Okmok

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  • Country
  • Subregion Name
  • Primary Volcano Type
  • Last Known Eruption
  • 53.43°N
  • 168.13°W

  • 1073 m
    3519 ft

  • 311290
  • Latitude
  • Longitude

  • Summit
    Elevation

  • Volcano
    Number

18 March-24 March 2009

On 20 March, AVO lowered the Volcano Alert Level for Okmok to Normal and the Aviation Color Code to Green. Seismic activity had been at low to near background levels and satellite views showed no activity during the previous two weeks. The last confirmed ash emission at Okmok occurred on 19 August 2008.

Source: US Geological Survey Alaska Volcano Observatory (AVO)



 Available Weekly Reports


2009: February | March
2008: July | August
2001: May


18 March-24 March 2009

On 20 March, AVO lowered the Volcano Alert Level for Okmok to Normal and the Aviation Color Code to Green. Seismic activity had been at low to near background levels and satellite views showed no activity during the previous two weeks. The last confirmed ash emission at Okmok occurred on 19 August 2008.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


25 February-3 March 2009

On 2 March, AVO raised the Volcano Alert Level for Okmok to Advisory and the Aviation Color Code to Yellow because of increased seismicity. Short bursts of volcanic tremor at an average rate of one per hour had been detected during the previous 24 hours, an increase above the typical background level. The events were the first sign of significant seismic activity at the volcano since the cessation of the last eruption in August 2008.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


27 August-2 September 2008

On 27 August, AVO lowered the Volcano Alert Level for Okmok to Advisory and the Aviation Color Code to Yellow because seismicity had declined steadily during 20-27 August and ash plumes had not been observed since 19 August. On 28 August, a thermal anomaly and a steam plume were detected on satellite imagery. Cloud cover prevented observations on the other days.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


20 August-26 August 2008

AVO reported that seismic activity at Okmok decreased on 19 August to near pre-eruption levels and remained low during 20-24 August. Occasional bursts of volcanic tremor were detected. Although satellite views were hindered due to cloud cover, a possible thermal anomaly in the crater was present on 21, 24, and 25 August. The Volcano Alert Level remained at Watch and the Aviation Color Code at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


13 August-19 August 2008

AVO reported that on 13 August low-level steam-and-ash plumes from Okmok were visible on satellite imagery drifting SE at altitudes of 3-4.6 km (10,000-15,000 ft) a.s.l. During 14-17 August satellite observations were hindered due to cloud cover; seismic levels fluctuated possibly indicating that steam-and-ash emissions continued. During 18-19 August, ash plumes were seen on satellite imagery at altitudes of 3-4.6 km (5,000-15,000 ft) a.s.l. The Volcano Alert Level remained at Watch and the Aviation Color Code at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


6 August-12 August 2008

AVO scientists observed the eruption of Okmok during a visit to Umnak Island on 2 and 3 August. They saw significant ashfall that had accumulated in the caldera and on the upper flanks, lahars and lahar deltas that formed in drainages from the SE to the NE flank, and continuous ash jets being emitted from three or more vents in the vicinity of Cone D in the NE sector of the caldera. Elevated seismicity was detected during 6-8 August, and declined on 9 August. According to observations of satellite imagery, steam plumes possibly containing ash rose to altitudes below 4.6 km (15,000 ft) a.s.l. during 9-10 August. On 11 August, steam plumes rose to altitudes of less than 6.1 km (20,000 ft) a.s.l. and drifted SE. Ash plumes that rose to around a hundred meters above the crater were reported by an observer in Nikolski (80 km SW). These plumes were not detected on satellite imagery due to cloud cover. On 12 August, a pilot reported an ash plume at an altitude of 4.6 km (15,000 ft) a.s.l. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


30 July-5 August 2008

Based on observations of satellite imagery and pilot reports, AVO reported that ash plumes from Okmok rose to altitudes of 4.6-10.7 km (15,000-35,000 ft) a.s.l. and drifted SW, W, N, NNE, and SE. On 30 July, seismicity alternated between continuous and pulsating volcanic tremor. The Volcano Alert Level was lowered to Watch and the Aviation Color Code was lowered to Orange. On 31 July, reports from a fishing boat 11.3 km N indicated no visibility due to ashfall.

