Okmok CFE II
Start: 2050 yBP ± 50 Years [1]
Event Type: Explosive
Max VEI: 6 [2]
- Geomorphologic change [7]
- Tsunami [4]
- Phreato-magmatic [8]
Description: From Miller and Smith (1987): "The Okmok volcanic center, Umnak Island (Fig. 1 [in original text)], is a large basaltic composite volcano with a caldera system marking the summit area. This system consists of two large partially overlapping calderas (Byers, 1959), each with an estimated diameter of about 10 km. Postglacial ash-flow tuffs exposed in sea-cliff exposures on the northwest side of the volcano are locally separated by a lava flow 6-9 m thick (Miller and Smith, 1975). A hiatus between extrusion of the lava and deposition of the upper ash-flow sheet is indicated by a well-developed erosional surface on the lava flow; in places, stream channels were cut through the lava flow and later filled by the younger ash flow. The occurrence of two major ash-flow units strongly supports the probability of two major caldera-forming eruptions in Holocene time."
"Charcoal directly beneath the upper ash-flow tuff unit on the north side of the island yielded a 14C age of 2400±+/-200 yr (sample 1, Table 1 [in original text]). This is a maximum age for the second ash-flow tuff eruption, presumably the second caldera forming eruption (Table 2 [in original text]). A minimum of about 5500 yr separates the two caldera-forming eruptions, assuming the correlations are correct."
From Beget and others (2005): "The second large caldera-forming eruption produced a pyroclastic flow deposit that buried the older pyroclastic flow and almost all other older volcanic and non-volcanic sediments on all flanks of Okmok Volcano. The deposits of this caldera-forming eruption are as much as 80 meters thick near the caldera rim, and 30 to 40 meters thick at some coastal exposures (fig. 7 [in original text]). The most distal deposits preserved on land are found 25 kilometers to the southeast across Umnak Pass on southern Unalaska Island. Sea cliff exposures on the north, east, and south flanks of the volcano show where the pyroclastic flow advanced into the sea in these directions. New radiocarbon dating on plants found buried and incinerated at the base of the pyroclastic flow indicates the second caldera-forming eruption occurred about 2,050+/-50 14C years ago (Begét and Larsen, 2001). This eruption generated a tsunami which affected the westernmost part of Unalaska Island."
From Larsen and others (2007): "...the Okmok II deposits are usually significantly thinner than Okmok I, which leads to significantly lower eruptive volume estimates. Burgisser (2005) notes the total volume of the initial rhyodacite and andesite Plinian fall deposits are ~0.5 km3, whereas the main pyroclastic flow deposits are estimated to be ~15 km 3 on-island."
The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 6.7, bulk eruptive volume of 50 cubic km and a dense rock equivalent eruptive volume of 29 cubic km for the eruption.
"Charcoal directly beneath the upper ash-flow tuff unit on the north side of the island yielded a 14C age of 2400±+/-200 yr (sample 1, Table 1 [in original text]). This is a maximum age for the second ash-flow tuff eruption, presumably the second caldera forming eruption (Table 2 [in original text]). A minimum of about 5500 yr separates the two caldera-forming eruptions, assuming the correlations are correct."
From Beget and others (2005): "The second large caldera-forming eruption produced a pyroclastic flow deposit that buried the older pyroclastic flow and almost all other older volcanic and non-volcanic sediments on all flanks of Okmok Volcano. The deposits of this caldera-forming eruption are as much as 80 meters thick near the caldera rim, and 30 to 40 meters thick at some coastal exposures (fig. 7 [in original text]). The most distal deposits preserved on land are found 25 kilometers to the southeast across Umnak Pass on southern Unalaska Island. Sea cliff exposures on the north, east, and south flanks of the volcano show where the pyroclastic flow advanced into the sea in these directions. New radiocarbon dating on plants found buried and incinerated at the base of the pyroclastic flow indicates the second caldera-forming eruption occurred about 2,050+/-50 14C years ago (Begét and Larsen, 2001). This eruption generated a tsunami which affected the westernmost part of Unalaska Island."
