Grand Prismatic Spring

The Yellowstone supervolcano's iconic hot springs and other hydrothermal features are evidence that there is still hot and active magma beneath the surface. © Filip Fuxa/ Shutterstock

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Supervolcanoes and their enormous eruptions

There are more than 1,000 active volcanoes dotted around our planet, but only a select few have produced unimaginably colossal eruptions, earning themselves the title of supervolcano. 

Find out more about supervolcanoes - what they are, what their eruptions are like and how they impact our planet.

How do we measure volcanic eruptions?

Volcanic eruptions range in magnitude from steady lava flows to the massive explosions with superheated, umbrella-shaped plumes of ash, rocks and gas that many of us associate with volcanoes.

Many volcanoes around the world exhibit explosive eruptions. How explosive each eruption is can be measured on a scale known as the Volcanic Explosivity Index. This is similar to the Richter Scale, which is used to measure the magnitude of earthquakes.

The Volcanic Explosivity Index runs from zero to eight, from non-explosive to colossal explosive eruptions. Where a volcano appears on the scale is linked to the amount of material an eruption emits and how frequently eruptions occur. 

A lava flow

Hawaiian eruptions may look gentle, but they are still dangerous. They can form intense lava flows that destroy everything in their path. © Thijs Peters/ Shutterstock

At the low end of the scale sits the relatively gentle, though still deadly, Hawaiian eruption - named after the style of eruption typically seen in this state. These eruptions often result in 1,100°C lava fountains that can reach speeds of 100 metres per second, rise to heights of a few hundred metres and form intense lava flows that cover the landscape. Though these lava flows destroy everything in their path, they do also form new land in the process.

Then there's Strombolian eruptions. Named after the stratovolcano Stromboli, these eruptions are short in duration and have explosions that generate small plumes known as fire fountains that reach hundreds of metres in height and throw out ash, volcanic bombs and ballistic blocks.

At the other end of the scale are the explosive Plinian eruptions, which are known for their eruptive columns that can travel up 55 kilometres into the atmosphere as well as their high-speed jets of magma and volcanic gases.

Plinian eruptions are named after Pliny the Younger, who documented the catastrophic eruption of Mount Vesuvius, which famously destroyed Pompeii and Herculaneum in 79 AD. On the Volcanic Explosivity Index this eruption had a magnitude of five. 

A gouache painting of Vesuvius erupting

The infamous eruption of Mount Vesuvius in 79 AD is categorised as a magnitude five eruption on the Volcanic Explosivity Index. This gouache painting by Mauton shows the volcano erupting in 1836. Image courtesy of Wellcome Collection, Public Domain.

What is a supervolcano?

While the exact number isn't certain, there's thought to be around 20 supervolcanoes on Earth.

However, 'supervolcano' isn't a scientific term. It was first used in the 1940s and since the 2000s has gained popularity and is now widely accepted by the scientific community.

A volcano is categorised as a supervolcano if it can produce a magnitude-eight eruption on the Volcanic Explosivity Index, discharging more than 1,000 cubic kilometres of material. These eruptions are sometimes called 'super eruptions' and are the biggest and most explosive of all.

Before an eruption, magma collects in a chamber below the surface and pressure builds until the magma eventually ruptures Earth's crust. During an eruption the chamber empties of magma and this can result in the overlying rock becoming less supported. Without enough support the ground can collapse, forming a depression on the surface known as a caldera.

Super eruptions produce giant calderas that can be more than 50 kilometres wide. 

Lake Toba

Lake Toba occupies the caldera of a supervolcano that last erupted 74,000 years ago. Some suggest this event may have had a profound impact on the course of human evolution. © Yudhapohan (CC BY-SA 4.0) via Wikimedia Commons

Eruptions of this magnitude are extremely rare. The general trend along the Volcanic Explosivity Index is that the larger the eruption, the less frequently it occurs.

But supervolcanoes don't always produce gigantic eruptions, sometimes they produce smaller, more 'normal' eruptions. Naples in Italy might be best known for Mount Vesuvius, but the region is also home to the supervolcano Campi Flegrei – one of the closest supervolcanoes to the UK. These days it's famous for smelly fumaroles and emitting sulphuric acid.

