Novarupta: a deep dive into the largest eruption of the 20th century
Novarupta is the most consequential volcano almost no one has heard of. In June 1912, in a remote corner of the Alaska Peninsula, it produced the single largest eruption of the 20th century — bigger than Pinatubo, bigger than Mount St. Helens, bigger than anything since Tambora in 1815. It carved a new valley, drained a neighbour's magma chamber and changed the chemistry of the atmosphere for years.
The largest eruption of the 20th century
The 1912 Novarupta eruption released around 13 to 15 cubic kilometres of dense rock equivalent of magma over about 60 hours. By volume it is roughly thirty times larger than the 1980 Mount St. Helens eruption and three times larger than the 1991 Pinatubo eruption. Only a handful of eruptions in the last 1,000 years match it.
A volcano people did not expect
The eruption was not where geologists assumed. Mount Katmai, 10 kilometres away, lost its summit in 1912 — but Katmai had not actually erupted. Magma had drained from Katmai's chamber and travelled laterally through the crust to erupt at a new vent now called Novarupta. The collapse at Katmai was a secondary effect.
The Valley of Ten Thousand Smokes
The eruption deposited a vast ignimbrite — a thick blanket of hot, gas-rich ash that filled a 60-square-kilometre valley to depths exceeding 200 metres. For years afterwards the valley was riddled with steam vents and is still called the Valley of Ten Thousand Smokes. The few that remain mark the gradual cooling of the deep deposit.
The Griggs expeditions
In 1916, a National Geographic expedition led by Robert Griggs became the first scientific party to reach the eruption site. His reports — and the dramatic photographs of an entire valley steaming — captured the public imagination and led directly to the creation of Katmai National Monument in 1918, later expanded into Katmai National Park.
A lava dome at the source
At the new vent itself, the eruption ended with the extrusion of a small but visually distinctive rhyolite dome. The dome is only about 90 metres high and 400 metres across, oddly small compared to the cataclysm that preceded it — a reminder that the visible result of an eruption does not always match its scale.
Atmospheric and climatic effects
The sulphur injected into the stratosphere produced a noticeable cooling effect in the Northern Hemisphere for several years. Coloured sunsets and pale solar coronae were reported across Europe and North America. It is one of the best-documented examples of how a single eruption can briefly cool the planet.
Why almost no one knows about it
Novarupta erupted in a remote, unpopulated corner of Alaska in the middle of summer. There were no Twitter feeds, no real-time satellite images, and the world's attention in 1912 was on the sinking of the Titanic and the lead-up to the First World War. The eruption's true scale was only understood after the Griggs expeditions made the geology legible.
Why Novarupta matters
Novarupta is the modern world's most underrated volcanic catastrophe and a textbook case for how magma can move through the crust without erupting where it formed. The site is now part of one of the most beautiful and least-visited national parks in the United States.
On the map
Open the map and find Novarupta on the upper Alaska Peninsula, west of Kodiak Island, inside Katmai National Park. The Valley of Ten Thousand Smokes stretches northwest from the vent.