Alternative History
1936 Yellowstone Eruption
Yellowstone Ash Deposits (Yell.1936).png
The volcanic ashfall region in the United States where ash reached a thickness of 1 cm
Volcano Yellowstone Caldera
Date July 18, 1936
Type Ultra Plinian
Location Yellowstone National Park, Wyoming
VEI 8.5
Impact Plunged Earth into a twelve year volcanic winter, led to failed harvests and famine worldwide

The 1936 Yellowstone Eruption, also known as the Yellowstone Event or Eruption Day, was a super-volcanic eruption that occurred on the morning of July 18, 1936 at the site of the Yellowstone National Park. It has been considered the largest eruption in recorded history, and perhaps the largest volcanic eruption in history. Tens of thousands died over the first number of days following the eruption, and the ash deposited in the atmosphere led to a worldwide volcanic winter lasting for twelve years (with cooling effects lasting well beyond then), as well as mass crop failure and famine, which furthered the death toll into the hundreds of millions; the most deadly natural disaster in human history.

Since its eruption 77 years ago, it has been a major watershed moment for the human species, with most governments worldwide still suffering the effects of the volcano (such as the failure of crop growth worldwide due to the cooling of Earth's temperature), and economic, technological and social growth being retarded due to the collapse of many governments in the subsequent years/decades following the eruption.

Build-up to the eruption

Prior to the 1936 eruption, the Yellowstone caldera had remained dormant for almost 640,000 years, when the Lava Creek eruption took place, whilst other eruptions are believed to have occurred 2.1 million and 1.3 million years ago. The minor tremors began in early June, leading some scientists working at the national park to believe that extensive magma activity was occurring beneath ground level. Over the next month, especially in the days leading up to the eruption, the ground in the park appeared to be moving up-and-down at a pace faster than it has normally done in the past. Shocks of magnitude of 2.5 were recorded in the lead up to July, and further minor tremors on a magnitude of 3.0 and above were recorded in the final days before the eruption. 

Diagram of the Yellowstone caldera prior to the eruption

Between the 10 July and the 13 July, over 500 minor shocks were recorded in the Yellowstone area, the magnitude of the strongest being 5.1. After July 6, the park was closed down after a major tremor struck the north of the park, with a foul smelling sulfur scent being detected after that date. Scientists began to clamber for those in the immediate area to leave until the tremors ended, some believing a eruption was due in two to three months time. During the last week prior to the eruption, thousands of tremors were detected beneath the YellowstonePark area, with a rise in geyser activity due to the underground magma heating the groundwater systems, and dozens of phreatic eruptions (steam driven eruptions) being noted prior to 18 July.

By this time, many people living close to the national park were beginning to flee or had already fled, with many professionals declaring the area within 50 km (31 miles) out of bounds. On the night of the 17 July, many scientists and officials still working in the area noted that small granitic magma flows were spotted in the extreme north regions of the Yellowstone park, and most were fearing that an eruption would come within the fortnight to at least three months (allowing most civilians to leave the area), but the final overnight tremors and the extensive land movement would prove them wrong the following morning.


At 5:03 am, on the morning of 18 July, another minor tremor woke those who remained in the Yellowstone, and around thirty seconds later, another, the most major earthquake hit the national park. Registering as a 9.1 on the Richter Scale (one of the largest in US history), it caused the ground of Yellowstone to fall into itself, releasing toxic gases, and the first minor pyroclastic flows. After almost a minute, the earthquake subsided, however, ash and gas continued to rise from the fissures that had formed. By now, all those who remained during the first month after the first tremors were beginning to pack their belongings and flee, however, it would only be twenty minutes until the first, most deadly eruption would occur.

Around 5:30 am, the first of the three explosions (as well as the most climatic phase of the eruption) began; with the ground of the park quickly and completely falling in on itself. Suddenly, a extreme loud and violent explosion rocketed dust, ash and debris into the atmosphere as Yellowstone finally erupted. Extensive pyroclastic flows flooded the immediate area surrounding the park as lava began to pour out of the large fissures. As the largest of the explosions, the sound it created could be heard as far away as places such as India, Australia and even North Africa, with reports saying the explosion could be heard from eastern Europe. As hundreds of cubic meters of fallout (hot ash and dust) fell during the first minutes after the event, town after town quickly became engulfed in asphyxiating material belched out by Yellowstone. Hot pumice (pieces of volcanic glass and dust) fell upon towns and cities, the largest of such stones having a reported size of 80 cm.