A2 Yellowstone (ours)

Introduction: What If Yellowstone Woke Up? 0:00

0:00Imagine waking up to this. Breaking news. A massive eruption has begun at Yellowstone National Park in the United States. Ash is falling across Wyoming, Montana and Idaho. If you are in the region, go indoors now. Close every window. Stay off the roads.

0:17This is Yellowstone.

0:19Most people know it as a park of geysers, bison and hot springs. But the middle of the park is a giant crater, about 70 km long and 45 km wide, almost the size of Rhode Island. Under it sits one of the largest active volcanic systems on Earth. If it ever erupted at full size, it would not matter whether you live in Wyoming, Mexico City or Mumbai. Sooner or later, it would reach you.

0:53Between 5 and 17 km down lies a body of hot rock about 90 km long, longer than two marathons end to end. Heat from deep inside the Earth keeps part of it molten, and that molten rock is packed with gas, held in by the weight above it, like the fizz in a sealed bottle of soda. Release the pressure fast, and it all comes out at once.

1:24So what if that happened tomorrow? What if Yellowstone erupted at full size, the way it did 631,000 years ago? How far would the ash fall? What would happen to the sky? And would anywhere on Earth be out of its reach?

1:43Let's find out.

1:44First, an honest warning. This is not a prediction. Scientists see no sign that Yellowstone is about to erupt, and a real eruption this big would most likely give weeks or months of warning: earthquake swarms and fast-rising ground. So we are going to bend the rules. Tomorrow at dawn, with no warning, Yellowstone erupts at the size of that last great eruption: about 1,000 cubic km of rock and ash, enough to bury all of Rhode Island under a layer almost as tall as the Eiffel Tower.

2:23Scientists at the U.S. Geological Survey have run this experiment on a computer. They fed a Yellowstone supereruption into the same software they use to forecast real ash clouds, and let it run for three days, for a week, and for a month. We will take the middle one. One week of eruption. That is our clock.

2:49Now, you might expect a disaster like this to hit hardest in the first minutes and then fade. Near the volcano, it does. But what makes this one different is not how hard it hits. It is how far it reaches. In the first hour, it belongs to the park. After a day, to the region. After a week, to a continent. And within a year, to the sky over the whole planet.

Hour 1: The Column & the Cloud That Beats the Wind 3:21

3:21At first light, the floor of the crater cracks open. Magma rushes upward, the pressure on it drops, and the gas inside expands so violently that the molten rock is blown apart into dust. A column of ash and gas punches into the sky, 30 to 50 km high. That is three to five times higher than passenger jets fly, straight through the weather and into the stratosphere.

3:52The first to know are the machines. Yellowstone is watched day and night by nine agencies, with seismometers, GPS stations, gas sensors, thermometers and cameras. In a normal month, they record dozens, sometimes a hundred or more, small earthquakes. This morning, every screen fills with one long, unbroken roar. Within minutes, aircraft across the West are turned away, and in our scenario, the warning you heard at the start goes out to every phone in three states.

4:23To follow what happens next, we need to know what is really inside that column, because everything else depends on it. When most people hear the word ash, they picture the soft grey powder in a fireplace. Volcanic ash is nothing like that. It is rock and glass, shattered into jagged grains the size of sand and dust. It is hard. It does not dissolve in water. It scrapes like sandpaper. And when it gets wet, it conducts electricity.

4:59And this is the part that matters most. Small eruptions make ash clouds that drift with the wind, like smoke from a chimney. A cloud this big does something else. When it can rise no higher, it spreads sideways in every direction at once, like an umbrella opening over the continent. In the USGS model, it spreads so hard that it pushes ash more than 1,500 km to the west, straight into the oncoming wind. That is far enough to reach the Pacific Ocean.

5:36Close to the vents, the column grows too heavy to hold itself up. Parts of it collapse and pour across the ground as a glowing avalanche of hot gas, ash and rock, hundreds of degrees hot and moving tens of metres every second, as fast as a car on a highway. In Yellowstone's past eruptions, flows like these raced out in all directions from the vents.

6:06Then the ground itself begins to give way. As magma pours out from below, the roof of rock above it loses its support. Over the hours and days that follow, a block of land almost the size of Rhode Island starts to sink, swallowing the mountains on top of it. This is how a caldera is born.

