We here in Alaska tend to pay attention to volcanic activity, partly because parts of the state get regularly dusted, and partly because Alaska was home to one of the largest eruptions in the 20th Century – Katmai – Novarupta a century ago. As you travel down the Alaska Peninsula into the Aleutians, you run across a number of calderas, indicating massive eruptions in the not so distant past.
Carl wrote up a very dangerous mountain, Aniakchak which had its caldera-forming eruption in the vicinity of 1645 BC. Aniakchak lies around 400 nm (740 km) SW of Anchorage. Fly another 400 nm (740 km) and you find the Okmok Caldera. In some ways it looks like a lunar crater. Like Aniakchak, it also measures in the neighborhood of 5 nm (9 kilometers) across. Unlike Aniakchak, it appears to have had two caldera forming eruptions in the last 12,000 years ago. The most recent around 2,050 years ago. The older one was 10,000 years before that.
Okmok is pretty active with 17 known or suspected eruptions since 1800. The most recent eruption of Okmok took place in 2008. It was andesitic in nature and lofted a plume estimated between 30 – 35,000’ (9,000 – 11,000 m). Activity took place in the second half of July that year. There was little warning of the eruption via installed seismic monitoring system, which may mean that this vent tends to wake up quickly and quietly. One person was evacuated from the island as the eruption began. AVO has a good write-up of the eruption on its web site. You can also find webicorders and a webcam there. http://www.avo.alaska.edu/activity/Okmok.php
The hazard assessment of Okmok by the State of Alaska can be found here.
It turns out that the most recent caldera forming eruption of Okmok took place around 40 BC. A few researchers at the University of Alaska Fairbanks have done papers and an unpublished dissertation on the eruption. Most of the work was done in the early to mid 2000’s.
I found a copy of a paper entitled “Physical Volcanology Of The 2050 BP Caldera-Forming Eruption Of Okmok Volcano, Alaska.” It is available here.
The following description is mostly based on this paper.
Total ejecta during the eruption was on the order of 50 cubic kilometers, or about half the size of what Anikchak had done 1600 years earlier. The eruption took place in two main phases. The first was Plinian in nature, laying down three tephra falls indicating strong SSE winds. Depth of the first tephra fall has been measured over a meter thick in places; the second at just over 20 cm thick; and the third at up to 20 cm.
After a period of days to months of relative quiet, explosive activity began again, this time with strong westerly winds in place. The venting started as phreatomagmatic explosions. Magma composition shifted over time from rhyodacite to andesite as the volume ejected per unit time picked up. After less than a half a cubic kilometer was ejected vertically, the caldera and plume collapse generated pyroclastic density currents that carried nearly 30 cubic kilometers out of the caldera. The northeastern half of Umnak Island was covered with pyroclastic flows. Depth of the main deposit has been measured at 60 m near the vent and tails off to around 30 m at the western shoreline where the flow entered the water. They also extended east to the neighboring Unalaska Island.
The pyroclastic flows separated into two parts, a dense lower layer that ended up blanketing the northern part of Umnak Island. It also entered the ocean into Umnak Pass and created a tsunami that inundated part of Unalaska Island. Thickness of the flow as it entered the eight kilometer wide Umnak Pass is estimated at 300 m thick, the lower third of which was the dense layer. The upper layer continued across the water to the east and left deposits on successive ridgelines on the island. It made it more than 20 km into the neighboring island. AVO believes that it went as far as 25 km in places. There is little but Bering Sea north of Okmok, so no estimate of how far the flow made it into the ocean to the north was made.
Okmok is a Full Service volcano, having demonstrated the full variety of volcano hazards. Not only does it erupt explosively with Plinian plumes and pyroclastic density currents. But there is also evidence of flank collapses and debris flows from them. The most recent took place within the last 2000 years when a flank collapse from Tulik volcano reached the Pacific Ocean coastline on the south side of the island.
Okmok also gets no small amount of snowfall and has a partly covered glacier in the caldera near one of the active cones. Over time, this snowfall and spring melt forms lakes within the caldera that periodically breach their natural dams and flood toward the ocean. Lahar and flood hazards are also caused when pyroclastic flows hit snow pack during the colder parts of the year. A flood in 1871 destroyed an Aleut village on the north shore of the island. There is evidence that the caldera was filled with as much as 500’ (150 m) of water for several hundred years after the most recent caldera forming eruption. That lake drained catastrophically to the north when the crater rim failed, nicely rearranging the land along the northern coast.
Okmok appears to be an increasingly active volcano as we look at what it has done in the geologic past versus what it has done over the last 200 years. This very well may be due to significantly better monitoring over that time and a larger number of people in the area on a full time basis.
It is capable of the widest possible array of eruptions and has demonstrated same. It periodically resurfaces significant parts of its island with pyroclastic flow deposits measuring tens to hundreds of meters thick. Those deposits are then cut by the runoff of the significant precipitation the island gets during the year.
Finally, Okmok sits under main air traffic and air freight routes over the North Pacific, particularly those that transit through Anchorage, and any significant eruption will likewise cause significant disruption to air traffic in this part of the world.