also served as a valuable teaching refresher: witnessing active boiling, steam-heated alteration, and evolving fluid pathways gives new ways to communicate these processes to students.