It’s been a while since my last post in VolcanoCafe. I have been working on multiple ideas and gathering data, but not been able to finish anything. Today, I bring something exciting. I’m using GIS data (Geographic Information Systems) to explore in depth a volcanic arc, specifically the Aleutian Arc. There’s a lot of potential hidden in GIS data to understand volcanoes, their history, and behaviour. My initial motivation was to get volume estimates of volcanic edifices to better understand how much, where, and how magma is produced, but the project has taken a life of its own and expanded, so we will also be looking at lava fluidity, stress fields, and chemistry. It will be a brief yet profound look into the characteristics of Aleutian volcanism.
The Aleutian Arc
We go to Alaska, amid stormy seas and rugged mountains. Covered in vibrant green grass in summer and white snow in winter, these volcanoes are remote and often hard to access. It is this remoteness that causes the lack of data that many of them experience when compared to other US volcanoes, yet they are still better studied than those in many other places of the world. Ironically, the extreme climate also helps make exposures more available. Past glaciers have cut into old volcanic edifices, revealing piles of stratified lava flows and tephra hundreds of meters thick. Widespread peatland occurrences preserve tephras in carbon-rich material that can help date them in great detail (wiggle matching should be possible, though yet unused technique, in peat sections). These same tephras reach Greenland in trace amounts, even those of subplinian eruptions, where they might one day be dated to the precise year using ice cores. There’s a lot to be learned in the future from it, but in the meantime, a lot can be learned without going to the place.
Pulling and squeezing
Volcanoes often use vertical sheets of magma known as dikes to erupt, which intersect the surface as linear fissures, and these sheets of magma act as compasses of the surrounding stress field, meaning the tension and/or compression the rock is under and their directions. Below is a map showing the direction of all the fissure swarms I’ve been able to spot (shown in red), and also the direction of motion of the Pacific Plate (green), which moves towards the northwest, where it “impacts” the North American Plate and subducts under it.
Red lines represent the direction of dike/fissure swarms of those Aleutian volcanoes that have them. Green lines represent the direction of motion of the Pacific Plate, which subducts under the North American Plate along the Aleutian Trench. Note how the fissure swarms mostly run parallel to the motion of the Pacific Plate, except at Seguam volcano, where it rotates to acquire a parallel direction to the trench instead, and this happens in the “Seguam Rift”.
The fissure swarms, as can be seen, are most of them aligned with the Pacific Plate motion. Why is this? The Pacific Plate is the main player here because it moves at about 7 cm/yr towards the subduction zone, while the North American Plate motion happens at only 3-8 mm/yr, towards the south. It squeezes the arc and forces magma to intrude in the same direction as the pressure is being applied from. If the dike direction (the long axis) were perpendicular to it, then the flat face of the dike (needs space to open) would be oriented towards and directly experience this crushing force, therefore unable to intrude. By aligning their direction to this compressive stress, they can avoid its effects. In other words, the dikes show that much of the arc is being compressed by the plate convergence.
But why do the volcanoes have fissure swarms if there is compression? Many of these swarms are quite weak, but the volcanoes that have more prominent ones are also very prominent in terms of elevation. Shishaldin or Veniaminof, for example, have marked fissure swarms, with abundant volcanic vents, and they also happen to be very tall edifices towering over 2 km above the surrounding area. It’s likely this height that provides the dikes with enough pressure (supported by the magma column under the volcano summit) to rip open the ground.
There is one marked exception in the stress directions, which reaches its maximum expression in the area of Seguam volcano, where dikes rotate to become parallel to the trench. There’s a very different stress field here, one that seems to be caused by none other than rifting. A dike intrusion has to push the ground to the sides, away from it. If a dike’s direction is parallel to the trench, then the ground will have to be pushed towards the trench, yet that’s where pressure normally comes from, so it’s the hardest direction to do so. This is unless the subduction zone is actually experiencing a suction force from the subducting plate at the trench. This suction is created by rollback, the subducting slab dropping downwards into the mantle and retreating away from the overriding plate, but also inevitably dragging the overriding plate with it, pulling it towards it. This creates stretching. Magmatism along the arc often exploits this tension, intrudes into it, and causes the arc to split open, along its length (may eventually turn into a back-arc basin). This seems to be happening at Seguam.
