An underground geothermal system that never stops

Partially molten magma reservoirs lie beneath Yellowstone, fed by a flow from the mantle at great depths. The heat emitted warms rainwater and snowmelt that seeps into the ground. This water can reach temperatures exceeding 200 °C while remaining liquid due to the high pressure.

When conditions are right—with sufficient water supply, narrow enough fractures, and stable heat—the superheated water is suddenly expelled to the surface: this phenomenon is known as a “geyser.” More than half of the world’s geysers are found in Yellowstone, making it a natural laboratory for geologists.

The most famous, Old Faithful, erupts every 60 to 90 minutes, shooting a column of water and steam that can exceed 50 meters in height. Other geysers are much less predictable, reminding us that these systems are fragile and sensitive to slight variations in pressure or fluid circulation.

In geothermal regions such as the Norris Geyser Basin, hydrothermal activity is not randomly distributed within the rock: it is largely guided by the network of fractures in the rock, which acts as a veritable “highway” for hot water, regulating fluid circulation and determining the location of geysers and hot springs. Studies conducted at Steamboat Geyser demonstrate that even small fractures or simple interconnected pores can be sufficient to rapidly expel water during eruptions.

The subsurface therefore resembles less a system of well-defined conduits than a complex environment, consisting of a tangle of interconnected fractures and pores of varying sizes. This structure controls the circulation of water and steam at depth and determines the triggering of hydrothermal eruptions, highlighting how the structure and composition of rocks govern the functioning of hydrothermal systems.

Hot Springs and Travertine: When Geology Becomes Art

Geysers are just one aspect of the spectacle. The vibrantly colored pools are primarily formed by the abundance of hot springs. The most famous and largest of these in Yellowstone (60 to 90 meters wide) is Grand Prismatic Spring; it is also the most frequently photographed to illustrate these hot springs.

Contrary to what one might think, these colors do not come from minerals, but from extremophile microorganisms. Each microbial community, adapted to a specific temperature range, forms distinct color rings around the pools: species thriving in the cooler outer zones display yellow and orange hues, while the higher temperatures at the center favor blue or green microorganisms. In the former cases, the colors result both from the carotenoid pigments specific to the bacteria and from iron and manganese oxides precipitated by their metabolic activity. These bacteria and archaea thus owe their hues to their photosynthetic pigments (chlorophylls or carotenoids) whose expression varies depending on temperature and light conditions. When the temperature of the springs drops, bacterial growth intensifies.

Travertine deposits can also form from springs rich in calcium carbonate. Travertine is a porous limestone that forms on the surface when hot water—having dissolved CO₂ and calcium carbonate deep underground due to pressure and temperature—rises to the surface: as it cools and decompresses, it releases the dissolved CO₂, causing calcite to precipitate.

This calcite forms white or cream-colored mineral terraces and cascades, while iron oxides precipitated simultaneously by bacterial activity give them their characteristic orange hue; such as those at Mammoth Hot Springs, which are constantly changing in response to shifts in hydrological and temperature conditions.

A Volcano Under Surveillance: What Are the Risks?

Yellowstone is often described as a “supervolcano”, a term used in the media to refer to volcanoes capable of producing massive eruptions, involving the release of at least 1,000 km³ of material. Two of Yellowstone’s three major eruptions reached this threshold, approximately 2.1 million and 640,000 years ago. The chances of such an eruption occurring at Yellowstone in the coming millennia are considered negligible by USGS geologists.

However, the park is the site of nearly constant seismic activity: every year, thousands of small earthquakes indicate fluid movement and crustal adjustments. Scientists from the U.S. Geological Survey (USGS), equipped with a sophisticated network of instruments, closely monitor these signals to detect the slightest anomaly.

Thus, the most immediate dangers in Yellowstone do not stem from a devastating eruption, but rather from local hydrothermal explosions capable of propelling rocks and boiling fluids from a few meters to several dozen meters around the point of emission. Several incidents in the park’s recent history illustrate this risk: a series of powerful explosions occurred at Excelsior Geyser in the 1880s and 1890s, hurling large boulders up to 15 meters away, while a notable explosion took place at Porkchop Geyser in 1989. In July 2024, the explosion at Biscuit Basin destroyed a tourist walkway, causing no injuries, while the explosion at Echinus Geyser a few days ago caused no major damage.

Furthermore, the park’s forests remain vulnerable to wildfires, whose frequency and intensity are increasing with climate change—a risk factor distinct from volcanic activity, but just as real for Yellowstone’s ecosystems.

Yellowstone: An Open-Air Geology Lesson

Yellowstone reminds us that our planet is alive and constantly evolving. The geysers, hot springs, and mineral terraces that amaze visitors are not mere natural curiosities: they are the visible manifestation of deep processes at the interface between the Earth’s mantle, crust, surface water, and even living organisms.

Yellowstone is no exception: in Iceland, the interaction between a hotspot and the Mid-Atlantic Ridge produces similar landscapes, such as the Geysir site. In Italy, the Phlegraean Fields, west of Naples, offer another demonstration of the connection between volcanism, fracturing, and fluids.

These comparisons show that the phenomena observed in Yellowstone follow universal laws: wherever heat, water, and fractures come together, geological phenomena sometimes manifest themselves in spectacular ways. In the face of these ever-changing landscapes, geology is no longer an abstract science confined to laboratories and computer models. It is concrete, perceptible, and integral to our understanding of the Earth’s internal mechanisms.