The Yellowstone volcanic system is one of the largest and most closely monitored volcanic regions on Earth.

Beneath the national park lies a vast reservoir of partially molten rock, while thousands of earthquakes, changes in ground level and fluctuations in hydrothermal activity are recorded across the region each year.

Most of this activity is normal and does not indicate that an eruption is approaching.

But experts have now revealed a blow-by-blow account of what would happen if a super-eruption did take place.

In a piece written for The Conversation Ilan Kelman, professor in disasters and health at University College London and Matthew Blackett, reader in physical geography and natural hazards at Coventry, have detailed the warning signs, the eruption itself and the chaos that would ensue after it.

‘Around 631,000 years ago, catastrophe unfolded in what is now north-western Wyoming,’ they said.

‘During what is thought to have been a series of immense volcanic eruptions, life within hundreds of kilometres of the blasts was shattered.

‘Despite the scale, no known human witnessed the cataclysm. Today, humanity would certainly notice. So what exactly would we experience if Yellowstone erupted? We are experts in disasters and volcanoes and what follows is a thought experiment into what might happen in the worst case scenario.’

‘Shortly after 6am, an analyst at the University of Utah Seismograph Stations sees a series of sharp spikes interrupting the usually quiet wavy lines recorded by Yellowstone’s earthquake monitors,’ the experts said.

‘The measurements represent earthquakes detected beneath Yellowstone overnight.

‘The scientist raises an eyebrow and takes a deep breath – these observations don’t fit with Yellowstone’s usual behaviour.’

The earthquake swarm would continue to migrate upwards from deep underground, the experts explained.

This would alarm scientists, who would raise the volcano alert from ‘normal’ to ‘advisory’.

‘As the volcano’s activity continues to ramp up, high-level decision-makers are alerted that this situation could become a major catastrophe,’ they wrote. ‘The probability of it happening within three months increases from 6-9 per cent to 21-27 per cent.’

At this point volcano unrest would escalate dramatically, with geyser activity becoming increasingly erratic.

‘Gas measurements and changes in spring water chemistry indicate an increased input of carbon dioxide and sulphurous gases into Yellowstone’s hydrothermal system,’ the experts said.

‘Taken together with the shallow earthquakes and rapid uplift, the concern is that magma may be moving upwards.’

New observations trigger scientists to announce an 85-92 per cent probability of a cataclysmic eruption within three weeks, with the volcano alert raised to ‘watch’.

‘An evacuation zone extending 100 kilometres beyond Yellowstone National Park is announced, affecting roughly 200,000 residents, as well as thousands of visitors in the region,’ the experts explained.

‘Charities step in to help some people leave on chartered planes, as well as via long, excruciating journeys on overcrowded trains and buses. The most marginalised and poorest may have no option but to stay behind. Some might join the minorities who do not believe the eruption will happen, who think their well-stocked underground bunkers will protect them.’

As the volcano begins to erupt, expanding clouds of steam rise from newly opened cracks. Rising magma sends superheated steam, mud and shattered blocks of rock miles into the air.

A few hours later, gas-rich magma, 650-800°C in temperature, breaks through the fractured surface. A vast eruption column would rise up to 31 miles (50km) into the atmosphere.

‘The immense umbrella cloud that forms does not behave like a normal ash plume that simply drifts with the prevailing wind,’ the researchers explained.

‘Its momentum initially drives ash outwards in every direction – spreading hundreds of kilometres before high-altitude winds take over and carry fine ash thousands of kilometres downwind within a day.’

Extremely hot, fast-moving mixtures of gas, ash and rock fragments surge tens of miles, splintering trees, igniting forests and burying the remnants beneath hot ash and pumice.

‘Anyone within this zone is pulverised by the blasts, crisped by the heat, or asphyxiated by the ash,’ the team wrote. ‘There may be up to 1,000 immediate human deaths, including people who refused to move.’

By this point, much of North America would be hidden in shadow. Within Wyoming and neighbouring states, conditions are catastrophic.

Billings, Montana, the nearest large city, is disappearing beneath what will eventually exceed a metre of ash.

‘For southern Canada, much of the US and northern Mexico, day becomes twilight and breathing outdoors without masks becomes increasingly difficult,’ the scientists said.

‘People on other continents watch in trepidation under vivid, red-orange sunrises and sunsets.’

Electricity networks begin failing. So do water pumps, sewage treatment, heating systems, fuel stations and mobile phone and internet networks.

People are scared and supermarket shelves empty nationwide as regional and national supply chains begin to break down.

‘Over the following weeks, the crisis becomes one of survival,’ the experts explained. ‘Repeated ashfall blocks roads, overwhelms drainage systems and damages roofs. During rain, dry ash turns into a dense slurry that makes clearance work harder.

‘Airports remain closed or severely disrupted across much of North America, while railways, freight depots and farms struggle to operate. Livestock and crops die where pasture and water supplies are buried or contaminated by ash.’

Eye and throat irritation are widespread. People with asthma and other respiratory conditions face serious health risks under a reduced and overworked health service.

The unreliable airspace and transport networks – coupled with agricultural losses, business disruption and reconstruction costs – trigger banking and insurance crises, producing a global recession.

‘Meanwhile, the atmospheric effects become global,’ the scientists said. ‘Most volcanic ash has fallen out, but sulphur dioxide injected into the stratosphere produces sulphate aerosols which reflect a significant proportion of the sun’s energy.

‘Modelling suggests it’s unlikely to cool the global average by more than about 1.5°C, although particular regions and seasons could experience much larger changes. Notably, central North America remains far cooler than before the eruption.’

A decade after the blast, people across the world have adapted to a more agrarian lifestyle, learning to till the land with root vegetables, grains and legumes.

People still eating meat typically either hunt big game or trap smaller birds and mammals.

‘For the eruption’s survivors, disease and death are still rising,’ the team wrote.

‘Excess mortality analyses could suggest 0.7-1.3 billion premature deaths due to the eruption and its impacts.

‘The good news, at least for our species, is that this unstoppable power of volcanism changed the planet, but did not end humanity – or come close to doing so.’

The scientists suggest that a futuristic species could visit Earth, which by this point has been abandoned by humans for 150,000 years.

It notices a large roughly ellipse-shaped ridge made of rock appears prominently underneath the top layers around Yellowstone, indicating an ancient volcanic eruption.