From Larsen and others (2007): "...the Okmok II deposits are usually significantly thinner than Okmok I, which leads to significantly lower eruptive volume estimates. Burgisser (2005) notes the total volume of the initial rhyodacite and andesite Plinian fall deposits are ~0.5 km3, whereas the main pyroclastic flow deposits are estimated to be ~15 km 3 on-island."
The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 6.7, bulk eruptive volume of 50 cubic km and a dense rock equivalent eruptive volume of 29 cubic km for the eruption.
Impact: McConnell and others (2020) argue that the Okmok II caldera-forming eruption caused widespread climate disturbance around the globe. They especially focus on the effects in Ancient Rome, where political instability surrounding Caesar’s murder the year prior was already a destabilizing force. Famines in Italy, Greece, and Egypt may well have been caused by the eruption. [5]
References Cited
[1] Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska, 2005
Burgisser, Alain, 2005, Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska: Bulletin of Volcanology, v. 67, n. 6, p. 497-525.[2] Volcanoes of the World, 2013
Global Volcanism Program, 2013, Volcanoes of the World, v. 4.5.3. Venzke, E (ed.): Smithsonian Institution. Downloaded 2017. http://dx.doi.org/10.5479/si.GVP.VOTW4-2013[3] Magmatic differentiation at an island-arc caldera: a stratigraphically constrained multi-isotope study of Okmok Volcano, Aleutian Islands, Alaska, 2004
Finney, B.M., 2004, Magmatic differentiation at an island-arc caldera: a stratigraphically constrained multi-isotope study of Okmok Volcano, Aleutian Islands, Alaska: University of Bristol Ph.D. dissertation, 296 p.[4] Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska, 2005
Beget, J.E., Larsen, J.F., Neal, C.A., Nye, C.J., and Schaefer, J.R., 2005, Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2004-3, 32 p., 1 sheet, scale 1:150,000.
full text of report 3.4 MB
map sheet 53.2 MB [5] Extreme climate after massive eruption of Alaska's Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom, 2020
McConnell, J.R., Sigl, Michael, Plunkett, Gil, Burke, Andrea, Kim, W.M., Raible, C.C., Wilson, A.I., Manning, J.G., Ludlow, Francis, Chellman, N.J., Innes, H.M., Yang, Zhen, Larsen, J.F., Schaefer, J.R., Kipfstuhl, Sepp, Mojtabavi, Seyedhamidreza, Wilhelms, Frank, Opel, Thomas, Meyer, Hanno, and Steffensen, J.P., 2020, Extreme climate after massive eruption of Alaska's Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom: Proceedings of the National Academy of Science of the United States of America, 7 p., www.pnas.org/cgi/doi/10.1073/pnas.2002722117.
[6] Geomorphic consequences of volcanic eruptions in Alaska: A review, 2015
Waythomas, C.F., 2015, Geomorphic consequences of volcanic eruptions in Alaska: A review: Geomorphology, v. 246, p. 123-145, doi: 10.1016/j.geomorph.2015.06.004[7] An overview of break-out floods from intracaldera lakes, 2010
Manville, V., 2010, An overview of break-out floods from intracaldera lakes: Global and Planetary Change, v. 70, p. 14-23, doi:10.1016/j.gloplacha.2009.11.004.[8] Late Pleistocene and Holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska, 2007
Larsen, J. F., Neal, Christina, Schaefer, Janet, Beget, Jim, and Nye, Chris, 2007, Late Pleistocene and Holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska, in Eichelberger, John, Gordeev, Evgenii, Izbekov, Pavel, Kasahara, Minoru, and Lees, Jonathan, eds., Volcanism and Subduction: The Kamchatka Region: Geophysical Monograph 172, American Geophysical Union, p. 343-364.Complete Eruption References
Magmatic differentiation at an island-arc caldera: a stratigraphically constrained multi-isotope study of Okmok Volcano, Aleutian Islands, Alaska, 2004
Finney, B.M., 2004, Magmatic differentiation at an island-arc caldera: a stratigraphically constrained multi-isotope study of Okmok Volcano, Aleutian Islands, Alaska: University of Bristol Ph.D. dissertation, 296 p.
Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska, 2005
Burgisser, Alain, 2005, Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska: Bulletin of Volcanology, v. 67, n. 6, p. 497-525.