As we advance westwards along the chain, the changes seem to emerge gradually, at the same time as the subduction angle becomes steeper. Westdahl is the first to show a slight rotation in the dike direction. By the time we reach Okmok there are some normal faults in the seafloor, and the field keeps rotating. On reaching Yunaska, a full graben develops along the axis of the arc, which continues as about three graben segments to Seguam, where the field seems purely extensional. This graben complex is known as the Amlia and Amukta Basins, though I think a name like “Seguam Rift” could work better. To the west of Seguam, the setting reverts immediately to compressive.
Map showing the Amlia Basin where Seguam volcano is located and the continuing graben, along the crest of the arc to Yunaska. General Bathymetric Chart of the Oceans, from NOAA.
Locally, other dike orientations exist that are related to the stress fields of inflating magma bodies, most notably Emmons Lake (but also Tana and Vsevidof), which has a fissure system aligned with the long axis of its caldera and magma body. As the magma builds up, it arches the overlying rock, and the shape of the inflating area favors intrusions to run along this axis.
As can be seen in the first map, the movement of the Pacific Plate becomes parallel to the trench to the west of Buldir volcano (last subaerial volcano to the west). With the southward motion of the North American Plate also being quite slow, this area experiences very little subduction and instead is largely a shear boundary. As such, while the full length of the Aleutian Trench is 3800 km, the westernmost 1,000 km have practically no volcanism. Some minuscule volcanic fields can be seen in the seafloor between Piip volcano and Buldir, often referred to as western cones.
Piip volcano stands lonely and mysterious near the west end of the Aleutian Trench and at the south end of the so-called Komandor Basin. This whole basin is formed by very recent seafloor spreading that was active until only 10 million years ago, according to current ocean crust age models. However, Piip at the southern end of this system is an active stratovolcano (has fumarolic activity and recent volcanic deposits), and also features some young-looking spreading ridge structures under and around it. Too young-looking to be anywhere as old as 10 Ma. I believe the Komandor spreading ridge still continues to be active under Piip. A junction between a feeble volcanic arc and a dying ridge. It probably accommodates some rollback exerted by the Aleutian Trench. The rest of the former Komandor spreading may perhaps have come from the now-dead subduction rollback along the nearby Shirshov Ridge.
Map showing the Komandor Basin formed by seafloor spreading until about 10 Ma, and also Piip volcano, the mountain at its southern end. General Bathymetric Chart of the Oceans, from NOAA.
Piip is an unusual volcano, not just due to its isolated location and arc-spreading combination, but also due to its strong high-Mg andesite chemistry that I will mention later, and this draws some parallels to the West Mata volcano of the Tonga Arc that is in a similar setting and erupts similar lava.
Eruption rates and viscosity
I’ve analysed the eruption rates and viscosities of volcanoes from Buldir eastwards. They comprise a total of 50 volcanic systems, spaced at mean intervals of 52 km. That said, I’ve treated volcanic complexes as one system that GVP often lists as multiple volcanoes. These two, eruption rate and viscosity, are fundamental parameters of volcanism related to the processes that give rise to the volcanoes and also fundamental factors controlling the volcano’s behaviour. And yet they are rarely ever given the importance that they have, probably because it has previously proven difficult to obtain estimates. Even now, with the wealth of GIS data that exists, what I’ve done is partly an art rather than a precise science, but I think nonetheless I’ve been able to apply a consistent technique that is quite useful and close enough to the actual values.
