On June 6-9th, 1912, the most spectacular Alaskan eruption in recorded history and one of the two largest eruptions in the world in the twentieth century (the other being Mount Pinatubo in 1991) resulted in the formation of a large summit caldera at Katmai volcano. The over 60-hour-long eruption actually took place at a vent about 6 mi (10 km) to the west of Mount Katmai (now marked by Novarupta dome) from which an estimated 30–35 km³ of ash flows and tephra were ejected rather than at Mount Katmai itself. Based on geochemical and structural relationships, it has been suggested that magma drained from beneath Katmai Volcano to Novarupta via the plumbing system beneath Trident Volcano. The withdrawal of magma beneath Katmai resulted in the collapse of the summit area, forming the caldera. Following the subsidence, a small dacitic lava dome known as Horseshoe Island was emplaced on the floor of the caldera; this is the only juvenile material erupted from Katmai caldera during the historical eruption. It was visible at the time of the expedition in 1916, but has since been submerged by the crater lake. Still, the eruption from Katmai had a VEI of 3, and possibly involved phreatic eruptions.[2]

Approximately 12–15 km³ of magma was vented during the 1912 eruption producing about 35 km³ of tephra. An estimated 11–15 km³ of ash flow tuff traveled 12 miles (20 km) northwest covering an area of about 120 km² in what subsequently came to be known as the Valley of Ten Thousand Smokes. The ash flow tuff produced in the 1912 eruption is made up of a silica-rich volcanic rock called rhyolite. In fact, this is the only major Quaternary eruption of rhyolite to have occurred in Alaska. Maximum thickness of the ashflow is estimated to be about 800 feet (240 m). About 20 km³ of airfall tephra was carried east and southeast with a minor lobe to the north covering 77,000 km² with more than 1 in (2.5 cm) of ash. Light ash fall was reported as far away as the Puget Sound region 1,500 mi (2,400 km) away. Extremely fine ash blown into the stratosphere remained in suspension as aerosols for months and caused spectacular red sunsets in many parts of the world.

June 6, 1912 began as a beautiful day with bright blue skies and sunshine. Kodiak residents were aware of the numerous volcanoes on the Alaska Mainland, but Kodiak Island was separated from the mainland by the 40 mile wide Shelikoff Straits. When the eruption came, the sound was heard in Juneau, 750 miles east of Kodiak Island and northeast across the Alaska Range at Fairbanks (500 miles distant) and even Dawson, Yukon Territory (650 miles distant. But unbelievably, it was not heard in Kodiak. A few rumbles, yes, but an explosion - no! The first hint that Kodiak residents got was the appearance of an odd fan-shaped cloud to the Southwest. The cloud climbed higher and higher and lightening storms began to flash within its midst. The black cloud grew closer and it began to get dark. Ash began to fall and the earth rumbled non-stop from earthquakes. 5 inches of ash fell on Kodiak, Alaska that first day.

On the following day, June 7th, the ash fall ceased for several hours and then resumed. By 1:00 in the afternoon it was pitch black and sulphurous fumes saturated the air. Bright lights could only be seen a few feet away. Ash avalanches could be heard crashing down the gullies above the town. A large log building burned to the ground and residents a scarce 200 feet away did not see the fire. The naval station at Woody Island was hit by lightening and burned to the ground. Static in the air made the wireless radios inoperable.

The priest of the Greek Orthodox Church in Kodiak told his church followers that if the church bells began to ring, the people were to go to the dock where the US Revenue Cutter Manning was docked. At 4 am on June 8th the church bells began to ring and the 800 souls descended to the docks. 500 were placed on the Manning and the remaining 300 were put aboard the barge, St. James. At 10 am the ship and barge put to see and docked near Woody Island where an escape to open seas could be made if necessary.

On June 10th the ashfall stopped and the people on the Manning and St. James returned to a much changed home town. The flat areas were coated with 18 inches of ash. Shallow lakes and springs on the island had been filled in and clogged with ash. New wells had to be dug for the livestock that survived. It was many, many years before Mother Nature could heal Alaska's Emerald Island.