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Mount Okmok

Okmok

Shield volcano · United States · 1073m

The summit of 35-km-wide Okmok has two largely overlapping 10-km-wide calderas. Both calderas formed by voluminous eruptions of tephra and pyroclastic flows during the Holocene, one about 8,250 years ago and the other about 2,400 years ago. Numerous cones and lava domes formed on the caldera floor and flanks. Historical explosive eruptions and lava flows have originated from cones within the caldera.
The summit of 35-km-wide Okmok has two largely overlapping 10-km-wide calderas. Both calderas formed by voluminous eruptions of tephra and pyroclastic flows during the Holocene, one about 8,250 years ago and the other about 2,400 years ago. Numerous cones and lava domes formed on the caldera floor and flanks. Historical explosive eruptions and lava flows have originated from cones within the caldera. · Photo: Photo by John Reeder (Alaska Division of Geological & Geophysical Surveys). · Wikimedia Commons
Type
Shield volcano
Country
United States
Region
North America Volcanic Regions / Aleutian Ridge Volcanic Arc
Elevation
1073m
Coordinates
53.430, -168.130
Last eruption
2008
Tectonic setting
Subduction zone / Intermediate crust (15-25 km)
Landform
Shield
Major rock type
Basalt / Picro-Basalt
Geological summary

The basaltic Okmok shield volcano forms the NE end of Umnak Island in the Aleutian Islands. 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 2,050 years ago when dacitic pyroclastic flows reached the coast. More than 60 tephra layers from Okmok have been found overlying the 12,000-year-old caldera-forming tephra layer. Numerous cones and lava domes are present on the flanks down to the coast, including the SE-flank Mount Tulik, which is almost 200 m higher than the caldera rim. Some of the post-caldera cones show evidence of wave-cut lake terraces; more recent cones were formed after the caldera lake, once 150 m deep, disappeared. Eruptions have been reported since 1805 from cinder cones within the caldera, where there are also hot springs and fumaroles.

From Wikipedia

Mount Okmok is a volcano on eastern Umnak Island, in the central-eastern Aleutian Islands of Alaska. Part of the Aleutian Volcanic Arc, it was formed by the subduction of the oceanic Pacific Plate under the North American Plate. Okmok is a large shield volcano capped by a 10-kilometer (6.2 mi) wide caldera. The caldera contains numerous cinder cones, their lava flows, and a few lakes. Okmok erupts mainly basaltic lava, mostly from the cones within the caldera.

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Eruption history

Summary (VEI over time)
Click a bar to see individual eruptions
6310 BCE~6033 BCE · 1 eruptions · max VEI 4210 BCE~67 · 1 eruptions · max VEI 61731~2008 · 17 eruptions · max VEI 46310 BCE4369 BCE2151 BCE210 BCE1731

Detailed timeline

  1. 2008VEI 4Observed
    2008-07-12 – 2008-08-19
    NE caldera floor
  2. 1997VEI 3Observed
    1997-02-11 – 1997-05-19
    SW part of caldera (Cone A)
  3. 1986VEI 2Observed
    1986-11-18 – 1988-02-26
    SW part of caldera (Cone A)
  4. 1983VEI 2Observed
    1983-07-08 – 1983-07-08
    SW part of caldera (Cone A)
  5. 1981VEI 3Observed
    1981-03-24 – 1981-03-24
    SW part of caldera (Cone A)
  6. 1960VEI 3Observed
    1960-10-15 – 1961-04-15
    SW part of caldera (Cone A)
  7. 1958VEI 3Observed
    1958-08-14 – 1958-08-25
    SW part of caldera (Cone A)
  8. 1945VEI 2Observed
    1945-06-04 – 1945-12
    SW part of caldera (Cone A)
  9. 1943VEI 1Observed
    1943-06 – Ongoing
    SW part of caldera (Cone A)
  10. 1938VEI ?Observed
    1938 – Ongoing
    SW part of caldera (Cone A)
  11. 1936VEI ?Geological estimate
    1936 – Ongoing
    SW part of caldera (Cone A)
  12. 1931VEI 2Observed
    1931-03-21 – 1931-05-13
    Cone A?
  13. 1899VEI 3Observed
    1899 – Ongoing
    SW part of caldera (Cone A)
  14. 1878VEI 2Observed
    1878 – Ongoing
    Cone A?
  15. 1824VEI ?Observed
    1824 – 1830
    Cone B?
  16. 1817VEI 3Observed
    1817-03-01 – 1820
    Cone E? or B?
  17. 1805VEI ?Observed
    1805 – Ongoing
  18. 100 BCE (±50 yrs)VEI 6Geological estimate
    BCE 100 – Ongoing
    Okmok II caldera
  19. 6310 BCE (±500 yrs)VEI 4Geological estimate
    BCE 6310 – Ongoing

External links

⚠ For reference only. Not for emergency response.