According to KVERT a significant eruption at Sheveluch began at 0110 on 11 April, local time. During the most intense phase of activity ash plumes possibly rose as high as 15.8 km (52,000 ft) a.s.l., a significant sulfur dioxide signature was detected in the plume, pyroclastic flows traveled notable distances, and ash-and-lapilli-fall impacted residents. Strong explosions continued during the morning of 12 April. At 0730 on 12 April satellite images showed ash plumes rising to 8 km (26,200 ft) a.s.l., though parts of the large ash plume generated earlier extended 600 km SW and 1,050 km ESE. The explosions weakened by 1710 when ash plumes were only rising to 4.5 km (14,800 ft) a.s.l. and drifting ESE; at 1801 KVERT issued a Volcano Observatory Notice for Aviation (VONA) lowering the Aviation Color Code to Orange. By 2310 ash extended as far as 3,000 km E. KVERT noted that ash deposits in Klyuchi were as deep as 8.5 cm, and ashfall was reported in Kozyrevsk, Maiskoye, Atlasovo, Lazo, and Esso during 10-12 April. According to news sources, the ash-and-gas plumes drifted E toward the Aleutian Islands and reached the Gulf of Alaska by 13 April, causing flight disruptions. More than 100 flights involving Alaska airspace were cancelled due to the plume. Flight cancellations were also reported in NW Canada (British Columbia) during 13-14 April. Alaskan flight schedules were mostly back to normal by 15 April, with only minor delays and far fewer cancellations; a few cancellations continued to be reported in Canada.

On 13 April Kamchatka Volcanological Station (KVS) volcanologists inspected pyroclastic flow deposits that had stopped about 600 m from the Klyuchi-Ust-Kamchatsk federal highway. They walked about 1 km through deep snow (1 m) covered in 6 cm of ash and noted that some parts of the deposits were hot. Steam rose from downed smoldering trees. One picture showed a large block lodged high up in a bare tree. They also noted that the pyroclastic flow deposits were thin with very few large fragments, different from previous flows from Sheveluch. Clearing weather on 15 April revealed that most of the previous lava-dome complex was gone and there was a new crater 1 km in diameter from which voluminous steam-and-gas plumes were rising. Evidence suggested that there had been a directed blast to the SE, and pyroclastic flows traveled more than 20 km.

Geological Summary. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Sources: Kamchatkan Volcanic Eruption Response Team (KVERT), Kamchatka Volcanological Station, Institute of Volcanology and Seismology (IVS) of the Far Eastern Branch of the Russian Academy of Sciences (FEB RAS), Anchorage Daily News, Canadian Broadcasting Corporation (CBC), Cabin Radio