Day 1: Darkness Over the Mountain West 6:31

6:31By the end of the first day, the cities closest to the volcano are under the cloud. In Billings, Montana, about 230 km away, and Casper, Wyoming, about 390 km away, noon looks like midnight. Ash falls like grey snow that never melts. By the time the eruption is over, the USGS model leaves cities this close under anywhere from a few centimetres to more than a metre of it. In Billings, the pile could stand taller than a kitchen counter.

7:08Then comes the weight. A layer of dry ash only 10 cm deep weighs about as much as a grown adult standing on every square metre of a roof. Soaked by rain, it weighs twice as much. Flat roofs give way first.

7:26And the sky above North America shuts. Ash can stop a jet engine in mid-air, so air travel across the continent grinds to a halt. For scale, in 2010 a far smaller eruption in Iceland closed more than 300 airports in Europe and cancelled more than 100,000 flights in a single week. This one is happening in the middle of America.

Week 1: A Continent Under Ash & Could We Stop It? 7:54

7:54Over the next few days, the umbrella keeps spreading until it covers almost all of North America. In the model, Minneapolis and Des Moines, about 1,400 km away, get centimetres of ash. New York and Washington, on the far side of the country about 3,000 km away, get millimetres. Seattle and Portland, which sit upwind, still get up to a few centimetres.

8:21A few millimetres does not sound like much. It is enough. Roads turn slick. Wet ash shorts out electrical transformers, so the power starts to fail, and phone networks falter with it. Breathing the dust is dangerous, especially for children and older people.

8:41And the ash lands on America's farm belt. The United States grows and exports more corn than any other country on Earth, most of it in the Heartland, from the Great Plains to Ohio. Even a light dusting of ash can damage crops, and buried pastures leave cattle with nothing to graze.

9:04Which brings up the question everyone would be asking by now. Could anyone stop it?

9:11No.

9:11The National Park Service puts it plainly: nothing can be done to prevent an eruption. Drilling a hole to let the pressure out would not work: at those temperatures, the USGS says, the hole would seal itself with new crystals. In 2017, a NASA engineer did propose something bolder: drilling 10 km down into the edges of the system and pumping in water to carry the heat away. But even on paper, cooling it that way would take tens of thousands of years. Once it starts, all anyone can do is wait it out.

9:53By the end of the week, the eruption weakens. The column shrinks. The ground stops shaking. For a moment, it looks as if the worst is over. It is not.

Month 1: A Veil Around the World 10:07

10:07The most dangerous thing Yellowstone sent into the sky was never the ash. Ash is heavy, and it falls back to the ground. The real threat is a gas you cannot see: sulfur dioxide, carried up the column into the stratosphere, more than 10 km above our heads. Up there, it reacts with water and turns into a thin haze of tiny droplets of sulfuric acid. And the stratosphere has no rain to wash them out. They stay for years, bouncing sunlight back into space before it can warm the ground.

10:47We know this is real, because we have measured it. In 1991, Mount Pinatubo in the Philippines blasted about 17 million tonnes of sulfur dioxide into the stratosphere. Satellites watched the haze wrap all the way around the Earth in about three weeks. Within a year it covered the whole planet, and the world's average temperature fell by about half a degree Celsius for two years. Yellowstone would throw out well over a hundred times as much rock and ash as Pinatubo did.

11:24And as the haze spreads, people far from America would see it for themselves. Sunrises and sunsets would glow in strange, deep colours. The midday sun would look pale and white, as if seen through thin milk. After Pinatubo, people in Hawaii watched skies like this on and off for almost two years. After Yellowstone, it would be the sky over every continent. It would be beautiful. It would also be a warning.

Year 1: The Summer That Never Comes 11:56

11:56To understand that warning, we need to go back to 1815. That year, a volcano called Tambora in Indonesia produced the largest eruption in recorded history. Yet it was more than five times smaller than even the smallest of Yellowstone's three great eruptions.

12:16The next year, 1816, became known as the Year Without a Summer. In May, frost killed crops across New York and New England. In June, heavy snow fell on Albany. In July, lakes were still frozen as far south as Pennsylvania. Harvests failed across Europe and the United States, and the price of grain soared.

12:40So how much colder would the world get after Yellowstone? Here, the honest answer is that scientists disagree. Older climate models put the global drop anywhere from 2 to 8 °C. The top of that range would be colder, for a few years, than the depths of the last ice age, which was about 6 °C below today. A 2024 study from NASA came out much gentler: probably no more than 1.5 °C, because it depends on the size of those droplets, which no one has ever measured for a supereruption. But even that gentle number is three times what Pinatubo did.

