Mount Okmok is the highest point on the rim of Okmok Caldera (Unmagim Anatuu in

Aleut The Aleuts ( ; russian: Алеуты, Aleuty) are the indigenous people of the Aleutian Islands, which are located between the North Pacific Ocean and the Bering Sea. Both the Aleut people and the islands are politically divided between the U ...

) on the northeastern part of Umnak Island in the eastern

Aleutian Islands The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi language, Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a cha ...

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...

. This wide circular

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

truncates the top of a large

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more v ...

. The volcano is currently rated by the Alaska Volcano Observatory as Aviation Alert Level Green and Volcanic-alert Level Normal.

Crater lakes

A crater lake once filled much of the caldera, but the lake ultimately drained through a notch eroded in the northeast rim. The prehistoric lake attained a maximum depth of about and the upper surface reached an elevation of about , at which point it overtopped the low point of the caldera rim. Small, shallow remnants of the lake remained north of Cone D at an altitude of about : a small shallow lake located between the caldera rim and Cone D; a smaller lake (named Cone B Lake) farther north near the caldera's gate. After the 2008 eruption, the

hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquif ...

of the caldera was greatly changed with five separate sizable lakes now emplaced. In addition to the caldera lakes, Cone A, Cone E, Cone G and the new 2008 vent on Cone D contain small crater lakes.

Eruptive history

Major eruptions of Okmok—with a Volcanic Explosivity Index (VEI) strength of 6—occurred approximately 8,300 and 2,050 years ago (i.e., BP). In 2020,

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...

deposits found in ice cores from the

Greenland ice cap The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice covering , roughly near 80% of the surface of Greenland. It is sometimes referred to as an ice cap, or under the term ''inland ice'', or its Danish equiva ...

were found to have come from the eruption of Okmok Caldera previously dated between 150-50 BCE. Analysis of the ice cores resulted in a new, more precise date of 43-41 BCE. Based on the eruption date and global climate models, investigators hypothesize that the 43 BCE eruption led to crop failures and famine around the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

, potentially influencing events surrounding the fall of the

Roman Republic The Roman Republic ( la, Res publica Romana ) was a form of government of Rome and the era of the classical Roman civilization when it was run through public representation of the Roman people. Beginning with the overthrow of the Roman Kin ...

and the end of

Pharaonic Pharaoh (, ; Egyptian: ''pr ꜥꜣ''; cop, , Pǝrro; Biblical Hebrew: ''Parʿō'') is the vernacular term often used by modern authors for the kings of ancient Egypt who ruled as monarchs from the First Dynasty (c. 3150 BC) until the ann ...

rule in Egypt.page 15, ''

The Daily Telegraph ''The Daily Telegraph'', known online and elsewhere as ''The Telegraph'', is a national British daily broadsheet newspaper published in London by Telegraph Media Group and distributed across the United Kingdom and internationally. It was fo ...

'' Wednesday 24 June 2020 After the caldera formed, numerous satellite cones and lava domes have formed on the flanks of the volcano. They include Mount Tulik (), Mount Idak (), and Jag Peak. At least seventeen eruptions of Okmok have been noted since 1805.

July 2008 eruption

On Saturday, July 12, 2008, Okmok Caldera exploded without warning, sending a plume of ash to into the air. map of alaska volcanoes okmok

During the following five and a half days of nearly continuous eruption, residents of Nikolski (47 miles southwest) were stranded for periods of up to three weeks; Unalaska (73 miles northeast) was repeatedly dusted with ash and flights into and out of this major fishing hub were frequently disrupted; floating rafts of scoria and low visibility prompted the Coast Guard to close

Umnak Pass Umnak Pass is a strait between the Bering Sea and the North Pacific Ocean in the Aleutian Islands in Alaska. It lies between Unalaska Island to the northeast and Umnak Island Umnak ( ale, Unmax, Umnax; russian: Умнак) is one of the Fox Isl ...

to marine traffic; and the Bering Pacific cattle ranch on the flanks of Okmok was periodically evacuated, once during noon-time darkness caused by heavy ashfall. The eruption ended in August 2008. It had a VEI of 4, giving it a rating of "cataclysmic." No lava flow was reported in this eruption, though the volcano has issued flows in the past.Alaska Volcano Observatory, Okmok Reported Activity, 2008
/ref> The eruption took place at a new vent in the northeast part of the caldera, creating a roughly 800 ft high cone, dramatically altering caldera hydrology, and discharging huge

lahar A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. Lahars are extreme ...

s, or volcanic mudflows, running from the caldera to the coast. In contrast, all 20th-century eruptions were from a cone near the southern rim of the caldera. The 2008 eruption was by far the largest eruption at Okmok since at least the early 13th century.

References

External links

Google Earth view
*
Volcanoes of the Alaska Peninsula and Aleutian Islands-Selected Photographs

''"Sulfur Dioxide from Okmok Volcano"'', JPL/NASA image

Okmok Volcano Ash Cloud
Photo page of the Okmok Volcano ash could, July 22, 2008

{{DEFAULTSORT:Okmok, Mount Shield volcanoes of the United States Active volcanoes Landforms of Aleutians West Census Area, Alaska Mountains of Alaska Volcanoes of Alaska VEI-6 volcanoes Calderas of Alaska Volcanic crater lakes Subduction volcanoes 21st-century volcanic events Umnak Mountains of Unorganized Borough, Alaska Volcanoes of Unorganized Borough, Alaska Polygenetic shield volcanoes Pleistocene shield volcanoes Holocene shield volcanoes Pleistocene calderas Holocene calderas