Volcanoes of the World, 2013
Global Volcanism Program, 2013, Volcanoes of the World, v. 4.5.3. Venzke, E (ed.): Smithsonian Institution. Downloaded 2017. http://dx.doi.org/10.5479/si.GVP.VOTW4-2013
Global database on large magnitude explosive volcanic eruptions (LaMEVE), 2012
Crosweller H.S., Arora, B., Brown, S.K., Cottrell, E., Deligne, N.I., Guerrero, N.O., Hobbs, L., Kiyosugi, K., Loughlin, S.C., Lowndes, J., Nayembil, M., 2012, Global database on large magnitude explosive volcanic eruptions (LaMEVE): Journal of Applied Volcanology, v. 1, n. 4, unpaged.
Catalog of the historically active volcanoes of Alaska, 1998
Miller, T. P., McGimsey, R. G., Richter, D. H., Riehle, J. R., Nye, C. J., Yount, M. E., and Dumoulin, J. A., 1998, Catalog of the historically active volcanoes of Alaska: U.S. Geological Survey Open-File Report 98-0582, 104 p.
intro and TOC PDF 268 KB
references PDF 43 KB 
Volcanoes of North America: United States and Canada, 1990
Wood, C. A., and Kienle, Juergen, (eds.), 1990, Volcanoes of North America: United States and Canada: New York, Cambridge University Press, 354 p.
Hard Copy held by AVO at FBKS - CEC shelf
Geomorphic consequences of volcanic eruptions in Alaska: A review, 2015
Waythomas, C.F., 2015, Geomorphic consequences of volcanic eruptions in Alaska: A review: Geomorphology, v. 246, p. 123-145, doi: 10.1016/j.geomorph.2015.06.004
An overview of break-out floods from intracaldera lakes, 2010
Manville, V., 2010, An overview of break-out floods from intracaldera lakes: Global and Planetary Change, v. 70, p. 14-23, doi:10.1016/j.gloplacha.2009.11.004.
Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska, 2005
Beget, J.E., Larsen, J.F., Neal, C.A., Nye, C.J., and Schaefer, J.R., 2005, Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2004-3, 32 p., 1 sheet, scale 1:150,000.
full text of report 3.4 MB
map sheet 53.2 MB 
Late Pleistocene and Holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska, 2007
Larsen, J. F., Neal, Christina, Schaefer, Janet, Beget, Jim, and Nye, Chris, 2007, Late Pleistocene and Holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska, in Eichelberger, John, Gordeev, Evgenii, Izbekov, Pavel, Kasahara, Minoru, and Lees, Jonathan, eds., Volcanism and Subduction: The Kamchatka Region: Geophysical Monograph 172, American Geophysical Union, p. 343-364.
The significance of volcanic ash in Greenland ice cores during the Common Era, 2023
Plunkett, G., Sigl, M., McConnell, J.R., Pilcher, J.R., and Chellman, N.J., 2023, The significance of volcanic ash in Greenland ice cores during the Common Era: Quaternary Science Reviews v. 301, 107936. https://doi.org/10.1016/j.quascirev.2022.107936
Full-text PDF 4.1 MB The sun of Rome is set! Volcanic dust veils and their political fallout, 2020
Oppenheimer, C., 2020, The sun of Rome is set! Volcanic dust veils and their political fallout: PNAS v. 117, no. 30, p. 17470-17472. https://doi.org/10.1073/pnas.2011054117
Extreme climate after massive eruption of Alaska's Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom, 2020
McConnell, J.R., Sigl, Michael, Plunkett, Gil, Burke, Andrea, Kim, W.M., Raible, C.C., Wilson, A.I., Manning, J.G., Ludlow, Francis, Chellman, N.J., Innes, H.M., Yang, Zhen, Larsen, J.F., Schaefer, J.R., Kipfstuhl, Sepp, Mojtabavi, Seyedhamidreza, Wilhelms, Frank, Opel, Thomas, Meyer, Hanno, and Steffensen, J.P., 2020, Extreme climate after massive eruption of Alaska's Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom: Proceedings of the National Academy of Science of the United States of America, 7 p., www.pnas.org/cgi/doi/10.1073/pnas.2002722117.