Mount Katmai has long been recognized for its caldera collapse. Mount Katmai rises at the head of the Valley of Ten Thousand Smokes on the upper Alaska Peninsula, along the volcanic front of the Aleutian arc. It’s a large compound stratovolcano that consisted of two contiguous cones, both beheaded by the caldera collapse of 1912.
Still many people don’t even hear its name. Mount Katmai is 6.3 miles in diameter with a central lake-filled caldera about 3 by 2 mi in area. Mount Katmai was formed during the massive Novarupta eruption in 1912. The sound of the explosion would be heard in Atlanta and St. Louis, and the fumes observed as far away as Denver, San Antonio, and Jamaica." (Robert F. Griggs, National Geographic Magazine, 1917, v. 81 no. 1, p. 50)
In June 1912, the most spectacular Alaskan eruption in recorded history and the 20th century's largest measured volcanic eruption formed a large summit caldera at Katmai volcano. The volcano has caused ten known fatalities due to gas exposure. The caldera rim reaches a maximum elevation of 6,716 feet. In 1975 the surface of the Crater Lake was at an elevation of about 4,220 feet, and the estimated elevation of the caldera floor is about 3,400 ft. The flows were still hot enough to release gases and boil water and would be until the 1980's
Mount Katmai is centered on the regional drainage divide, the edifice is amazingly asymmetrical. It was one of largest eruption in the 20th century, ejecting almost 30 cubic kilometers of ash and debris into the atmosphere. It is well beating 1991 Pinatubo by a factor of 3 in volume and, for comparison, 1980 Mt. St. Helens by a factor of 30.
The magnitude of the eruption can perhaps be best realized if one could imagine a similar outburst centered in New York City. All of Greater New York would be buried under from ten to fifteen feet of ash. Philadelphia would be covered by a foot of gray ash and would be in total darkness for sixty hours. And Washington and Buffalo would receive a quarter of an inch of ash, with a shorter period of darkness.
The mountain is located in Kodiak Island Borough, adjacent to its border with Lake and Peninsula Borough. The massive eruption happened at a vent about 10 km to the west of Mount Katmai. More than 60 hours, the volcano eruption took place 28 km³ of ash flows and tephra representing 13 km³ of magma volume. Mount Katmai consists mainly of lava flows, pyroclastic rocks, and non-welded to agglutinated air fall.
However, very little is known about the historical activity of Katmai volcano before 1912 eruption. The Quaternary volcanic rocks at Mount Katmai and nearby cones are less than 5,000 feet thick. Much of the volcano is mantled by snow and ice and some valley glaciers radiate out from the flanks. The gigantic eruption produced a cloud of suffocating gas and ash that blackened the sky for the inhabitants of the town of Kodiak. The ash falling on them were quickly led to the harbor and evacuated by boat.
In 1919, geologists noted a lake covering a large part of the caldera floor. However at the end of 1923 the lake was gone and many fumaroles, mud pots, and a large mud geyser had replaced it. Please keep in mind that Mount Katmai should not be climbed without mountaineering skills and equipment. Because high winds, regular rain and drizzle, brown bears, icy stream crossings, crevassed glaciers, and particularly its remoteness make the area a true wilderness. But it’s exhilarating and risky, occasionally wonderful, rarely comfortable, and never to be trifled with.
The withdrawal of magma beneath Katmai resulted in the collapse of the summit area, forming the caldera. After the subsidence, a small dacitic lava dome recognized as “Horseshoe Island” was emplaced on the floor of the caldera. That is the only juvenile material erupted from Katmai caldera during the historical eruption. It was visible at the time of the expedition in 1916.
After that it has since been submerged by the Crater Lake. Still, the eruption from Katmai had a VEI of 3, and possibly involved phreatic eruptions. The nearby Trident, Griggs, and Snowy Mountain volcanoes became active long before Mount Katmai, and activity at Mageik volcano began about the same time as at Katmai.
The massive eruption had a decadal impact of wildlife, affecting the spawning of salmon in the rivers near Novarupta for years. The Mount Katmai cluster was built on a set of rugged glaciated ridges, marine siltstone and sandstone of the Jurassic Naknek Formation. The lake has since refilled to a depth of over 800 feet. Moreover, small glaciers have also formed on a bench within the caldera beside the lake. Pumice still floats on Naknek Lake nearby.
Access is difficult because the Katmai cluster lies completely within the Roadless wilderness of Katmai National Park. The only convenient access is by boat or amphibious aircraft from King Salmon to Brooks Camp on Naknek Lake. The fascination with the eruption goes beyond the size of the event.
Modern geology had never seen the deposition of such a large ash flow sheet. So it was the first time geologists could examine first-hand how the sheet cools and welds after an eruption. Overall, the 1912 eruption shows how interconnected magmatic systems in a cluster of volcanoes might be. The area of the Alaska Peninsula is still sparsely populated, but important things occurred nearby actually, directly above: air traffic.
Not only would the trans-Pacific lines be affected by ash of a Novarupta-scale event, but the ash fallout distribution shows that the potential exists for air travel across much of Canada and the northern United States (especially Seattle, Portland, Vancouver) to be halted. Thankfully, the eruption only lasted 60 hours, but it took at least a week for the ash to settle out of the atmosphere.
Today, only infrequent earthquake swarms occur in the Katmai Cluster and many of the hydrothermal features created during the 1912 eruption are dead. However, the eruption at Novarupta does show the potential for a new colossal eruption to occur where none has happened in the geologically-recent past. Hence a reminder of how vital volcano monitoring and research can be in helping notice the signs of such an event well in advance.