Strong volcanic tremor on 2 August prompted AVO to raise the Volcano Alert Level to Warning and the Aviation Color Code to Red. Cloudy conditions prevented satellite observations. Later that day, AVO geologists in the area reported that ash-and-steam plumes rose to minimum altitude of 6.1 km (20,000 ft) a.s.l. The seismicity decreased and the Volcano Alert Level was lowered to Watch and the Aviation Color Code was lowered to Orange. Observers in Ft. Glenn on Umnak Island reported smelling sulfur and seeing a larger ash plume than earlier that day. The plume drifted ESE. On 3 August, helicopter and ground-based observers indicated a lower-altitude ash plume along with a higher steam plume. Satellite imagery revealed that ash plumes at altitudes of 9.1-10.7 km (30,000-35,000 ft) a.s.l. drifted SSW. On 4 August, ashfall reported in Nikolski had accumulated to a depth of 3 mm. During 4-5 August satellite imagery and pilot observations indicated that ash plumes rose to altitudes of 3-7.6 km (10,000-25,000 ft) a.s.l. and drifted SW and W.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


23 July-29 July 2008

AVO reported that on 23 July, seismicity from Okmok changed from episodic volcanic tremor to nearly continuous mid-level volcanic tremor. Although cloud cover obscured views of Okmok, previously emitted ash plumes were observed to the ESE. On 24 July, a thermal anomaly was possibly present on satellite imagery. On 25 July, seismic amplitude increased. Based on pilot reports and observations of satellite imagery, AVO reported that ash plumes rose to altitudes of 10.7-12.2 km (35,000-40,000 ft) a.s.l. The Volcano Alert Level was raised to Warning and the Aviation Color Code was raised to Red.

On 26 July, seismic activity decreased and satellite imagery indicated that ash plumes rose to altitudes of 6.1-6.7 km (20,000-22,000 ft) a.s.l. The Volcano Alert Level was lowered to Watch and the Aviation Color Code was lowered to Orange. Seismicity increased again on 27 July. Satellite imagery possibly indicated another thermal anomaly; a possible plume at an altitude of less than 3 km (10,000 ft) a.s.l. was also noted. On 28 July, seismic tremor decreased. An ash plume at a possible altitude of 8.2 km (27,000 ft) a.s.l. drifted 90 km SE. Seismicity changed from nearly continuous volcanic tremor to episodic. Later that day and on 29 July, ash plumes possibly rose to an altitude of 10.7 km (35,000 ft) a.s.l. and drifted E to SE. The Volcano Alert Level was raised to Warning and the Aviation Color Code was raised to Red.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


16 July-22 July 2008

AVO reported that during 15-16 July seismicity from Okmok changed from nearly continuous to episodic volcanic tremor, and the overall seismic intensity declined. Satellite imagery indicated elevated surface temperatures in the NE sector of the caldera; meteorological clouds obscured views. On 16 July, the Volcano Alert Level was lowered to Watch and the Aviation Color Code was lowered to Orange. On 17 July, a pilot reported that an ash plume rose to altitudes of 4.6-6.1 km (15,000-20,000) a.s.l. and drifted E and NE. On 18 July, AVO indicated that the eruption was episodic, with occasional ash-producing explosions occurring every 15 to 30 minutes. The plumes from these explosions were limited to about 6.1 km (20,000 ft) a.s.l.

On 19 July seismicity increased markedly, interpreted as possibly indicating that ash plumes rose to altitudes of 7.6-9.1 km (25,000-30,000 ft) a.s.l. The Volcano Alert Level was raised to Warning and the Aviation Color Code was raised to Red. The next day, seismicity declined again and the Volcano Alert Level was lowered to Watch and the Aviation Color Code was lowered to Orange. Satellite imagery revealed that an ash plume about 20 km from Okmok drifted SE at an altitude of 3.7 km (12,000 ft) a.s.l. Additional ash plumes observed on satellite imagery and spotted by pilots rose to altitudes of 4.6-6.1 km (15,000-20,000 ft) a.s.l. During 21-22 July, ash plumes rose to altitudes of 6.1-9.1 km (20,000-30,000 ft) a.s.l. and drifted SE.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