Isn’t chemistry a good indicator of viscosity already, one for which there is plenty of existing data? Not really; while viscosity is indeed mainly controlled by the silica concentration in the melt, most existing chemistry data refer to the lava as a whole, which is not the same thing. Magmas/lavas include liquid, gas, and solid. They contain bubbles of volcanic gas that mostly escape during an eruption, but there are also crystals in the magma, often 50% if not more, that stay with it even after erupting. These crystals will have a lot less silica than the melt and will substantially shift the overall composition towards mafic. Andesites and basaltic andesites are the dominant lavas in the Aleutians, but a lot of these mafic-intermediate volcanoes have a melt fraction that is actually rhyolite, so they form viscous lava domes and coulees. That’s why I’ve looked directly at the products of the volcanoes to obtain their true melt viscosity and approximate silica content.
What I’ve done is measure the minimum lava channel width of lava flows; this is the minimum width for lava to form a free-flowing stream and is a rough indicator of viscosity. Fluid lavas may form free-flowing streams that are only tens of centimeters wide in places like Hawaii. On the other extreme, at their most viscous, lava channels will be no narrower than 200 meters. Using cases with reported glass silica content, I was able to get a rough relation between the two parameters, which is the following:
Melt silica%=48.438*Minimum lava channel width^0.0903
The SiO2 can be off by up to 8 wt%, but it’s still quite useful to get approximations of the silica content of the melt fraction of the magmas. There is some offset because the way I’ve estimated viscosity already is a rough one, and also because there will be some factors other than silica that may affect the viscosity of the lavas. 200 m channel width is sort of a limit because at this size the above equation yields 78 wt% silica, which is the most silica magma is known to reach. If I get channel estimates of over 200 m, or find volcanoes too viscous to flow at all (domes surrounded by PDC aprons), I assume an equivalence to 200 m and thus 78 wt% silica.
Minimum lava channel widths I got for the Aleutian volcanoes. Volcanoes of the central part of the arc can generally erupt more fluid lava. The three volcanoes of the “Seguam Rift”, which are Seguam, Amukta, and Yunaska, can erupt very fluid lavas akin to Etna volcano. Shishaldin and Gareloi also display, sometimes, similar “Etna-level” fluidity during their eruptions. Yellow circles are dome-only volcanoes that have effused during the Holocene but have not been able to flow, at least in part due to the high viscosity.
I obtained eruption rates by estimating the volumes of edifices younger than about 1 Ma, using their state of erosion, and helped by existing age data of volcanic products. Taking an interval this long helps ensure that two or so shifts in the locus of volcanic activity have taken place since, and thus separates edifices by their ages and also helps approach a geologic long-term, if such a thing exists. I filled glacier valleys to account for their powerful and relentless erosion.
There were some problems, however, when it came to pyroclastic shields. When a caldera covered in glaciers collapses, the ignimbrite produced immediately starts melting the ice, which creates enormous lahars that carry the pyroclastic material all the way to the sea. For this reason, it seems the frequently glaciated recurrent calderas have been unable to build substantial edifices. Powerful volcanoes like Fisher, Okmok, and Semisopochnoi have actually lost elevation over time, as glaciers dissected their long-lived shields and explosive caldera-forming eruptions were unable to deposit ignimbrites on the thick ice cover. I had to make guesses for the volumes of these three volcanoes based on nearby systems that seemed similarly vigorous. For Emmons Lake, I used the five known ignimbrite deposits and approximated their volumes based on the caldera size (100-150 km3 dense rock equivalent each, using the DRE for the volume estimate to try and keep consistency, because DRE often falls close to the ignimbrite/PDC volume, and it is this last component what I’m measuring in general with other volcanoes since distal ash won’t contribute to edifice growth).
The graphs below sum up the results of the investigation.
Cumulative eruption rate km3/kyr of all the volcanoes from east to west. Where the slope of the red line is steeper, it means the arc is more productive overall. In the eastern part, it can be seen how a few powerful volcanoes concentrate almost all the output, being surrounded by weak systems.
Overall, there’s probably some underestimation since I’m not counting the distal ash (which doesn’t contribute to volcanic edifice growth), but I don’t think the number would change too much when including it. In the last 1 million years, the Aleutian Arc seems to have erupted 12,000 km3 of volcanic material. This is equivalent to about 12 km3 every 1,000 years, averaged over the considered period. Is this a lot? I only have a similar estimate for the Lesser Antilles Arc: 1.3 km3/1000 years. True, the Antilles are a lot shorter, but still, when dividing by the length of the arc, the Aleutian Arc comes out to be more than twice as productive per unit of distance. Though other arcs like Sunda or Kamchatka are definitely more productive than Aleutian. The Aleutian Arc eruption rate would be about half that of the Reunion hotspot, and only a tenth or less that of Hawaii, but likely a lot stronger than many weak hotspots.
Combined graph showing the melt silica concentration I estimated from viscosity and the total volume erupted in the last 1 million years. It helps divide the arc into three main segments: eastern with lower overall production and higher viscosity but a few powerful isolated volcanoes, a central segment with high productivity and fluid lavas, and a western segment with low magma production and high viscosity.
When taking viscosity and eruption rates together, it becomes clear that the Aleutian Arc has some substantial variation in the way it performs volcanic activity. And this variation almost seems symmetrical around a point near the center of the arc. There’s a central segment where the eruption rate is generally higher, and lavas are fluid. This central segment more or less goes from Shishaldin to Atka. A lot of volcanoes in this area are recurrent caldera systems, and as we will see, they also have a characteristic chemistry. To the sides of the central segment, we can distinguish eastern and western segments that run all the way to the extremities of the arc. The main difference is that these two lateral segments erupt more viscous lavas, often lava domes or thick coulees, and are more drawn towards building stratovolcanoes with a single central vent. The volcanoes in the flanking segments are still dominated by basaltic andesite and andesite lavas like the ones in the central part, but they are more often crystal-rich; therefore have a more evolved and silica-rich melt fraction. The other difference is that lava eruption rates tend to drop from the center towards the extremities of the arc. This is particularly true for the west end, where volcanism is already quite feeble on reaching Buldir, but a wisp of volcanic activity still continues to its west, fading into weak submarine fields.
View of several Aleutian Arc volcanoes in line. From front to back: Shishaldin, Isanotski, Frosty, and Emmons Lake-Pavlof. Shishaldin is a beautiful basaltic stratovolcano formed in the last ~100,000 years, Isanotski is likely its predecessor and seems to have been a similar volcano back in the day. Both combined have a volume of 450 km3. One of the most voluminous expressions, if not the most, of pure stratovolcanism in the arc. Photo by Chris Barnes: https://avo.alaska.edu/image/view/194507
In the eastern segment, which spans the Alaska Peninsula, there’s a curious behaviour. Five volcanic systems concentrate almost the entire lava output: Hayes-Spurr, the Katmai Group, Aniakchak, Veniaminof, and Emmons Lake. These five systems vary in their eruption style, but they do have in common that they seem to have a more mature storage (either shallow chambers or more expansive areas of magma storage in the crust). True, this could be a consequence of the higher supply, yet it could also be the cause. The big five may, thanks to their greater development, have a stronger pull on magma production and starve the weaker systems around them. This volume disparity seems to strengthen eastward across the entire arc, likely related to the growing continental crust thickness. Emmons Lake yielded the largest lava production among the volcanoes in the arc, 1250 km3 in the last 1 Ma.
Topography map of Emmons Lake, the most powerful volcano of the Aleutian Arc. An old eroded volcanic edifice, probably a stratovolcano, of originally 400 km3 and likely over 3 km tall, stands on the northwestern side of the complex. This was followed by caldera volcanism that migrated to the SE and led to the emplacement of five ignimbrites at 295, 237, 123, 51, and 27 ka, likely all VEI 7 eruptions. The twin stratovolcanoes of Pavlof and Pavlof Sister have grown on the northeast flank of the complex to a volume of 90 km3 in the last ~100,000 years.
The Ancient Aleutians
Having calculated the 1 million years lava output creates a nice opportunity to compare it to the magma production over the entire history of the arc. The arc is young; the continental crust grown on top of Cretaceous oceanic crust, from Shishaldin westward, belongs to magma production along the current subduction zone. Because the oldest ages of volcanic arc products are about 47 Ma, it’s usually assumed that’s the moment when the arc first initiated. I used the Gareloi-Shishaldin section for the comparison. Turns out the total magma production since inception has been 3 million km3. This is a lot more than I was expecting to get, since it would imply rates of 71,000 km3/Ma over the last 47 Ma, compared to the 6,500 km3 this section has actually seen in the last 1 Ma. There’s definitely an important intrusive volume below the surface contributing to the continental crust growth, but it shouldn’t do much more than double the number, so that, even then, the gap would be too great.
The answer to this problem, I think, is that the arc may have actually started earlier. As it grew, the older products were probably buried by younger volcanism and subsided as material piled up on top. I think it works better, and also makes more sense tectonically, if the Aleutian Arc started with the opening of the same back-arc basin that created the Bering Sea oceanic seafloor, so about 85 million years ago, during rollback of the former Kula Plate. Would be a similar mechanism to how younger island arcs like Tonga, Vanuatu or the Marianas have formed; older arcs that split into seafloor spreading. Subduction rates must have been higher back in the day, and the Cretaceous was quite skilled at producing powerful volcanism, so this Late Cretaceous Arc may have been more productive than the current one and may explain the huge amount of continental crust.
Chemistry
Lastly, we will be going over a chemical review of the Aleutian Arc. There are two aspects I’ve found interesting to look at: silica concentration per given MgO and TiO2-Fe2O vs Al2O3-CaO per given MgO. The MgO is always a good way to judge how primitive a magma is. This is because, no matter what crystals are coming out of the magma, the MgO is always more concentrated in these crystals and depleted in the melt. Magma will always travel to less MgO as it cools and evolves. Also, evolution tends to end around the time MgO hits 0 and start around 10 wt% or more for alkaline magmas.
Both aspects I’ve looked at seem related to each other, as well as to the viscosity and eruption rates, but the exact reasons as to why this is seem unclear. In general, I think that a lot of aspects of volcanoes remain poorly understood.
SiO2 vs MgO shows that no volcanoes are silica-undersaturated relative to the subalkaline volcanoes of places like Iceland, so although some volcanoes will fall in the alkaline field of the TAS diagram, I think when looking at the silica content, none of them really have the low silica characteristic of alkaline magmas. Silica per given MgO increases from a low at the center to a peak at both ends of the arc. The volcanoes near the sides erupt strong high-Mg andesite/calc-alkaline magma. On one side, we find this magma at Buldir and the submarine fields to its west, plus Piip volcano, and on the east, we find it at Douglas-Fourpeaked, Augustine, and Hayes-Spurr. High-Mg andesites are magmas that have an inherently high silica content. In the more extreme cases, they may already form in the mantle with silica contents of 57 wt% SiO2 (between andesite and basaltic-andesite) while featuring high MgO comparable to primitive basalts.
Something very interesting, too, is what I call anorthite-pyroxene fractionation. I name it because I don’t think it has a name, and, if it does, I’m not aware of it. Basalts (and low-SiO2 basaltic andesites) that are stored at low pressures can undergo a specific kind of crystal fractionation in which the magma evolves while barely increasing in silica. Sometimes, over half the magma can crystallize, while the remaining half will only have risen by ~2 wt% SiO2 and remain almost as fluid. This kind of crystallization is probably what allows lava lakes that pond inside craters in Hawaii to break out downslope decades later and still erupt very fluid lava (like the Keaiwa 1823 flow). During the time this happens, the crystallizing minerals are about half of them pyroxene and half plagioclase.
Anorthite-pyroxene fractionation leaves a marked chemical imprint in that, as the magma evolves, Fe2O3 and TiO2 become enriched (unlike in other forms of fractionation), while CaO and Al2O3 are depleted. The main culprit of this process is probably the calcic variety of the mineral plagioclase, known as anorthite, with the formula CaAl₂Si₂O₈. Anorthite is more stable at lower pressures; it’s clearly what depletes the calcium and aluminium, and probably also helps increase the iron and titanium by displacing other mineral types that would take them.
