Yellowstone National Park is a wild and dynamic volcanic environment renowned for its extraordinary geothermal activity. Historically, estimates suggested there were about 10,000 thermal features throughout the park, including geysers, hot springs, fumaroles, and mud pots, accounting for over half of the world's total. However, recent satellite data have revised this estimate upward to nearly 30,000 thermal features, underscoring the immense scale and complexity of Yellowstone’s geothermal systems.
While many visitors associate the park primarily with Old Faithful, the iconic geyser, Yellowstone actually boasts more than 600 geysers in total, many of them larger than Old Faithful. Tracking these features is challenging because they are continuously changing. The shallow lava chamber beneath the park fuels these hydrothermal systems, causing them to heat up and cool down unpredictably, leading to fluctuations in their activity.
One remarkable location to witness this geothermal dynamism is the West Thumb Geyser Basin along the shores of Lake Yellowstone. Here, Abyss Pool has exhibited rapid temperature and volume changes over recent years. This sudden heating has had a noticeable impact on the surrounding area, resulting in the emergence of new thermal features, the death of trees due to heat damage, and the awakening of previously dormant geysers. The changes at West Thumb highlight the ever-evolving nature of Yellowstone’s geothermal landscape and the powerful volcanic forces at work beneath the surface.
The Four Varieties of Thermal Features
Yellowstone boasts an abundance of each of the four kinds of hydrothermal features. Before diving into what they are and how they function, let's look at the word hydrothermal. It breaks down into two parts: hydro, meaning water, and thermal, meaning heat. These hot waters are created by contact with hot rocks. The geothermal energy (geo for earth) comes from a volcanic hotspot, similar to the one that is making the Hawaiian Islands. This hotspot is a jet of incredibly hot material streaming up from the mantle core boundary and heating the material, molten rock, around it. This heating causes the magma to expand, become less dense, and thus more buoyant than the magma around it. This buoyancy, think of a raft in a pool, rises and heats the ground as it pushes towards the surface. As another result, it also pushes the entire Yellowstone plateau up nearly 2,000 feet.
This extra elevation gives the Yellowstone region enough height to cause its own weather. Orogenic snowfall, snow caused by the cooling of water vapor as it rises over mountains, accumulates, melts, seeps into the ground, and provides the water part of our hydrothermal equation. The water percolates through the porous volcanic rock and finds its way to lower elevations, where it is heated by the aforementioned geothermal energy. In lower elevation basins where the water table is close enough to the surface, this heated water can find its way to the surface, where it pools up. We call this a hot spring.
Heart springs with the lion geyser group in the background.
Where there is insufficient water pressure to force water to the surface, but there is adequate heat to boil water, we get steam vents. Geologists often refer to these as fumaroles. Fumaroles are common on hillsides just above hot springs because the mountainside acts to keep the ground surface just above the water table. In many places in the park, the water table varies seasonally, and as a consequence, hot springs may turn into fumaroles during drier times of the year. Because steam does not have an upper temperature limit like liquid water does, these fumaroles are the hottest features in the park. Also interesting to note, the “steam” we see and associate with fumaroles is just the condensed water vapor. What we see is only the coldest part of the water vapor; the rest remains invisible in its gaseous form and is over 200 degrees Fahrenheit.
Geysers are the rarest of the hydrothermal features, with only around 1,000 worldwide and only some 600 in Yellowstone. Many of these are small geysers shooting only a few feet into the air, but a handful are among the tallest in the world. Old Faithful is the most well-known and best studied among geysers, but it is not the largest. Old Faithful has become renowned because of its ideal mix of spectacular height, consistency, and predictability.
For a geyser to exist, a perfect storm of underground architecture must come together. First is the boiling chamber, which is a particularly porous or hollow cavity below the earth. The size of the boiling chamber will determine the volume of water a given geyser can erupt. This boiling chamber must be situated in an area with enough geothermal energy to boil water. Touching the rocks on the inside of this would be akin to touching your stovetop.
The boiling chamber must also have a way for water to infiltrate it. In the case of Old Faithful, there is a small underground waterfall that fills the chamber. The flow rate of water into the chamber is one factor that will determine the period of the geyser, which is the time between eruptions.
Finally, there needs to be an opening to the surface that is large enough for water to get through, but not so large as to allow free flow of water. We often call this a constriction point. This constriction allows water in the boiling chamber to become superheated, that is, water that is above the boiling point at atmospheric pressure. This process can happen because the boiling point of water rises as we put it under pressure. So water that would normally boil at 212 degrees at sea level becomes pressurized and remains in its liquid state even at 250 or 275 degrees F.
When the water finally overflows the boiling chamber, finds its way through the constriction, and splashes out on the surface, it causes the volume of water inside the boiling chamber to drop. The decrease in volume inside a fixed space causes a drop in pressure. The drop in pressure causes some of the superheated water to instantly turn to water vapor. The steam almost instantaneously expands to 1,700 times its volume and pushes more water out of the constriction. This causes another volume drop, followed by another pressure drop, followed by another flash to steam. The process is in a positive feedback loop until all of the superheated water is gone or cooled below the boiling point. The next time you watch a geyser, watch for this surging effect.
The height a geyser reaches correlates with the size of the constriction point and the temperatures reached in the boiling chamber. Taller geysers build up more pressure and have a smaller constriction. In this sense, the constriction also acts like a thumb on a garden hose. Channeling the water and allowing it to shoot higher.
Mud pots represent one of the most unusual hydrothermal features found in nature. Unlike geysers or hot springs, they occur in areas where minimal or no hot water reaches the surface, yet the ground temperature remains high enough to boil water. Beneath these sites, pockets of superheated water often act as the heat source, driving the processes that create these formations.
The formation of mud pots relies on several key factors:
Sufficient precipitation to keep the mud semi-fluid.
Hydrogen sulfide gas bubbling up through underground rock layers.
The presence of chemosynthetic extremophiles, which are bacteria and archaea capable of using chemicals—specifically hydrogen sulfide—for energy.
These extremophiles metabolize hydrogen sulfide and release sulfuric acid as a byproduct. Since water flow into these systems is limited, sulfuric acid accumulates, significantly acidifying the mud pool. This acidic environment breaks down surrounding rock into clay, creating the distinctive thick, bubbling mud characteristic of mud pots. Some pools in Yellowstone National Park exhibit a pH close to 2, which is slightly more acidic than lemon juice or soft drinks like Coke. This strong acidity gradually dissolves surrounding rocks, reducing them to the finest clast sizes. These ultrafine clay particles are responsible for the distinctive thick, viscous mud found in mud pots.
In sum, mud pots provide an intriguing window into extreme microbial ecosystems shaped by geothermal forces and chemical energy rather than sunlight. Their unusual appearance and distinct biological characteristics make them valuable subjects for both geological and microbiological research. Many researchers are currently studying these organisms in search of answers to what early life on Earth may have looked like and what alien life may look like when we find it.