9 July-15 July 2008

A strong explosive eruption at Okmok began abruptly at 1143 on 12 July after about an hour of rapidly escalating earthquake activity. Ash and gas from the initial explosions reached at least 15 km (50,000 ft) a.s.l on 12 July and drifted as a large cloud S and E above the North Pacific. Satellite tracking of the ash cloud by traditional techniques has been hampered by the high water content due to interaction of rising magma with very shallow groundwater and surficial water inside the caldera. Heavy ashfall occurred on the eastern portion of Umnak Island; a dusting of ash also occurred for several hours in Unalaska/Dutch Harbor.

As of 15 July, based on AVO analysis of satellite data, ash is continuing to erupt from a composite cinder and spatter cone called Cone D in the eastern portion of the 6-mile wide caldera. Seismicity reached a peak at about 1400 on 12 July and has been gradually declining since. The volcano is currently at aviation color code RED and alert level WARNING. All areas immediately around the volcano are considered hazardous.

News media reported that residents of Umnak Island heard thundering noises the morning of 12 July and quickly realized an eruption had begun. After calling the US Coast Guard for assistance, they began to evacuate to Unalaska using a small helicopter. A fishing boat evacuated the remaining residents after heavy ashfall made further flights impossible.

Sources: US Geological Survey Alaska Volcano Observatory (AVO), Associated Press


16 May-22 May 2001

AVO reported that the earthquake swarm centered near Okmok that was first detected on 11 May greatly diminished by 15 May.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


9 May-15 May 2001

During ~0800 to at least 1700 on 11 May AVO detected a small earthquake swarm that was centered near Okmok. Earthquakes in the swarm had magnitudes of approximately 2-3.6, but their locations could not be pinpointed because Okmok is not monitored by a local seismic network. AVO noted that the earthquakes may have been of volcanic origin, but earthquake swarms with similar sizes and character are not uncommon at Aleutian arc volcanoes and do not necessarily lead to eruptive activity.

Source: US Geological Survey Alaska Volcano Observatory (AVO)


Summary of eruption dates and Volcanic Explosivity Indices (VEI).

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
2008 Jul 12 2008 Aug 19 Confirmed 4 Historical Observations NE caldera floor
1997 Feb 11 1997 May 19 ± 4 days Confirmed 3 Historical Observations SW part of caldera (Cone A)
1986 Nov 18 1988 Feb 26 Confirmed 2 Historical Observations SW part of caldera (Cone A)
1983 Jul 8 1983 Jul 8 Confirmed 2 Historical Observations SW part of caldera (Cone A)
1981 Mar 24 1981 Mar 24 Confirmed 3 Historical Observations SW part of caldera (Cone A)
1960 Oct 15 ± 45 days 1961 Apr 15 ± 45 days Confirmed 3 Historical Observations SW part of caldera (Cone A)
1958 Aug 14 1958 Aug 25 ± 10 days Confirmed 3 Historical Observations SW part of caldera (Cone A)
1945 Jun 4 1945 Dec Confirmed 2 Historical Observations SW part of caldera (Cone A)
1943 Jun Unknown Confirmed 1 Historical Observations SW part of caldera (Cone A)
1938 Unknown Confirmed   Historical Observations SW part of caldera (Cone A)
[ 1936 ] [ Unknown ] Uncertain     SW part of caldera (Cone A)
1931 Mar 21 1931 May 13 (in or after) Confirmed 2 Historical Observations Cone A?
1899 Unknown Confirmed 3 Historical Observations SW part of caldera (Cone A)
1878 Unknown Confirmed 2 Historical Observations Cone A?
1824 1830 Confirmed   Historical Observations Cone B?
1817 Mar 1 1820 Confirmed 3 Historical Observations Cone E? or B?
1805 Unknown Confirmed   Historical Observations
0100 BCE ± 50 years Unknown Confirmed 6 Radiocarbon (uncorrected) Okmok II caldera
6310 BCE ± 500 years Unknown Confirmed 4 Radiocarbon (uncorrected)

The following references are the sources used for data regarding this volcano. References are linked directly to our volcano data file. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title. Additional discussion of data sources can be found under Volcano Data Criteria.