The diagram I used to sort Aleutian volcanoes into their respective chemical series. It’s meant to gauge the amount of anorthite-pyroxene fractionation the magma has undergone by comparing the concentration of titanium and iron (enriched in anorthite-pyroxene fractionation) to that of calcium and aluminum (depleted).
I’ve used the chemical data available to sort most of the volcanoes into different “iron” series depending on the amount of pyroxene-anorthite fractionation they’ve seen. Low Fe volcanoes will have seen no or very little. The following is the list of volcanoes and the group that they belong to; the numbers in parentheses are the melt silica contents, obtained earlier from the minimum lava flow viscosity.
Low Fe series: Spurr (66), Augustine (75), Douglas-Fourpeaked, Buldir (78), Ingenstrem Depression, Western Cones
Medium Fe series: Redoubt, Kaguyak, Snowy Mountain, Katmai Group (65), Yantarni (78), Emmons Lake (63), Amak (74), Frosty (78), Bogoslof, Rechesnoi-Vsevidof (62), Atka (57), Koniuji (77), Great Sitkin (76), Adak (78), Kanaga (68), Bobrof (78), Tanaga (64), Little Sitkin (75), Kiska (73).
High Fe series: Aniakchak (61), Veniaminof, Shishaldin (53), Fisher, Westdahl (60), Akutan (61), Makushin, Okmok (58), Seguam (55), Semisopochnoi (63).
Map showing the iron content of the Aleutian volcanoes. Blue marks the low-iron series, which are also volcanoes with a strong high-Mg-andesite/calc-alkaline chemistry, located near both ends of the arc. The green triangles are volcanoes with a medium-iron series, the most abundant, and probably making up almost the entirety of the eastern and western segments of the arc. And the red triangles indicate volcanoes of the high-iron series, dominant in the central segment, almost all of which are proficient explosive caldera producers that build shields of lava flows and ignimbrites around their central calderas.
I found the results to be surprising. Once again, the differences between the central segment and the side segments emerge. The volcanoes with a strong high-Mg andesite chemistry also have low Fe; this may be because their inherently high silica or other chemical characteristics block anorthite-pyroxene fractionation altogether.
But what’s most interesting is that the high-Fe series volcanoes, all of them except Shishaldin and Westdahl, have collapsed calderas during the last ~13,000 years. What’s more, some of them have done so twice. Aniakchak, Veniaminof, and Okmok have each collapsed twice during this time, and their calderas are all about 9 km across! And even better, the areal-largest nine Aleutian caldera-forming events (those at or beyond 4 km diameter) of the last 13,000 years have ALL come from the few high-Fe group volcanoes. Why is this?
Well, while I don’t know all the details of the anorthite-pyroxene fractionation, it seems to benefit from low pressure (shallow magma storage) and basaltic melts. I speculate that the volcanoes that belong to the high Fe series have basaltic/basaltic-andesite melts reaching their shallow chambers, only a few km below the surface, where they can crystallize in this style for a long time and undergo the corresponding chemical changes. The primitive Al-rich basalts rarely make it to the surface, except at Seguam. This low-silica melt reaching their shallow chambers will be highly fluid, similar to volcanoes with known lava lake activity, like Masaya or Ambrym, and will thus allow magma convection even in relatively narrow conduits. As such, convecting mafic melts between the asthenosphere and the shallow storage of these volcanoes, I think, could be helping sustain magma chambers that stay molten thanks to this steady supply of heat. And thanks to their thin roofs, collapse easily, producing caldera-forming ignimbrite eruptions at short intervals.
The ignimbrites are sometimes mafic-intermediate (Veniaminof, Okmok, Akutan, Makushin and the western caldera of Seguam), sometimes silicic (eastern caldera of Seguam and Semisopochnoi), or sometimes both (Aniakchak and Fisher). As another reflection of their mature caldera style, some of these volcanoes have a ring of vents within their caldera along what must be the location of their outward-dipping ring faults. A heavily intruded ring fault. Their edifices can be lava shields, with massive piles of lava flows coming from flank and summit vents (Veniaminof and Westdahl), or almost pure pyroclastic shields with an output that largely goes into the caldera-forming eruptions (Aniakchak, Fisher, Okmok, or Semisopochnoi). The central segment, understudied and data-lacking volcanoes of Amukta and Yunaska, I suspect, also to be in the high-Fe group if samples were available, and Yunaska is definitely a Holocene caldera-forming eruption producer.
