
Mount Denali: Not a volcano.
I recently stumbled across an article about the Denali Gap, a region along the Alaska trench where there are no volcanoes generated by subduction. The gist of the article is that Denali lies right where volcanoes would be expected, but it is not a volcano. However, some magma movement was recently detected. The geologist quoted mentioned that the reason that there are no volcanoes there is due to “flat slab subduction,” where the subducting plate does not immediately sink and melt, but rather stays mostly flat, while being overridden. This got me to thinking about a number of things that can affect subduction zone volcanism that aren’t covered in most peoples’ education about geology.
For those who need to be reminded of the basics, here they are: When two tectonic plates that are moving towards each other collide, one of the two is pushed below the other and sinks into the mantle. This plate carries water and other volatile substances with it. Water and other volatile substances lower the melting point of rock and increase its buoyancy. Therefore, when the plate sinks into the mantle, it heats up, creating buoyant, molten rock. This buoyant, molten rock rises up, and eventually melts through the plate on top, creating volcanoes. This buoyant magma is created at the point where the subducting plate reaches approximately 100 km depth. The distance from the trench to the volcanic arc depends on the subduction angle, typically between 30-70 degrees, with a steeper angle creating an arc that is closer to the trench. Generally, this is about 100 km inland from the trench. The magmas produced are usually from the calc-alkaline series, which are highly oxidized and water rich. In contrast, the magma of midocean ridges consists of tholeiite, which is reduced (the opposite of oxidized), and generally contains less water. (If you want to read about oxidation states, go read some chemistry books.)
What is interesting is that along some subduction zones, there are large gaps where no volcanoes exist, and in some places, subduction volcanoes erupt tholeiitic lavas. Additionally, in some places, volcanism extends much further from the trench than one would expect. Last, but not least, there are areas where large ignimbrite flare-ups (lots of large caldera forming eruptions over a geologically short period of time) have occurred. While there are still plenty of volcanic mysteries that befuddle geologists, most of the aforementioned phenomena can be explained by flat slab subduction, slab rollback, delamination, slab windows, and back-arc basins. This is the first in a short series of articles explaining these.
Disclaimer: I am not a geologist. This is only based on what I have learned from reading on these topics. I may not have interpreted things perfectly, and I may have missed some important theories. If you ARE a geologist, I welcome you to come here and prove me wrong or argue for or against the theories presented here.
Flat Slab Subduction
As mentioned at the beginning of this article, one phenomenon that can create gaps in a volcanic arc is flat slab subduction. This is pretty much exactly what it sounds like. Instead of the subducting plate immediately sinking into the mantle at a 30-70 degree angle, the plate remains pretty much horizontal while the lighter plate simply overrides it. Not surprisingly, this tends to lift up the overriding plate, creating great mountain ranges, like the Rocky Mountains of the American cordillera, or the Andes of Peru. (For further reading, look up the Laramide Orogeny, which is one of the best studied instances of flat slab subduction.) Interestingly, though, much of the mountain building may actually occur after flat slab subduction ends. (We will get to this in the next article.) Since the plate does not dive into the mantle as soon as it is overridden, it goes much further inland before it reaches the depth where the volatile substances begin creating melt. Without hot, buoyant, liquid magma, there is nothing to fuel the volcanoes of the existing arc, so these volcanoes become extinct. Volcanism may push back to where the subducting plate finally reaches sufficient depth to create melt, but often, the lower plate simply stops sinking for a time. In this case, there is a complete stoppage to volcanism, at least for a while. This may be the calm before the storm, though, as we will see in the next article of this series.

