Mount Westdahl, also known as Westdahl Peak, is a relatively young, glacier-capped volcano with a summit elevation of 5,118 feet (1,560 m) situated at the southwest end of Unimak Island in the eastern Aleutian Islands, about 527 miles (848 km) southwest of Kodiak and 87 miles (140 km) northeast of Dutch Harbor, Alaska. In 1902, O.H. Tittmann named the volcano for Ferdinand Westdahl of the US Coast and Geodetic Survey, who determined its geographic position in 1901. The volcano comprises four summits—Westdahl, Faris Peak, Pogromni, and Pogromni’s Sister. Although Pogromni is the highest at 6,531 feet (1,991 m), the most active summit is Westdahl. Pogromni derives its name from the Russian word for ‘violent outbreak,’ and five unnamed cinder cones lie nearby. Westdahl sits on a gently sloping plateau that may represent the surface of a truncated ancestral cone composed of thick basalt lava flows dating to the Pleistocene or earlier. The northeast slopes are steeper, and erosion from more than 50 streams radiating from the summit has extensively dissected the ancestral cone. With a base spanning 11 miles (18 km), the cone is among the largest volcanoes in the Aleutians; consequently, some volcanologists classify Westdahl as a shield volcano. Stratovolcanos, however, are more common in subduction zones such as the nearby Aleutian Trench, where oceanic crust subducts beneath continental crust. Magma forming stratovolcanoes rises when water trapped in hydrated minerals and porous basalt from the upper oceanic crust is released into the mantle above the sinking slab. Unlike shield volcanoes, stratovolcanoes are marked by steep profiles, summit craters, and periodic explosive eruptions. Lava from these volcanoes typically cools and hardens before spreading far because of its viscosity.

Documented eruptions at or near Westdahl Peak date to 1795, 1796, 1820 and from 1827 to 1830. Additional events may have been missed because of the volcano’s remoteness and persistent cloud cover. Ivan Popov-Veniaminov described a 1795 eruption on Unimak Island’s southwest end—likely at Westdahl. Although four late-18th- and early-19th-century eruptions were once attributed to Pogromni, later studies reassign them to Westdahl. Westdahl was active in the latter half of the 20th century, with eruptions in 1964–65, 1978–79 and 1991–92. Their volcanic explosivity index falls in the moderate to moderate-large range, with 13 to 1,300 million cubic yards (10 to 1,000 million cubic meters) of erupted material. In March–April 1964, a fissure eruption produced lava flows on Westdahl’s east flank. A 1978 vulcanian eruption through glacier ice lasted about six days, forming a crater 0.9 miles (1.5 km) wide and 0.3 miles (0.5 km) deep near the 1964 vent. The eruption column reached 6 miles (10 km) high, with 7‑inch (18‑cm) ash deposits observed 9 miles (15 km) southwest. Ash reportedly fell on a ship 621 miles (1,000 km) southeast, and a lahar swept down the southwest flank to the sea. In 1979, a satellite captured an eruption cloud over 5 miles (8 km) high and 31 miles (50 km) wide, although the nearby US Coast Guard lighthouse at Cape Sarichef—less than 16 miles (25 km) away—reported no activity as it was automated that year. In November 1991, pilots observed a fissure eruption through ice that produced a lava flow about 1.9 miles (3 km) wide, 16 to 33 feet (5 to 10 m) thick and 5 miles (8 km) long along the northeast flank. Lava fountaining and phreatic activity generated ash plumes rising 4 miles (7 km), prompting the Federal Aviation Administration to divert air traffic. Most ash remained low and dissipated over the Bering Sea, though light ashfall reached Unimak Island, including the village of False Pass 55 miles (88 km) northeast of the vent.

Ferdinand N. Westdahl, the volcano’s namesake, spent 52 years with the U.S. Coast and Geodetic Survey on the Pacific Coast and in Alaska, often commanding survey ships. Born in 1843 in Visby—a medieval city in Gotland, Sweden—he immigrated to the United States in 1865 and worked for the Western Union Telegraph, which sought to build the Russian-American Telegraph (also known as the Collins Overland Telegraph) from San Francisco to Moscow. Western Union funded Perry M. Collins to survey and construct the line, while the Smithsonian Institution and the Chicago Academy of Sciences backed Robert Kennicott’s pioneering study of Alaska’s flora and fauna. Kennicott, accompanied by hundreds of men and many naturalists, sailed from San Francisco to Alaska aboard the barkentine Golden Gate on March 21, 1865 under Captain Charles M. Scammon; Westdahl served as the ship’s first officer. In spring 1866, Westdahl’s expertise in astronomy and surveying earned him a post under Captain William Ennis at Unalakleet. His journal—sent to his father in Sweden and serially published in a local Visby newspaper—vividly described the survey’s hardships. Kennicott died in the field in 1866 and was succeeded by William Healey Dall. In 1867, a successful transatlantic cable prompted the abandonment of the trans‑Russian effort, which, despite its economic failure, is now regarded as a valuable exploratory undertaking. Westdahl soon joined the US Coast and Geodetic Survey, eventually becoming assistant superintendent and commanding the steamer McArthur during surveys from the south coast of Unimak Island and the Sanak Islands to Montague Island and Prince William Sound. His accounts appeared in the Survey’s Annual Reports for 1901–03 and were incorporated into its charts. A World War II Liberty ship built by Kaiser Permanente shipyards—and a US Coast and Geodetic Survey ship—were named in his honor. Read more here and here. Explore more of Mount Westdahl and Unimak Island here: