Okmok – Another Aleutian Caldera

This eruption of Cone A in Okmok Volcano shows volcanic ash and lava flows - http://www.volcano-explorer.alaska.edu/46/airphoto/okmok-2.html

This eruption of Cone A in Okmok Volcano shows volcanic ash and lava flows – http://www.volcano-explorer.alaska.edu/46/airphoto/okmok-2.html

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.

Photo courtesy Alaska Volcano Observatory, (AVO) Image 13283

Photo courtesy Alaska Volcano Observatory, (AVO) Image 13283

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

Image of the eruption of Okmok, taken Sunday, July 13, 2008, by flight attendant Kelly Reeves during Alaska Airlines flights 160 and 161. Picture Date: July 13, 2008 Image Creator: Kelly Reeves – Image courtesy of Alaska Airlines.

Image of the eruption of Okmok, taken Sunday, July 13, 2008, by flight attendant Kelly Reeves during Alaska Airlines flights 160 and 161. Picture Date: July 13, 2008 Image Creator: Kelly Reeves – Image courtesy of Alaska Airlines.

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.

Map of Umnak Island, showing Okmok volcano. Credit: Jessica Larsen, Alaska Volcano Observatory / Alaska Division of Geological & Geophysical Surveys.

Map of Umnak Island, showing Okmok volcano. Credit: Jessica Larsen, Alaska Volcano Observatory / Alaska Division of Geological & Geophysical Surveys.

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.


View to the east of the new cone that formed during the 2008 eruption in the caldera at Okmok volcano. The new cone is on the left, and the older Cone D, is the higher peak on the right. Photograph by Janet Schaefer, Alaska Volcano Observatory / Alaska Division of Geological & Geophysical Surveys.

View to the east of the new cone that formed during the 2008 eruption in the caldera at Okmok volcano. The new cone is on the left, and the older Cone D, is the higher peak on the right. Photograph by Janet Schaefer, Alaska Volcano Observatory / Alaska Division of Geological & Geophysical Surveys.

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.



131 thoughts on “Okmok – Another Aleutian Caldera

  1. Thank you for a very interesting post Agilmarc!
    It is interesting how many large eruptions Alaska has seen since deglaciation. If I have counted correctly there has been more than 1 caldera forming VEI-6 per 1 000 years, and at least 1 VEI-7. To boot it off there has also been a couple of non-explosive caldera formations (Katmai) and most likely a couple of non-caldera forming VEI-6s.

    Alaska to me seems to be a very disproportionate state, like my own. High yield of natural resources and a small population. If we then pair it off with a bewildering amount of massive volcanoes you have a scenario that can get out of hand if it is not monitored properly and mitigated. In this aspect it might actually be more important to mitigate the effect on the natural resources extraction than on people.
    This might sound cynical, but the GDP effect on the US would be beyond belief if the flow of natural resources was halted even for a short timespan.
    I think that the US would be well advised to divert quite a few assets from the hghly dormant Yellowstone into Alaska.
    Later down the line I will get back into the two volcanoes I see as most likely to go full on caldera, and they would not be pretty for the natural resources extraction, nor would they be fun for anyone living in the state.

    Once again, thank you Agilmarc!

  2. For those who wonder about this earthquake.
    04.11.2013 13:29:16 63.858 -19.825 10.2 km 1.6 90.04 16.7 km SSW of Hekla

    It is not a Hekla Earthquake, it is at the extreme end of the Vatnafjöll fissure. Vatnafjöll is an independent volcano that last erupted more than 1000 years ago.
    It is still seismically active, and lately it might have shown signs of increased activity.

    • Just got updated to a 1.9
      04.11.2013 13:29:16 63.857 -19.820 11.4 km 1.9 99.0 16.7 km SSW of Hekla

      • Ah, that was very interesting. Apparantly Vatnafjöll reactivated seismically a couple of days prior to the 1987 M5.8 earthquae. I did not know that. Seems like I will have reason to return to Vatnafjöll in a post soon.

        • A normal mode Mag 5.8 fault can easily have a 7.76 km down-dip rupture width and 0.16 meter average displacement. (gap formation).

          [calc using Wells-Coppersmith]

          • Well. IF the chamber is under enough pressure, and the quake can open a path to it… and it stays open long enough for magma to surge up into it…. showtime.

            The dynamics of a pressurized chamber pushing a feeder dike by itself require the over pressure to exceed the hoop stress limit of the chamber wall… with the overlying lithospheric stress helping to reinforce the tensile strength of the rock… and to maiintain that until the dike reaches the surface. As soon as the pressure drops below that limit, the dike slams shut.