Within its giant caldera sits Monte Nuovo - a cinder cone volcano that formed in 1538 in an eruption that buried the medieval town of Tripergole. Between 1982 and 1984, volcanic activity in the region led to fears of another eruption, a reminder that there's still hot and active magma below the surface.

What's a super eruption like?

A magnitude-eight eruption is almost unimaginable. No one alive today has ever seen a supervolcano erupt. The most recent super eruption was of New Zealand's Taupō volcano, which occurred around 26,500 years ago.

The 1991 eruption of Pinatubo in the Philippines is one of the largest eruptions in living memory. This event only ranked six on the Volcanic Explosivity Index, making it around 100 times smaller than the benchmark for a supervolcano.

Mount Pinatubo's giant eruption column

The eruption of Mount Pinatubo in 1991 was one of the largest eruptions of the twentieth century, categorised as magnitude six on the Volcanic Explosivity Index. Image courtesy of Dave Harlow / United States Geological Survey via Wikimedia Commons, Public Domain.

Despite this, we can hypothesise what a super eruption might be like.

For example, Yellowstone, one of the world's most famous supervolcanoes, has super erupted three times relatively recently in terms of our planet's history. These eruptions formed the Yellowstone Caldera, which is 72 by 55 kilometres in size and is now the site of the USA's Yellowstone National Park. The largest of these eruptions was about 2.1 million years ago, when the supervolcano released some 2,450 cubic kilometres of material.

If Yellowstone were to produce another super eruption, the surrounding states of Wyoming, Idaho and Montana would be directly impacted by massive pyroclastic flows. These hot avalanches of volcanic ash, pumice, gases and rocks can reach 400-500°C, move at more than 300 kilometres per hour and travel for more than 100 kilometres.

You can't outrun a pyroclastic flow - they pick up and destroy everything in their path. Take the eruption of Martinique's Mount Pelée in 1902. A pyroclastic flow completely destroyed the town of St Pierre when it hit in the early hours of 8 May killing all 28,000 residents and that was a much smaller, magnitude-four eruption. 

A building destroyed by a pyroclastic flow

St Pierre on Martinique was destroyed by the 1902 eruption of Mount Pelée. This image shows what remained of the town's bank in the wake of the eruption. Image courtesy of Yorkshire Museum (York Museums Trust) via Wikimedia Commons, Public Domain.

As for our hypothetical Yellowstone super eruption, it's not just the area directly surrounding the volcano that would be affected, so too would the continent of North America. It's possible that the surrounding 800 kilometres, beyond the reach of the pyroclastic flows, would be covered in a thick blanket of ash, with the thickness decreasing further from the volcano.

In fact the super eruption would be felt around the world!

How do volcanoes affect the climate?

Volcanic eruptions can have an impact on our planet's temperature.

When volcanoes erupt explosively, they shoot gases - mainly water vapour, carbon dioxide and sulphur dioxide - into Earth's stratosphere. Sulphur dioxide is the most significant, as it reacts with water vapour to create sulphate aerosols. These increase the reflection of radiation from the Sun back into space and can soak up terrestrial radiation. This has a cooling effect on the planet's lower atmosphere, which in turn can have huge potential impacts on life around the world.

An aerial image taken of Soufrière Hills erupting on Montserrat

Soufrière Hills is a stratovolcano on Montserrat in the Caribbean. It erupted in 1995 after centuries of dormancy, with pyroclastic flows and mud-flows destroying human settlements and important habitats. Image courtesy of NASA Earth Observatory via Wikimedia Commons, Public Domain.

It's been claimed that the Young Toba Tuff super eruption 74,000 years ago was so great that it might have altered the course of human history. This eruption resulted in a massive caldera, measuring 100 by 30 kilometres and which is now filled by Indonesia's Lake Toba.

The Toba Catastrophe Theory proposes that this gigantic eruption sent the world into a decade-long volcanic winter and caused the climate to be cold and dry for thousands of years after that. The resulting famine is theorised to have reduced the ancient human population down to just a few thousand individuals, with a dramatic effect on human evolution.