Figure: The volcanoes of Mexico are far inland from the trench due to flat slab subduction. (Compare to the volcanoes of Guatemala, El Salvador and Honduras.)
While the consequences of flat slab subduction are well understood, the causes are still somewhat under contention. The simplest explanation is that the rate of movement of the overriding plate and the subducting plate have little to do with each other. In this case, the overriding plate is moving rapidly in comparison to the rate of subduction. There is some support for this based on the opposite occurrence, slab rollback and back-arc basin formation, which I will get to later. Also, flat slab subduction occurs primarily where there is rapid convergence between the two plates. This is not the only theory, though.
Another major theory stems from the observation that almost every instance of flat slab subduction occurs near where an oceanic plateau or aseismic ridge (a ridge that is not a spreading center) is being subducted under a continent. Of course, the presence of a ridge makes for thicker, stronger crust, meaning that it is harder to deform it and make it bend down into the mantle. This is not the only force at play, though. While a ridge may not be proper continental crust, most aseismic ridges consist of lighter material than typical ocean crust. This makes them more buoyant, and thus resist sinking into the mantle. Along the Andes, you can see two major gaps that fit this description: The Peruvian gap, apparently caused by the Nazca ridge and the Pampean gap, associated with the Juan Fernandez ridge.

Figure: The Pampean gap occurs between the Southern volcanic zone and the central volcanic zone. The Peruvian gap is between the central and Northern volcanic zones.
The last theory is that flat slab subduction takes place where the mantle is cooler and therefore more viscous. In this case, the subducting plate is held in place by the thicker, more solid mantle material, while the overriding plate pushes over it. Suction also plays a role in this model. In order for the lower plate to fall away and sink into the mantle, mantle material must move up into the newly formed gap between the plates. If the mantle material is too thick to be sucked up into this space, then the lower plate can’t sink! It’s notable that none of these theories are mutually exclusive of each other. Multiple factors may be involved in flat slab subduction.
So, what’s really going on at the Denali gap? Well, it appears that this gap is due to the subduction of the Yakutat microplate. This microplate seems to be made of thick, low density material. It may be part of the continental shelf that became attached to the Pacific plate when transform faulting began along the Queen Charlotte fault. Originally, the plate margin was an arced trench, but when the Pacific plate arrived, it turned into an essentially linear fault. Parts of one or both plates would have to have broken off and welded to the other in order to change the shape of the plate margin. Alternatively, it could simply have originated as a typical oceanic plateau. In either case, this lighter material creates an area of low density that is subducting, leading to flat slab subduction and a volcanic gap.
Matt
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?
LikeLike
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.
LikeLike
(Midocean ridges can actually be traced by the earthquakes they create.)
LikeLike
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.
LikeLike
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 –
http://americastectonics.weebly.com/yakutat-plate.html
Click to access elliott_etal.pdf
LikeLike
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.
LikeLike
Also, keep in mind that this model of what’s going on could be completely wrong!
LikeLike
I am tempted to put tongue firmly in cheek and end the discussion with “… your mileage may vary.” Cheers –
LikeLike
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.)
LikeLike
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.
LikeLike
Mount Lokon, one of Indonesia’s most active I guess.
LikeLike
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.
LikeLike
APOD has a very nice photo of Fuego in Guatemala last month. Worth the visit. Cheers –
http://apod.nasa.gov/apod/astropix.html
LikeLike
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!
LikeLike
From Volcano Discovery, latest activity report. Cheers –
http://www.volcanodiscovery.com/volcano-activity/news/51681/Volcanic-activity-worldwide-9-Mar-2015-Fuego-volcano-Kliuchevskoi-Soputan-Shiveluch-Turrialba-.html
LikeLike
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.
LikeLike
And clues have been appended for the two remaining riddles!
LikeLike
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.
LikeLike
What do you think of this? These are earthquakes 5-10 km roughly north of Sabancaya volcano, Peru. The four greater ones are near or over M 4, and 8-11 km deep. Does that look like magma working hard to erupt soon? I am also looking at the many small ones, and the small long things, which are LP quakes if I am not wrong.

http://i.imgur.com/SFicx0T.png?1
http://volcams.malinpebbles.com/pubweb/Peru.htm
LikeLike
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!
LikeLiked by 1 person
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!
LikeLike
Decent BBC article on the Hunga Tonga eruption
http://www.bbc.co.uk/news/world-asia-31848255
LikeLike