View of Okmok as perhaps the most characteristic Aleutian high-Fe volcano. It has a 10 km caldera surrounded by a shield edifice. Some 2 million-year-old eroded remnants show that this shield was present and likely taller back then. The reason for this must be that the ignimbrites usually interact with thick ice cover, generating lahars that go into the ocean; a lot of the pyroclastic material also directly flows into the ocean, given the small size of the island, and doesn’t contribute to the edifice growth. Okmok collapsed 2,000 and 13,000 years ago during massive eruptions that may have had around 20-40 km3 DRE. The 2,000-year-old event was one of the worst historic volcanic winters, described in Roman and Chinese documents, and obscured the Sun for several months. The lava in these explosions was basaltic-andesite, very fluid, which reveals the common misconception about fluid magmas and their supposed inability to cause large explosive eruptions. Post-caldera vents are arranged in a ring within the caldera; one of them produced a VEI 4 in 2008. Photo by Cyrus Read: https://avo.alaska.edu/image/view/13283
Concluding remarks
The Aleutian Arc is a mid-productivity arc. Volcanism seems organized into an interesting structure that changes from the center to the sides. The central area has higher eruption rates and more fluid lava. I speculate that many of the central systems have long-lived shallow magma chambers and convection of fluid magmas into their base, which allows them to become recurrent sources of caldera eruptions. Eruption rate decreases towards the edges and gradually fades into nothing towards its western extremity.
Volcanoes are varied. I haven’t had a chance to look much into each of them separately, but I may do a future article doing so. There’s still much to learn from them. I hope you all enjoyed this short but dense overview of its volcanism.
Sunset over Shishaldin and Isanotski stratovolcanoes. Photo by Cyrus Read: https://avo.alaska.edu/image/view/4309
I’ll need more time to dive deep in this excellent, full of data, post! Congrats, Hector!
Thanks, Renato!
Fascinating, and thanks for all of this!
Curious to get your opinion on a few related items.
1. The Fisher caldera formed supposedly around 9300 BP, and produced a massive 11×18 caldera. But there doesn’t seem to be other enormous signatures in the volcanic record an eruption of that size around that time. I think I’ve even seen you speculate that the dating is not correct. Is it possible / likely that this was the largest Holocene eruption instead of what we currently believe to be the largest (Kikai)?
2. The High FE series volcanoes seem fairly unique to the Aleutians in their character. But one volcano that came to mind that feels somewhat similar to these is Masaya. Similar to the Aleutians calderas, it’s a pyroclastic shield in an arc that has a very basaltic magma source, yet still produces large caldera eruptions. Are there similarities here in magma chemistry? Asking as a very chemistry illiterate person.
3. Volcanoes like Shishaldin that frequently erupt somewhat effusively are quite large and have a lot of the characteristics you mentioned to be similar to the big caldera eruptors. Shishaldin also sits next to the most prolific of those calderas in Emmons lake. Do you think there is a high likelihood that Shishaldin will become similar to the other caldera forming neighbors in time, and is just a more juvenile iteration of the the more mature caldera systems?
4. There was a research project a while back indicating the presence of a very large caldera, of likely high VEI-7 or maybe even VEI-8 size centered around the cluster of volcanoes where Cleveland currently exists. Any thoughts on how that would fit into everything here?
I was retaking a look at the sulfur spike chart Héctor provided in his section of the ‘Crisis on Ioto’ article (
?resize=700%2C338&ssl=1) and I noticed there were no sulphate spikes as high as Kikai’s supposedly were.
Could it be that Fisher was bigger than Kikai but was much less sulphuric content in it’s erupted material than Kikai? Or, and this might understandably be a LOT less likely, perhaps the Fisher caldera wasn’t formed in a single eruption?