            • that is a neat little assessment – I like your turn of phrase there (possibly complex for some non physic types though – so here’s my version).

              The jam inside the squashed doughnut only shoots out if the pressure is enough to rip through the wall, if there is other stuff reinforcing the doughnut (eg more doughnuts piled over it holding the doughnut walls together) you have to squash even harder to make the jam shoot out. (hope I got that right:) )

  3. Great article Agilmarc! Another great bit of armchair volcanology for us all to enjoy.

    I like this bit:
    “After a period of days to months of relative quiet, explosive activity began again,”

    I don’t know how many times I have read that on papers about caldera forming eruptions and it always stays at the back of my mind when there is a small eruption at a volcano that could potentially harbour a large shallow “body of locked crystal mush” (my currently preferred term for the old “magma chamber”). Often these small eruptions are not just an expression of a change in a magmatic system. I reckon they can unsettle the stability of a system in their own right and trigger a sudden change in locked crystal melt to eruptible magma as the pressure in the system drops… at least that is how I’ve understood the stuff I’ve read. Erik linked to paper at the AGU that sounds really intriguing in this respect where crystalization in itself can lead to a sudden tipping point withn a system. BTW it’s worth checking out Erik’s last article on Laguna del Maule:

    • Thank you, Bruce.

      One of the things I ran across in some of the papers on this and Aniakchak was a suggestion that there was an injection of basalt into a crystal mush mix leading to a very quick and very explosive and very large eruption. So the new basalt becomes the trigger for the eruption. Why? I would guess that there is a temperature, chemistry and dissolved gas difference between the two substances and apparently the new stuff and the old stuff don’t play well together. I’ll see if I can dig up the citations for you.

      Had thought about the depressurization trigger a bit. The thing I have wondered is what destabilizes the eruptible stuff in the first place. Much head scratching remains about all of it.

      Saw the Laguna article. Many thanks again for the pointer. Cheers –

      • This would be worth an article in its own right. I can think of three mechanisms working possibly in conjunction.
        1. (Basaltic) intrusion from depth. This is probably the most commonly documented and makes a lot of sense as the instrusion brings a lot of heat and more importantly is fluid so this can destablize a locked body of crystal mush and tip it into eruptible melt.
        2. Changes in the local stress regime. This relates to the evolution of the stress forces acting in the vicinity of the volcano or even in the volcano itself. Extensional quakes can open up dykes for melt to move (fault propagation) or even develop (decompression melting). Conversely melt on its own accord can change the stress regime and “force” an eruption (common in basaltic eruptions).
        3. Crystallization.. whereby the evolution of a body of mush over time leads to separation and concentration of volatiles that could reach a critical tipping point and form melt from an existing body of locked crystal mush. Recent papers have suggested this could occur over really short timescales

        What have I forgotten?

        Finally, once you get an eruption happening, all sorts of things can happen. The main point being that a magma body will not go caldera on us unless the depth to width ratio is low enough which I understand as being an expression of the fact that explosive fragementation will not happen until the top pressure gets released to a certain critical point whereupon it suddenly cascades throughout the body.

  4. Highly OT, but John Vidale posted a link that relates to work I did back in the nineties. Nice to see that it has ended up as a practical thing. I am though a bit reticent about the concept since there is a risk of the private sphere being unduly invaded by various government agencies.

    • On the other hand if getting cops was the objective it would be a method of getting targets for a sniper in the zone, not a thing i would encourage but with the over board shootings happening by self appointed judge jury executioners (law enforcement officers) some one some day will do the revenge thing

    • It must be playing an old recording or something. All the other cams are pitch black. And the light here looks like just before sunset.

    • No it’s real. But what is it. The camara is clearly very light sensitive. Even a car down the base of the mountain blinds out half the image with the headlights. The lighter sky on the left/west could be city light from Kagoshima.
      The red spot is always the same a the bottom (crater rim) but does change in hight. And from the sse you would look straight at showa.
      Could that cam show hot steam as red? Must be pretty superheated i’d say. I’ll try and capture a shot with headlights.

      • Have seen this glow several times in the dark. Seems to be the Showa crater. I cannot get the cam you use and the one I look at (view from E or NEE) seems to be a little less light sensitive. I thought it is hot material deep in the crater. The faint glow shows now and then. Will make a screenshot when I see the glow again.

        • Here it is, faint but visible:
          Image and video hosting by TinyPic
          The location of the bridge down left (see Google maps / streetview) helps navigating. It is the bridge between the “island” and the “mainland”.

