Yellowstone supervolcano: What happened 631,000 years ago and what if it erupts again?

Yellowstone's last giant eruption happened about 631,000 years ago. It was so powerful that the ground above the underground magma system collapsed, creating the massive Yellowstone Caldera seen today. There were no modern humans around to witness it.

But what would happen if Yellowstone erupted again today?

Yellowstone, located primarily in northwestern Wyoming, is one of the world's most closely studied volcanic systems. Beneath its famous geysers, hot springs and vast landscape lies a complex volcanic system capable of producing eruptions far larger than those of conventional volcanoes.

What happened during Yellowstone's last giant eruption?

About 631,000 years ago, Yellowstone experienced its most recent caldera-forming eruption. Magma stored deep beneath the surface was expelled on an enormous scale.

The eruption was powerful enough to cause the ground above the emptied magma reservoir to collapse, forming the Yellowstone Caldera.

Scientists classify the largest volcanic eruptions using the Volcanic Explosivity Index, or VEI. A magnitude-8 eruption, often called a super-eruption, can release more than 1,000 cubic kilometres of volcanic material.

A Yellowstone super-eruption today would therefore be unlike anything experienced in recorded human history.

What if Yellowstone erupts again?

Despite the terrifying images often associated with Yellowstone, scientists say a future eruption is far more likely to be small than a giant super-eruption.

Research indicates that only a fraction of Yellowstone's magma system is molten, and the US Geological Survey says there is currently no evidence that enough eruptible magma has accumulated to support another super-eruption.

If Yellowstone erupts again, a lava flow or a smaller explosive eruption would be more likely.

Since the last giant eruption, Yellowstone has experienced numerous smaller volcanic events. Its most recent lava flow occurred about 70,000 years ago.

What would a Yellowstone super-eruption look like?

A full-scale super-eruption would be an entirely different scenario.

More than 1,000 cubic kilometres of material could potentially be expelled, with enormous quantities of ash and volcanic gases entering the atmosphere.

The areas closest to Yellowstone in Wyoming, Montana and Idaho would face the greatest immediate danger. Fast-moving pyroclastic flows — mixtures of scorching gas, ash and rock — could devastate areas near the caldera.

But the effects would not stop there.

Volcanic ash could spread across large parts of the United States. Heavy ash deposits could damage buildings, contaminate water supplies, disrupt electricity networks, damage machinery and bury crops.

Even relatively thin layers of volcanic ash can create serious problems for aircraft because ash can damage jet engines. A major eruption could therefore disrupt air travel, roads, railways and supply chains across a much wider area.

Could Yellowstone change the world's climate?

The biggest global concern would come from volcanic gases reaching the upper atmosphere.

Sulfur-rich gases can form sulfate aerosols that reflect sunlight back into space. This can temporarily cool the planet and alter rainfall patterns.

A sufficiently large Yellowstone eruption could therefore cause major agricultural disruption, crop failures and food-supply problems in different parts of the world.

The 1815 eruption of Mount Tambora in Indonesia offers a historical example of how volcanic activity can affect climate. It contributed to the period known as the "Year Without a Summer," when global temperatures fell and harvests failed in parts of the Northern Hemisphere.

However, the effects of a hypothetical Yellowstone super-eruption remain difficult to predict precisely.

Is Yellowstone overdue for an eruption?

No.

The fact that roughly 631,000 years have passed since Yellowstone's last caldera-forming eruption does not mean another one is due.

Volcanic eruptions do not operate like clockwork. The intervals between Yellowstone's giant eruptions have varied greatly, and scientists do not consider the volcano to be "overdue."

The USGS estimates the annual probability of another Yellowstone super-eruption at roughly one in 730,000.

As of August 2026, Yellowstone's volcanic alert level remains at Normal, with seismic activity and ground deformation at background levels. USGS monitoring recorded 68 earthquakes in Yellowstone during July, with the largest measuring magnitude 3.3.

What is Yellowstone's biggest realistic threat?

A super-eruption may grab headlines, but it is not considered Yellowstone's most likely hazard.

Hydrothermal explosions, earthquakes and smaller volcanic eruptions are considerably more realistic possibilities.

Hydrothermal explosions happen when extremely hot water underground rapidly turns into steam and blasts apart surrounding rock. Small events occur in Yellowstone periodically and can pose localized dangers.

Scientists also closely monitor Yellowstone for signs that could indicate increasing volcanic activity. These include earthquake swarms, rapid ground deformation, changes in volcanic gases and changes in heat flow.

So, could Yellowstone erupt again?

Yes, Yellowstone is an active volcanic system and could erupt again someday.

But the scenario of Yellowstone suddenly producing a civilization-ending super-eruption is extremely unlikely in the foreseeable future.

The last giant eruption 631,000 years ago reshaped the landscape and left behind the enormous caldera that exists today. If a similar event ever occurred again, the consequences would extend far beyond Yellowstone, potentially disrupting agriculture, transportation, aviation and climate across large parts of the world.

For now, however, scientists say there is no evidence that a catastrophic Yellowstone super-eruption is imminent.