Hydrothermal Features

Blue badge with white silhouette of an eruption geyser over an image of a steaming pool of water.
Yellowstone is home to more than 10,000 hydrothermal features, including more than 500 geysers. That's about half of the world's geysers and the largest concentration of active geysers in the world!

Hot Springs, Geysers, Mudpots, and Fumaroles

Deep blue pool of hot water surrounded by a barren, gray ground.
Abyss Pool

NPS/Diane Renkin

Hot Springs

  • The most common hydrothermal feature in Yellowstone.
  • Plumbing has no constrictions.
  • Superheated water cools as it reaches the surface, sinks, and is replaced by hotter water from below.
  • This process, known as convection, prevents water from reaching the temperature needed to set off an eruption.
An eruption of water and steam rises from a crater on the bank of a small river.
Cliff Geyser

NPS/Jim Peaco


  • Hot springs with a constriction in their plumbing system.
  • This allows pressure to build to the point where an eruption occurs to release the built up pressure.
  • Over half of the world's geysers are found in Yellowstone.
Thick, whitish mud-like substance bubbles and pops.
Artists' Paintpot

NPS/Jim Peaco


  • Acidic hydrothermal features with limited water supply.
  • Some thermophiles use hydrogen sulfide as an energy source.
  • This helps convert gas into sulfuric acid, which breaks down rock into clay minerals.
  • Various gases escape through the wet clay, causing it to bubble.
Steam rises up from a barren, rocky hillside.
Roaring Mountain

NPS/Jim Peaco


  • The hottest hydrothermal features in the park.
  • Limited amount of water in these features causes water to flash into steam before reaching the surface.
  • Also known as steam vents.

Hydrothermals Underground

Cross-section of the earth showing some of the paths water takes to make hydrothermal features on the surface.
Diagram of water flow through a hydrothermal system.

Geyser & mudpot (NPS/Jim Peaco), hot spring (NPS/Janine Waller), and fumarole (NPS/Tom Pittenger)

  • A - Snow melt and rain water move down through rock layers.
  • B - Magma below the surface heats the water.
  • C - Weight of water above causes pressure and heat to build below.
  • D - Constrictions in the plumbing system cause water and steam to explode with force.

Extreme Environments

The geology of Yellowstone influences habitats for all of the wildlife in the park, from the tiniest microorganism to the biggest bison.

THERMO = heat

PHILE = lover

Thermophiles are organisms that survive and thrive in extreme conditions, such as the heat and acid in Yellowstone's hydrothermal pools. In 1966, Dr. Thomas Brock "discovered" the first thermophile in a hot spring in the Lower Geyser Basin.

View of a strip of Grand Prismatic showing the color range, with five different thermophiles shown by color.
Grand Prismatic Spring with five types of thermophiles living in the water.

Grand Prismatic (NPS/Jim Peaco); A, B, C, & D (Montana State University), and E (Connecticut College)


Color often provides a clue as to which organisms are living in hydrothermal features.

  • A: Green non-sulfur bacteria (Chloroflexus) or Archaea
  • B: Deinococcus-Thermus (Thermus aquaticus)
  • C: Cyanobacteria (Synechococcus)
  • D: Cyanobacteria (Phormidium)
  • E: Cyanobacteria (Calothrix)

This in turn can offer a clue as to the temperature of the water (because thermophiles tend to live in specific ranges of temperature).

  • A: 95–185°F (35–85°C)
  • B: 104–174°F (40–79°C)
  • C: 122–166°F (50–72°C)
  • D: 95–122°F (35–50°C)
  • E: 86–113°F (30–45°C)

How acidic or basic the water is also plays a role in what organisms can live in it. This is measured by the pH scale.

  • A: 7–9
  • B: 5–9
  • C: 7–9
  • D: 6–8
  • E: 6–9

Grand Prismatic Spring happens to be a fairly neutral hot spring. Mud Volcano, however, has features with a pH of 2, while the Heart Lake Geyser Basin has features with a pH of 10.

pH scale with some common household items and hydrothermal features listed.

Another way to look at it, if you measure the temperature and pH of the water, then you can get a good sense of the thermophiles that are living there.


Listen, Look, and Smell

We can learn new things through the power of observation. In 1938, Ranger Harry Woodward observed Old Faithful for a summer. He looked at Old Faithful over time and noticed something unusual: how long Old Faithful erupted was connected to when the next eruption happened. He and other rangers recorded many observations of Old Faithful. Now, rangers can accurately predict Old Faithful eruptions.

  1. To observe something means to pay close attention to it by using your senses.
  2. Look at hydrothermal features from different angles, places, or over time. Listen to the sounds they make. Be mindful of the smells that are released. Just be sure to remain on the boardwalk or marked trails at all times.
  3. Use tools to learn more. Magnifying lenses, binoculars, rulers, cameras, and watches are great tools.
  4. Record your observations. Take photographs, write notes, or draw pictures.


Every geyser has its own unique sound. Take a moment to listen to Spouter Geyser and Beehive Geyser. Take notes about what you notice about each geyser's sounds. How are they similar? How are they different? What parts of the sounds from each might you use to identify them?



Each geyser has its own style of eruption, as well as eruption frequency. In addition, all geysers are either cone geysers or fountain geysers.

  • Cone geysers erupt in a narrow jet of water, usually from a cone.
  • Fountain geysers erupt water in various directions, typically from a pool.

What other visual traits about a geyser's eruption can you think of using to identify it?


Cone and Fountain Geysers

Steam and water erupt out of a beehive-shaped cone of rock. Steam and water erupt out of a beehive-shaped cone of rock.

Left image
Beehive Geyser
Credit: NPS/Jim Peaco

Right image
Grand Geyser
Credit: NPS/Diane Renkin

Compare these two geysers—Beehive Geyser and Grand Geyser—found near each other in the Upper Geyser Basin. One is a cone geyser and one is a fountain geyser. Can you tell which one is which?


Clues in the Steam

Hot blue water issues steam. Hot blue water issues steam.

Left image
Excelsior Geyser Crater
Credit: NPS/Jim Peaco

Right image
Grand Prismatic Spring
Credit: NPS/Curtis Akin

Excelsior Geyser Crater and Grand Prismatic Spring, located next to each other in Midway Geyser Basin, is a great location to observe other visual clues. Take a look at these two hot springs and note what you see as similar and what you see as different. What might the color differences suggest? What might the shape of the surrounding rocks suggest? Any other similarities or differences?


Bubbling and Hissing

Steam rises from a cracked and contorted hole in the ground. Steam rises from a cracked and contorted hole in the ground.

Left image
Fumarole at Fountain Paint Pot
Credit: NPS/Frank Balthis

Right image
Fountain Paint Pot
Credit: NPS/Diane Renkin

Fumaroles and mudpots are two types of hydrothermal features that tend to lack water. What do you notice that is different between these? Are there any similarities that you can see?



Some hydrothermal features are smellier than others. Smells can be a good indicator of what is going on in an area. Notice a smell of rotten eggs? That's a clue that there is sulphur in the ground and that a hydrothermal feature might be more acidic. Mud Volcano, a hydrothermal area just north of Yellowstone Lake, is often referred to as the smelliest place in the park. It also happens to be a location of a lot of mudpots. Mudpots need acidic water to dissolve the surrounding rock.

Take a few moments and walk around your home. Smell each room. What did you notice? Could you identify each room by a different smell? Were some rooms smellier than others? Could you identify what the room might be used for by the smell?

Child wearing a winter hat and coat looking out across a deep, aqua-green hot spring.
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Last updated: October 16, 2023

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