          • Excellent, i was wondering what the light spot on the left down corner of my images was. It’s clearly the bridge that showes nicely in your image. Glow from the same spot for sure! and you’re cam is clearly E or NEE.

    • Thank you for your kind words. It is not uncommon up here for high winds to bounce around the webicorders a bit on monitored volcanoes (the lower half of your photo). Don’t precisely know why, as placement shouldn’t be that subjected to winds. Makes it tough for the uninitiated (like me) to figure out what is going on. Satellite photo shows a storm just to the east of the area. Cheers-


      • Thanks for the input. Unfortunately there is no Power density spectrum or fourier transform (like we have on the ign.es site for El Hierro for instance), so interpretation is more difficult especially as I have no backgound on this particular volcano.

      • Also there is a power issue on a relay station for the volcanoes out on the islands further away that sometimes cause interesting phenomena…

          • Great post,agimarc. Anchorage is my favorite
            Alaskan town Sitka is a close second. Trained on Red Bull’s Dc 6 sim earlier this
            Spring-just as the weather warmed up.

            • Interviewed with those boys a number of years ago. I didn’t mind hauling cargo or loading the plane. I did mind paying for my own training on a “Trust Me” basis. Ended up working on the Slope for some years afterwards. They are still flying our of ANC. Not much of a climb rate out (power) or a descent rate coming back into ANC (cooling the jugs). Cheers –

  5. Wow! Here’s an usual place for an earthquake. Indian Head Park, Illinois is a western suburb of Chicago.

    0km SE of Indian Head Park, Illinois
    2013-11-04 12:35:34 UTC-06:001.4 km

  6. Nevado Del Ruiz has started up another swarm. The last few swarms that have occurred at Ruiz have resulted in nothing more than steam or a phreatic eruption or two. I guess we’ll see if this is just a small swarm, or it becomes a prolonged swarm like we saw almost a year back.

  7. Hi

    Here is the October summary for El Hierro. Pretty regular except for the 31st when there were no quakes. Inflation is down also since the beginning of November. The vast majority of the quakes (329 for the month) are under the island.

    • I still wonder when the eruption there is going to start. I think Bob was something of a sideshow (literally). Not scaremongering (well at least not meaning to) but I reckon we will one day see an eruption from the main vent there one day. All of these deeper quakes must indicate a slow but constant infusion of new melt. It is kind of hard to imagine they are all just a product of resettling after all the prior activity.

      • Me too wondering about this. Will it be in this century? I check the IGN info every day. Too bad they removed the webcams. Something seems to be brewing 10-12 km beneath the middle of the island, but what?

        • “Something seems to be brewing 10-12 km beneath the middle of the island, but what?”

          Easy… a curtailment of funding for their green energy ambitions. That’s why they downplay it as hard as they do. Don’t want to scare off the money ya know.

          Hell, they built thier peaking reservoir in an old scoria cone. What happens if that particular vent becomes active again? How do you explain that to the insurers or the people backing the loan? “Okay, you put how much money into that reservoir? In a volcanic cone? And it erupted you say?” Sorry, you’re still liable for the money.

          “Due diligence” would eat them alive.

          • Locally, we have an old Westinghouse Plant. In days gone by, they used to make bomb casings that adhered to the high requirements of specialized applications… or so the story goes. No longer in operation, and for the most part defunct, the plant was sold to GE. Now they make bird chippers.

      • Hi

        For the time being, the situation is quite calm. However there are still quite a few quakes, with this level of activity being maintained over 3-4 months now. The quakes are mainly located under let’s say “Tanganasoga” (or in the vicinity of), except for a few stray quakes that are (IMHO) some resetting ones (for a minority) and some other ones (the majority) which are more like a new (but slow) infusion around 11-12 km depth. Should there be something larger brewing on, we would get a new earthquake crisis well before I think.

      • I agree with DFM, we would see a larger amount of earthquakes before anything happens, and also an increased uplift. Problem is just that I am not so sure it would be in Tanganasoga area it would happen. Remember that it is at least 3 eruptions that have happened out in the ocean or at the edge of the island. I would more suspect that the same thing happens again, the island is after all more like a swiss cheese than anything else.

        • Yep – I agree El Hierro remains interestingly simmering rather than on the boil, thanks dfm, and that the 12 km depth seems to be under the island, but then the quakes go quiet and a vent opens to the south. It did seem though that when Bob was being effusive there were also vents along the arms to the north, around El Golfo. I always have to remind myself how steep the island is, in deep water. And as you say, Carl, like a Swiss cheese!

