22 comments on “Things That Affect Subduction Volcanism: Flat Slab Subduction

  1. Thank you Matt, this a good, easy to understand article! I have often seen those gaps referred to as “aseismic”, and wondered how that could be. Well, if I understand right, they can not really be aseismic as a) subduction still takes place and needs to be accommodated under the continental plate, and b) if a volcanic zone is created further inland all seismic activity will also occur there. Additionally, c) I can imagine that the breaking up of the subducting oceanic crust to the right and left of a more rigid part (e.g. ridge) must cause some considerable seismic activity. Just thinking of the usually very strong earthquakes in the Iquique area, northern Chile, could they not be caused by the oceanic crust breaking off along the southern part of the Nazca ridge?


    • It’s not earthquake-free where the ridge is subducted. Aseismic refers to the ridge NOT being a spreading center. A spreading center that is subducted has its own affects on volcanism, which I will get to in a later article.


  2. New eruptions at Mount Lokon, Indonesia:
    Jakarta Post (March 9): Mount Lokon in Tomohon, North Sulawesi, has been showing increasing activity since Friday, when the frequency of eruptions was recorded at more than 15 times every six hours. […] “But this time, the activity escalated. We have informed the Tomohon city administration to take the necessary steps and to keep the people away from the danger zone,” Farid said. […] In September last year, the volcano erupted forcing many people to leave the area.


  3. Howdy Matt – good read. I saw the initial article about the gap and reacted with no small amount of skepticism. Looked at the Yakutat block as a terrane rather than a microplate, though expect is can be either / both / all 3. Had looked at it as another chunk of stuff transported by the Pacific Plate and plastered on the southern part of Alaska. Didn’t see enough at the time to consider flat plate subduction. Thanks for digging it out.

    The gap runs from Hayes Volcano some 135 km NW Anchorage to Wrangell some 320 km E of ANC. The major geologic feature along that line is the Denali fault, which is a strike – slip fault. Don’t know of much volcanic activity along major strike – slip faults. Lots to learn. A couple links for your consideration. The second is most interesting as it suggests the way the Pacific Plate is impacting southern Alaska is in the midst of rearranging itself into multiple chunks with a variety of impact morphologies, turning a complex collision into an even more complex one. Cheers –


    Click to access elliott_etal.pdf


    • Terranes are pieces of the underlying plate that get scraped off onto the overlying plate, often with the trench shifting. It’s absolutely no surprise that this is happening, and it’s entirely possible that flat slab subduction is common when terranes are added to a continent. Everything West of Colorado, including the entirety of Alaska, is built from terranes.


          • I am totally open to being wrong! I’m no expert, (and neither are the experts!) Plate tectonics is a newer area of science than genetics! (Seriously.)


  4. I’m wondering if in any of the zones without volcanism going on are areas where bigger earthquake can occur. I wonder also why I am talking earthquakes more than volcanoes.


  5. I just read that Mount Cristobal in Nicaragua had to ashy explosions last week. Not news anymore, but worth keeping an eye on:
    SINAPRED: Two explosions of gas and ash occurred Thursday afternoon in San Cristóbal Volcano, located in the department of Chinandega. Dr. Guillermo Gonzalez said at a press conference that the first explosion produced a column of volcanic material reached 200 meters, while the second explosion went up to 500 meters. San Cristóbal remains on Yellow Alert. In addition, the INETER is to monitor the phenomenon and performing measurement of gases and other studies to determine the cause of both explosions. Note that the San Cristóbal is the highest volcano in Nicaragua and one of the most active.


    • Thank you, agimarc! ” First, there was an unusual smell. Then there was a loud bang. But what appeared to the eye was the most amazing of all…” …this must be the best volcano picture I have ever seen!


  6. Name The Volcano (NtV) riddles are all dinged and a new batch is up!
    For reference, all the solved riddles with the volcano names can be viewed on an extra page. You find the link on the NtV page.


  7. There is an earthquake swarm going on near Guam in the Mariana Islands, Five quakes at +/- M 5 in two and a half hours, at depths of 40-47 km.


  8. About a perfect example of sensationalist journalism in the news – seen in The Japan Times and some other poor news columns who just copied the article:

    “Second Volcano Rumbles to Life in Guatemala – Two of Guatemala’s three active volcanoes have now rumbled to life, officials said Wednesday, one day after the Santiaguito volcano began belching […] The sleeping giant began to stir on Tuesday, but officials said that thus far, they have not had to evacuate populated areas nearby.”

    Hmm… not directly wrong, BUT:
    Guatemala has nine (9) volcanoes that erupted in historical times, and yet a a number more that are considered active. The three mentioned are in ONGOING eruption! Fuego just had a more explosive spell last week, while Pacaya erupted violently in 1965 and has been erupting continuously in Strombolian fashion since then. – Santiaguito, which is a relatively new crater (or better a lava dome) at the slope of Santa Maria volcano has now been continuosly erupting for nearly a century, with vertical ash eruptions and a lava extrusion rate ranging from 0.1 to about 2 m3/s.

    So, it has NOT rumbled to live two days ago. Santiaguito has NOT been “sleeping” and, far less is a “Giant”! To complement this article on Santiaguito with the photo of a 2012 Fuego eruption is the crown of lazy reporting. – The only “news” about Santiaguito is that it showed a hightened activity, which happens about every month. But journalists need to make elephants out of midgets to get their papers sold. What a pity, I would have expected more concise reporting from Japan, the land of volcoanoes!

    Liked by 1 person

  9. Volcano Ruang in North Sulawesi (not to be confused with Raung in East Java) has been raised to alert level two. It showed no visible signs yet, but the number of shallow volcanic earthquakes has risen considerably. This in itself is nothing to write home about, but Ruang is a small volcano (725 m a.s.l. on a 4×5 km island) that has produced an ash column of 20 km in its last eruption in 2002, together with pyroclastic flows and lahars. Eruptions of Ruang have caused casualties and evacuations. In 1871 a destructive tsunami with a height of 25 m occurred as a result of dome collapse and pyroclastic flow. Keep an eye on this one!


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