Yellowstone Supervolcano

Yellowstone is a famous national park in the western United States that sits on top of one of the most powerful supervolcanoes on Earth. Instead of a classic cone, Yellowstone has a giant caldera: a depression approximately 72 km by 55 km wide formed when an earlier huge eruption collapsed the original volcano into itself. The supervolcano powers the park's famous geysers (including Old Faithful), hot springs and bubbling mud pots. Yellowstone has produced three super-eruptions in the past 2.1 million years. It is currently quiet but very much alive: scientists carefully monitor it every minute, just in case.

  • TypeSupervolcano (caldera)
  • Caldera size72 x 55 kmA huge sunken depression
  • Number of past super-eruptions3In the last 2.1 million years
  • Most recent super-eruption640,000 years ago
  • Geysers in the parkApproximately 500Most in the world
  • Old Faithful intervalApproximately every 90 minutesRoughly predictable

The three past super-eruptions

  • 2.1 million years ago: the Huckleberry Ridge eruption. Approximately 2,500 cubic kilometres of material ejected. Approximately 2,500 times bigger than the 1980 Mount St Helens eruption.
  • 1.3 million years ago: the Mesa Falls eruption. The smallest of the three, around 280 cubic kilometres.
  • 640,000 years ago: the Lava Creek eruption. Approximately 1,000 cubic kilometres. Created the current Yellowstone Caldera. Ash fell as far away as Texas and California.

Each super-eruption was thousands of times bigger than any volcanic eruption in human recorded history. The intervals between them (roughly 800,000 years and 660,000 years) have led to occasional speculation that another is "overdue", but supervolcanoes do not erupt on a strict schedule.

What powers Yellowstone

Yellowstone sits on top of a deep hot spot: a plume of unusually hot mantle rock that has been rising from far below for at least 17 million years. The hot spot itself stays fixed, but the North American tectonic plate is slowly drifting southwest over it at approximately 4 cm per year. The result is a long chain of progressively older volcanic features stretching back across Idaho into Oregon, ending at the modern Yellowstone Caldera in north-western Wyoming.

Beneath the caldera lies a huge magma chamber: approximately 90 km long, 40 km wide and several kilometres thick. Most of it is solid hot rock with a small percentage of molten rock mixed in. Heat rising from this magma chamber drives all the famous surface features of the park.

The geothermal park

Yellowstone has the largest collection of geothermal features on Earth, including:

  • Geysers: hot springs that periodically shoot boiling water and steam into the air. Yellowstone has approximately 500 of them, more than the rest of the world combined.
  • Hot springs: pools of hot water heated underground. The Grand Prismatic Spring is famously rainbow-coloured due to different temperature-loving bacteria growing in concentric rings.
  • Mud pots: hot springs where the water has dissolved nearby rock into thick mud, which bubbles and burps as gas escapes.
  • Fumaroles: vents that release hot gas and steam without much water.

Old Faithful

The most famous feature in Yellowstone is Old Faithful, a geyser that erupts approximately every 60 to 110 minutes (averaging around 90). Each eruption shoots boiling water up to 56 m into the air for 1.5 to 5 minutes. The geyser was named in 1870 by an expedition that was impressed by how reliable it was. It has been erupting on a steady schedule since records began, and probably for centuries before that.

Rangers post the predicted time of the next eruption on signs around the park. The predictions are based on careful measurements of how long the previous eruption lasted: longer eruptions are followed by longer intervals. Visitors can plan their day around it and sit on the benches around the geyser waiting for the show.

Fact If Yellowstone produced another super-eruption like the one 640,000 years ago, the consequences would be continental in scale. Approximately 800 cubic kilometres of ash would be ejected, blanketing the western United States and much of central America in metres of ash. Global temperatures would drop by several degrees for years from sulphur aerosols in the stratosphere. Farming in much of the northern hemisphere would be severely disrupted. The good news is that there is currently no sign Yellowstone is preparing to do this, and even if it were, the warning signs would build up over decades or centuries.

Will it erupt again?

Yellowstone is closely monitored 24 hours a day by the Yellowstone Volcano Observatory. Hundreds of sensors track tiny earthquakes, ground movements, gas emissions and temperature changes. The current consensus among volcanologists is that another super-eruption is very unlikely in the foreseeable future. Possible eruption scenarios include:

  • Small hydrothermal explosions: when steam pressure breaks through the surface. These happen every few hundred years and are very localised.
  • Small to medium lava eruptions: the most recent at Yellowstone was approximately 70,000 years ago.
  • Another super-eruption: possible but no current signs.

If a major eruption were ever to start brewing, scientists believe the warning signs (huge ground swelling, swarms of earthquakes, increased heat) would build up over many years. The world would not be caught by surprise.

Did you know? The vibrant colours of Yellowstone's hot springs, including the famous Grand Prismatic Spring, are caused by different species of heat-loving bacteria called thermophiles. Different temperatures support different bacteria, which produce different colours. The hottest centre is sterile and pure blue (the natural colour of pure water reflecting the sky). Cooler rings around the edge support orange, yellow and green bacteria, creating a rainbow effect.
Deeper dive: extremophiles and the search for alien life

The bacteria that live in Yellowstone's hot springs are some of the most studied organisms on Earth. They belong to a group called extremophiles: living things that thrive in conditions that would kill most other creatures. Yellowstone's hot springs host bacteria that:

  • Live at temperatures of 70 to 90 degrees C, comfortable for them but lethal for most life.
  • Survive in highly acidic water with pH as low as 1 (more acidic than stomach acid).
  • Get their energy from chemicals (sulphur, hydrogen) rather than sunlight or food.
  • Have been around for billions of years, descended from some of the earliest life on Earth.

One of these Yellowstone bacteria, Thermus aquaticus, discovered in 1969, turned out to be one of the most important biological discoveries ever made. It produces an enzyme (called Taq polymerase) that copies DNA at high temperatures without breaking down. This enzyme is the key ingredient in PCR (polymerase chain reaction), the technique used in essentially every modern DNA test (including all the COVID-19 tests during the pandemic). One small Yellowstone bacterium has saved millions of lives and made multi-billion-dollar industries possible.

The discovery has also encouraged scientists to look for similar life elsewhere in the Solar System. Mars, Europa (a moon of Jupiter) and Enceladus (a moon of Saturn) may all have subsurface environments similar to Yellowstone's hot springs: hot or warm water with dissolved chemicals and possibly underground volcanism. If life exists anywhere else in the Solar System, it is most likely to be something like Yellowstone's extremophiles. Studying them on Earth is the best preparation for finding (or recognising) extraterrestrial life if and when we do.

For more, see types of volcano and volcanic eruptions.