13:26And the global average hides the worst of it, because the cold would not be shared evenly. In 2021, scientists ran 42 climate simulations of Toba, an Indonesian supervolcano that erupted 74,000 years ago, the largest eruption of the last two million years. Across most of Africa, the cooling almost never passed 4 °C. But in the runs with the most sulfur, North America and Asia were almost certain to cool by more than 4 °C, and parts of Europe and Asia by as much as 10 °C.

14:04Now picture Year 1 in those places. Spring arrives on the calendar, but not in the fields. Frost comes in June, and again in August. The growing season, the short warm window a crop needs to ripen, shrinks or disappears. And in the worst case, it happens to America's corn belt, Europe's wheat fields and Asia's farms in the same year.

14:30This is the moment the eruption finally reaches the whole world. Not with ash. Not with fire. Through food. A volcano in one corner of Wyoming, more than 1,000 km from the nearest ocean, ends up on the dinner tables of billions.

14:51Even in the harshest models, the cold would not last forever. But for a year, or two, or longer, the most important number on the planet would not be the depth of the ash or the size of the crater. It would be how many nights a farm could get through without frost.

Year 10: Recovery & a Very Old Story 15:15

15:15So does the world recover? It does, and sooner than you might expect. The haze cannot stay up forever. Year by year, the droplets sink and fall out of the sky. Pinatubo's haze lasted about three years. Yellowstone's would last longer, and USGS scientists describe the climate effects of a supereruption as lasting years to decades. But slowly, the strange sunsets fade, and summer comes back.

15:44In North America, the ash does not simply disappear. For years, heavy rains wash it off the hills as rivers of mud, called lahars. After Pinatubo, lahars like these did more damage than the eruption itself. And at Yellowstone, where the heart of the park used to be, there is now a new crater, tens of km across, still steaming. Over hundreds of thousands of years, slow lava flows ooze out and fill it, rivers carve new valleys through it, and the forests come back.

16:22And here is what surprises most people. Everything we just imagined has already happened here. Three times. 2.1 million years ago, the first great eruption left a crater more than 100 km across and threw out around 6,000 times as much magma as Mount St. Helens did in 1980. Its ash fell as far away as Missouri. A smaller eruption followed 1.3 million years ago. And 631,000 years ago came the one we copied today. Its ash still lies buried in layers as far away as Iowa, Louisiana and California.

17:03And even that is not the beginning. The heat source under Yellowstone, called a hotspot, has stayed in roughly the same place for 16.5 million years, while North America slid slowly southwest over the top of it. Like a candle held under a moving sheet of paper, it has left a scorched trail: more than 100 old calderas strung out over 800 km across Idaho, all the way back to the border of Nevada and Oregon.

17:37So the landscape that millions of people visit every year, its geysers, its hot springs, its lake, exists because of eruptions like this one. Every visitor watching Old Faithful is standing inside the crater of a supervolcano. What we imagined today is not science fiction. It is the next page of a story this place has been writing for more than 16 million years.

Reality Check: The Giant That Mostly Sleeps 18:05

18:05In reality, Yellowstone is not about to erupt. The USGS puts the yearly chance of another giant eruption at about 1 in 730,000, roughly the same as the chance of an asteroid 1 km wide hitting the Earth. Over an 80-year lifetime, that works out to about 1 chance in 9,000. And it is not overdue. The popular idea of an eruption every 725,000 years or so comes from averaging just two gaps, which the USGS calls meaningless.

18:39The magma underneath is only 5 to 15% molten, more like a warm sponge than a lake of fire, and scientists are not sure there is even enough liquid to feed a giant eruption. If Yellowstone does erupt again, the likeliest kind is a slow lava flow, creeping at most a few hundred feet a day, about a football field's length. A disaster for the park, but not for the planet.

19:11And it is watched very closely. Nine agencies track its earthquakes, its ground, its gases and the temperature of its water, and they publish an update every month. On 1 October 2026, the alert level was the lowest there is: Normal. Green.

19:30So tomorrow, in the middle of that ancient crater, Old Faithful will erupt, as it does every 60 to 110 minutes. The ground will rise and sink a little, as it always has. And a few kilometres down, the heat that built this whole landscape will carry on as it has for millions of years: slowly, quietly, and almost entirely out of sight. We live on a restless planet. Yellowstone is simply the place where we can watch it breathe.

20:06Thanks for watching. If you enjoyed this, subscribe, and tell us in the comments which disaster we should imagine next.