  8. I suspect that we in the next few days might see a paroxysm. A small swarm of earthquakes that seemed to cause a bump in the tremor a few hours later. Perhaps a sign of fresh magma moving into the system.
    Caution: I am not a speleologist, professional chef, nor am I owning a Matisse. So, over to Boris for a more professional view. 🙂


    • And it has caused a bump in the tremor, and it looks like a new increase is starting. Sadly EBCN tremor graph is down today.

      • Also, the very unscientific but quite often accurate Borisometer* is having a very low score today. That is often indicative of activity pointing towards a possible eruption.

        * = Borisometer is the uncscientific term for the amount of times Boris Behncke writes on Facebook. When Etna is doing something his writings diminish in direct proportion to the amount of activity in Etna. (Yes, I invented this due to my jelousy over his kitchen window view…) :mrgreen:

        • I have a kitchen window view also…. I look out the window, if the dog is out there, the squirrels disappear. As do anyone seeking to enter the yard. At night, when the neighborhood yappers get excited, along with these two rat dogs, They are joined by a deep throated booming “Woof, woof!” of the new one. It sends a clear message. ‘yeah, there is something there that can take your leg off.’

          • I have one too… but it does not involve a volcano 🙂

            Boris have commented the swarm that it is not caused by bubbling magma (which most in here understands), but says that it might be the sign of magma arriving.
            And we know what magma arriving often ends up in 🙂

  9. I was apparantly spot on… INGVs report on todays mini-eruptathingy.


    After 10 days of relative quiescence, Etna’s New Southeast Crater (NSEC) is showing signs of renewed eruptive activity. At 16:43 (GMT =local time -1) on 5 November 2013, the surveillance cameras of the INGV-Osservatorio Etneo recorded an ash emission from the NSEC, which was preceded by a brief glow, and which generated a strong signal on the seismograms of the summit seismic stations; the bad weather conditions have precluded observation of any possible further explosive events. More information will be provided soon.”

  10. I don’t know if this was posted before.
    Eruptions has a good read on Etna. A must view is this video by Boris Bhecke i believe.
    Watch it on a big screen and turn up the volume at 2:42 .

    • What a display! That video cheered me up arjanemm thanks. Only 3 days into being kitchenless and I am beginning to feel a little stressed. Tomorrow the concrete will be poured in and spread. So no way out of the back of the house until Thursday. Only access to back and wash house is to walk up the hill and back round the track behind our row of houses.
      However in the grand scale of things this is less than a minor hiccup and not life threatening so not complaining. I am lucky to have a house!
      I am content just lurking quietly here and enjoying reading the comments.
      Thank you AGIMARC for a very interesting post. I just keep on learning here. 🙂

      • Good luck with the building going on. This is exactly my job. Extending, improving and restoring existing buildings. Many times i work in a situation where the costumers are still living in the house.
        People like camping in te holydays for a few weeks but weeks of building going on in your house is intrusive and camping in your own house is not nice. I always try to get the job done within 4 weeks.After a month people get sick of it..

      • Heh…. some trivia for you.

        One of the greatest human contributions to Carbonyl Sulfide, that gas I keep yammering about, is from the curing of concrete. Extremely inactive in the troposphere, 200 to 270 nm light allows it to dissociate and take part in the formation of the Junge layer up above the tropopause. Ya have to have that short wavelength light to break it into something that can make a sulfate.

    • Buns of Plastic. It’s the new rage.

      They have even made arrests of unlicensed bun enhancers. Really horrible work. Left the “patient/victim” literally with a floppy butt.

      Warning, horrendous butt images about a certified plastic surgeon’s effort to salvage some girls arse from the effects of an illegal procedure.

    • Hello and welcome, nice to “see” you 🙂
      Do peruse Welcome to the Volcanocafe in the top menu for house rules, guides and tips etc.

    • Hello and welcome!

      I tried to put in a welcome picture, but it failed for some reason.

      Regarding Etnas small puff, I am still expecting something larger to happen within the next couple of days. This is just part of the run up.

      • Mornin’ Carl,
        I think you are right, tremor looks like it’s starting to rise, though still at low level and that swarm of eqs yesterday seems significant…
        I hope she will hold off for 24 hours or so, as I have to be at work (with limited interwebnet access) until 230 tomorrow…

        • I think you might just make it. Remember that this is most likely new fresh magma rising upwards, and it should take time untill it reaches a level where the volatiles dissolutes and we get something more impressive.
          Also, my newly invented Borisometer is having a low value 😉

  11. I always find Boris a good predictor of eruptions at Etna. Its simple, if he hasn’t posted on either Eruptions or VC for 3 days, something’s up 😉

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