Beget J E, Larson J F, Neal C A, Nye C J, Schaefer J R, 2005. Preliminary volcano-hazard assessment for Okmok volcano, Umnak Island Alaska. Alaska Dept Nat Resour Div Geol Geophys Surv, Rep Invest, 2004-3: 1-32.

Burgisser A, 2005. Physical volcanology of the 2,050 BP caldera-forming eruption of Okmok caldera, Alaska. Bull Volc, 67: 497-525.

Byers F M, 1959. Geology of Umnak and Bogoslof Islands, Aleutian Islands, Alaska. U S Geol Surv Bull, 1028-L: 267-365.

Byers F M, 1961. Petrology of three volcanic suites, Umnak and Bogoslof Islands, Aleutian Island, Alaska. Geol Soc Amer Bull, 72: 93-128.

Coats R R, 1950. Volcanic activity in the Aleutian Arc. U S Geol Surv Bull, 974-B: 35-47.

Henning R A, Rosenthal C H, Olds B, Reading E (eds), 1976. Alaska's volcanoes, northern link in the ring of fire. Alaska Geog, 4: 1-88.

IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior..

Keller F, Meuschke J L, Alldredge L R, 1954. Aeromagnetic surveys in the Aleutian, Marshall, and Bermuda Islands. Eos, Trans Amer Geophys Union, 35: 558-572.

Larsen J F, Neal C, Neal C, Schaefer J, Beget J, Nye C, 2007. Late Pleistocene and Holocene caldera-forming eruptions of Okmok caldera, Aleutian Islands, Alaska. In: Eichelberger J, Gordeev E, Izbekov P, Kasahara M, Lees J (eds), Volcanism and Subduction: the Kamchatka Region, {Amer Geophys Union, Geophys Monogr}, 172: 343-364.

Miller T P, McGimsey R G, Richter D H, Riehle J R, Nye C J, Yount M E, Dumoulin J A, 1998. Catalogue of the historically active volcanoes of Alaska. U S Geol Surv Open-File Rpt, 98-582: 1-104.

Motyka R J, Liss S A, Nye C J, Moorman M A, 1993. Geothermal resources of the Aleutian arc. Alaska Div Geol Geophys Surv, Prof Rpt, no 114, 17 p and 4 map sheets.

Patrick M R, Dehn J, Papp K R, Lu Z, Dean K, Moxey L, Izbekov P, Guritz R, 2003. The 1997 eruption of Okmok volcano, Alaska: a synthesis of remotely sensed imagery. J Volc Geotherm Res, 127: 87-105.

Smith R L, Shaw H R, Luedke R G, Russell S L, 1978. Comprehensive tables giving physical data and thermal energy estimates for young igneous systems of the United States. U S Geol Surv Open-File Rpt, 78-925: 1-25.

Wong L J, Larsen J F, 2010. The Middle Scoria sequence: a Holocene violent strombolian, subplinian and phreatomagmatic eruption of Okmok volcano, Alaska. Bull Volc, 72: 17-31.

The broad, basaltic Okmok shield volcano, which forms the NE end of Umnak Island, has a dramatically different profile than most other Aleutian volcanoes. The summit of the low, 35-km-wide volcano is cut by two overlapping 10-km-wide calderas formed during eruptions about 12,000 and 2050 years ago that produced dacitic pyroclastic flows that reached the coast. More than 60 tephra layers from Okmok have been found overlying the 12,000-year-old caldera-forming tephra layer. Numerous satellitic cones and lava domes dot the flanks of the volcano down to the coast, including 1253-m Mount Tulik on the SE flank, which is almost 200 m higher than the caldera rim. Some of the post-caldera cones show evidence of wave-cut lake terraces; the more recent cones, some of which have been active historically, were formed after the caldera lake, once 150 m deep, disappeared. Hot springs and fumaroles are found within the caldera. Historical eruptions have occurred since 1805 from cinder cones within the caldera.