[6] Umnak Island is largely uninhabited, but fishery vessels sail around it all year round[7] and a major North Pacific aviation route passes in the area.

[8] Dutch Harbor on Unalaska Island, the most important production place of seafood in the United States,[9] is 120 kilometers (75 mi) from Okmok.

[28] The summit is cut by a 10 kilometers (6.2 mi) wide caldera,[2] breached to the northeast by Crater Creek through a notch known as "Gates".

[41] As of 2008[update] there are two major lakes, one north of Cone D and a slightly smaller one at the centr of the caldera; both are partially or entirely within craters excavated by the 2008 eruption.

[22] In the central Aleutians these include from west to east Seguam, Amukta, Chagulak, Yunaska, Herbert, Carlisle, Cleveland, Uliaga, Kagamil, Vsevidof, Recheshnoi, Okmok, Bogoslof, Makushin, Mount Gilbert, Westdahl, Fisher, Shishaldin, Isanotski and Roundtop.

[55] Surface deformation has been recorded before and after eruptions, often continuing for years, implying underground magma movements[56] centered at 3–4 kilometers (1.9–2.5 mi) depth.

[77] The Okmok rocks contain only small quantities of phenocrysts, which include clinopyroxene, olivine and plagioclase;[78] rhyolites also contain augite pyroxene, hypersthene and titanomagnetite.

[21] The rocks have a typical composition for volcanic arc magmas,[78] with enrichment of elements presumably derived from subducted sediments[80] and sediment-derived fluids.

[81] The subducting plate releases fluids into the overlying mantle, which ascends under Okmok and melts at temperatures of 1,500–1,600 °C (2,730–2,910 °F) to form basaltic magmas.

[28] Tussock grass and tundra covers the lower parts of the volcano, with numerous flowers budding during late summer.

[6][25] Animals include red foxes, reindeer and numerous birds, and marine life occurs in the seas surrounding Umnak.

[90] Volcanic activity on northeastern Umnak Island commenced about 2.1–1.7 million years ago, eventually giving rise to Okmok volcano.

[45] This eruption was part of a wider surge of volcanic activity recorded in Greenland at the end of the last ice age.

Conceivably, the retreat of the glacial icecaps would have generated stresses on Earth's crust that drove increased volcanic activity.

[112] Between the caldera-forming eruptions, the so-called "Clear Creek Basalt" lava flows, and various scoria deposits were emplaced on the volcano.

[123] A[124] 200–600 °C (392–1,112 °F) hot[125] pyroclastic flow descended the slopes of Okmok, burning the vegetation buried by the preceding fallout,[123] and traversed ridges and topography.

Some flows crossed the 8 kilometers (5.0 mi) wide sea between Umnak and Unalaska to form deposits on the latter,[127] probably on top of pumice rafts.

[133] Humans abandoned a village on Carlisle Island west of Okmok as a consequence of the eruption,[134] allowing seabirds to reoccupy certain areas.

[136] The Okmok II eruption released about 15–16 teragrams of sulfur (but no chlorine or fluorine[137]) into the stratosphere,[138] causing a volcanic winter with 0.7–7.4 °C (1.3–13.3 °F) cooling across the Northern Hemisphere.

[143] In the Mediterranean, computer modeling and historical reports show that the eruption led to cold weather, snowfall, famines and a failure of the floods on the Nile,[139][130] causing an economic and social crisis in Egypt.

[146][147] After the Okmok II eruption, a crater lake filled the caldera[148] within a decade,[149] eventually reaching an elevation of 475 meters (1,558 ft) above sea level.

About 1,400–1,000 years ago, an intense eruption of Cone D[153] produced large waves that overtopped[151] the northeastern caldera margin.

The lava dam failed, causing a 2,000 cubic meters per second (71,000 cu ft/s) flood that destroyed an Aleut village at Cape Tanak.

[170][99] According to the geologist Constantin von Grewingk, the inhabitants had been away fishing while the eruption took place; when they returned, they abandoned the original site of the village in favour of a new one, presumably Nikolski.

The cloud height fluctuated between 2–13 kilometers (1.2–8.1 mi) and originated from multiple vents on the caldera floor before activity became limited to one crater west of Cone D by August.

[184] The Alaska Volcano Observatory (AVO) was alerted by the US Coast Guard after the latter received requests for assistance from a family living on the eastern side of Okmok.

[191] The cloud reached the stratosphere, leading to disturbances in air travel[14] that were however dwarfed by the disruptions caused by the eruption of Kasatochi that same year.

[207] A field of geysers and sinter deposits may be linked to Okmok,[211] and the caldera has been evaluated as a site for the generation of geothermal power.

Pyroclastic flows and surges can scour the island, overtop ridges and topographic obstacles, and advance at speeds reaching 100 meters per second (330 ft/s).

Ash or pyroclastic fall onto ice can produce mudflows and volcanic cones outside of the caldera may be a source of debris flows.