However, there are also strong arguments against this theory. Evidence from ice cores shows that sulphur emissions were lower than originally thought, meaning the effect on climate would have been less extreme than predicted. It's also been found that no species other than humans saw a population decline, and the Asian mammal fossil record doesn't support extinction. Together this suggests that the theory that we are the descendants of a small group of survivors resulting from this incident is somewhat unlikely.

But we do know that volcanic eruptions, even those not large enough to be considered super eruptions, have influenced the world's climate and impacted people globally.

It's estimated that three megatonnes of sulphur were released into the atmosphere during the 1991 eruption of Mount Pinatubo. This caused the global air temperature to drop by 0.5°C during the period of 1991-1993.

Anak Krakatau erupting

Krakatau is an active stratovolcano in the Sunda Strait. In 1883 it produced a magnitude six eruption that killed 36,000 people, making it one of the deadliest in recent history. This is Anak Krakatau, which began forming in the Krakatau caldera in 1927. © Belikova Oksana/ Shutterstock

Further back, Krakatau's eruption in August 1883 cooled Earth by 0.6°C for months and the tiny particles it released caused the Moon to appear blue for a year.

Prior to this, the massive 1815 magnitude-seven eruption of Tambora in Indonesia had an even greater effect on the planet. In fact the year that followed it, 1816, is sometimes known as the year without summer. The massive eruption killed an estimated 60,000 people and is now known to have been the cause of crop failure, famine and unusual weather.

When will a supervolcano next erupt?

Volcanoes don't work on a precise schedule, so it's hard to know exactly when a supervolcano will next erupt. However, we do know that the chances of it occurring in our lifetimes is extremely low.

Working out when volcanoes will erupt is a tricky business, but we're getting much better at predicting eruptions over the short term.

Volcanoes give off warning signals in the hours and sometimes years before an eruption. These signals include heat anomalies, changes in the amounts of fumarolic gases, earthquakes that we can detect with seismographs and ground deformation that can be seen using satellites and other surveying equipment. Detecting these warning signals has saved countless lives. 

Dragon's Mouth Spring

Dragon's Mouth Spring is one of Yellowstone National Park's many hydrothermal features. © Ken Larmon/ Shutterstock

It's much harder to predict if a volcano will erupt at some point in the distant future, but trends of past eruptions can provide volcanologists with clues.

Large Plinian eruptions occur with far less frequency than effusive Hawaiian and Strombolian eruptions, which can occur near-constantly. Super eruptions tend to only occur at intervals of tens of thousands of years.

The media delights in discussing whether we're 'due' for another eruption from supervolcanoes such as Yellowstone. This volcano last erupted 630,000 years ago, but based on the average spacing between its eruptions, we could have another 100,000 years to wait before its next super eruption. There's also every chance that if Yellowstone erupted again, it might not be a magnitude-eight, caldera-forming eruption, instead it might be a hydrothermal explosion or lava flow.  

But when another large eruption does occur, are we ready for it? Some argue that we're not and that we're actually more prepared for an asteroid strike, which is less likely to occur, than we are for a magnitude-seven or -eight volcanic eruption. 

A series of photos showing Hunga Tonga-Hunga Ha'apai erupting

A sequence of images showing the magnitude five eruption of the submarine volcano Hunga Tonga-Hunga Ha'apai in January 2022. Image courtesy of NASA Earth Observatory via Wikimedia Commons, Public Domain.

Researchers have pointed out that the destruction caused by the 2022 eruption of Hunga Tonga-Hunga Ha‘apai in the South Pacific should serve as a wake-up call. This magnitude-five eruption cut off Tonga's communications for several days when undersea cables were severed, caused tsunamis and global shockwaves and dropped ash over hundreds of kilometres, contaminating water supplies and making breathing difficult.

We live in a connected world and need our global food, energy, communications and other systems to be prepared for any disaster, whether it's asteroids, pandemics, climate change, biodiversity loss or even a volcano brewing up a super eruption.