It’s unclear how much warning organizations like the United States Geological Survey (USGS) would get, but shortly before the eruption happened, the ground around Yellowstone National Park would rise upwards somewhat. Hydrothermal system, including the geysers and geothermal pools, would rapidly heat to temperatures above boiling, and they’d likely become extremely acidic – more so than usual.
A swarm of earthquakes would be detected making their way towards a central point, indicating magma rising rapidly through the crust. Then, the roof rock would fail and the eruption would begin.
A vast column of ash and lava would shoot upward to heights of around 25 kilometers (16 miles). Sustained by both raw explosive energy and the release of heat through cooling lava blebs and bombs, it would sustain itself for days, pumping ash into jet streams that would transport it around the stratosphere.
When the eruptive column or parts of the column fail, enormous pyroclastic flows would blast their way across the park.
These mixtures of ash, lava blebs, and superheated gas exceed temperatures of 1,000°C (1,832°F) and can move at speeds of up to 482 kilometers per hour (about 300 miles per hour). If they hit anyone, they’d die within seconds; those nearby would be burned as the air heats up to around about 300°C (570°F).
Generally speaking, pyroclastic flows travel up to 15 kilometers (9.3 miles) out from their source, but they can theoretically reach up to 100 kilometers (62 miles).
This is basically the length of Yellowstone National Park, so if the vent emerged directly in the center, and the pyroclastic flows were particularly energetic, many in the park would die, either from the pyroclastic flows or the collapsing caldera roof itself.
On average, there’s about 11,000 visitors there at any one time, based on a yearly visitor count of 3.8 million. There are far more visitors in the summer months, so a summer eruption would be far more deadly.
When the pyroclastic flows and ash deposits settle and cool, they may seem harmless, but they’re not. If it rains heavily after the eruption, especially on any slopes, then these could mix with mud and turn into rapidly-moving, cement-like slurries called lahars. If you get stuck in one, there’s a good chance you’ll die.