Makes me wonder too how big Fisher must’ve been if it was indeed a single eruption compared to how big Emmons Lake’s eruption(s) were. One figure I’ve seen for what I think its most recent caldera forming blast is 220 km3 bulk tephra deposits. I think you’ve mentioned before that if your theory on Fisher’s eruption size is true, that puts a bulk tephra number well over 220.
Also makes me curious about the relationship between repose periods and CFE’s. Based on at least what I’ve read, virtually all of the time a long repose period is what helps produce the VEI 6< sized eruptions. But after Tonga, I am curious about if which volcanoes have exceptionally short repose times between eruptions but have in the past still produced major CFE's, or, if they haven't yet, are still capable of doing so. I wonder if it's only specific arcs are able to produce CFE's after a shorter rest periods between eruptions, or if it's commonplace on arcs across the globe. For example, I think I remember you saying in a past comment that Shiveluch may be capable of a CFE despite it being for a long time one of the most active volcanoes on the planet. I mention all this because I look at the High Fe volcanoes Héctor mentioned in this article that haven't produced a CFE and wonder how close they may be to producing one (more than likely not the geological near future but still).
Also I have some doubts about the area around Cleveland being a "supervolcano" theoretically capable of producing a low end VEI 7, let alone a high end 7 or even an 8, given how small the supposed caldera ring of the Island of Four Mountains is. Looks to me like a potential caldera much smaller than say Fisher, Emmons Lake, Aniakchak, Okmok, etc.
Looking at the above map the Cleveland cluster is in that gap as we transition into thicker crust just east of Seguam – notably next to Yunaska and Okmok. I would suspect high-Fe, though I doubt a VEI-8 has taken place (unless it was early-mid Peistocene) as this would surely be notable in samples taken from nearby islands, if not the mainland.
Hi Cbus, thanks. I’ll try to address these questions:
1: Be careful because there are two types of radiocarbon ages: conventional ages and calibrated ages. Conventional ages (Libby’s age) are the older ones, which are off, but still often quoted in articles, and if they say just BP then it’s likely that. At one point, it was realized that the carbon isotopic composition of the atmosphere has fluctuated over time, which led to a new slightly longer half-life and the need to use a calibration curve, and this correct age is a calibrated age, usually put as cal BP. I usually calibrate them in OxCal, where it’s rather simple and intuitive. When calibrated, the ages on Fisher fall nearly coincident with the massive 9225 BC spike, which is also high northern latitude, so quite fitting.
2: Indeed, Masaya is one of the most iron-enriched subduction zone lavas. It’s not an uncommon chemistry, though, and I’m planning on downloading a lot of data and checking which volcanoes do have it. I do recall, once, using Masaya or some similar case and looking for other chemistry data samples that fell close in composition (other iron-rich basalts), and they did show up in several arc volcanoes: Vanuatu, Llaima and Villarrica, Izu-Oshima and Miyakejima, Tofua, I think Gorely, among others. I do expect the high-Fe series is also quite common in Central Sunda, New Britain, and possibly nearly always present in Izu-Bonin and Tonga-Kermadec arcs.
3. I could see Shishaldin eventually turning into a caldera. However, it’s an odd volcano overall. It has more MgO than the other Aleutian volcanoes, occupying a unique position. I think the basalt rises through the lithospheric mantle and Mohorovic without ponding much, if at all, compared to the other volcanoes, and arrives quite primitive at minor crustal storage, where it crystallizes anorthite and pyroxene. Now extinct Isanotski seems to have been similar to Shishaldin and did not collapse, but maybe if magmatism stays longer at Shishaldin it can manage.
4. I don’t know. But I don’t see any indication of a caldera here. The group is not really too productive and it’s the high-viscosity, high-crystallinity exception in this area. Shallow bodies may exist at Tana and Kagamil but they would be low-end VEI 7 sized at best, if they do ever collapse.
Thanks for the replies.
Fisher definitely would warrant more research if it’s the largest Holocene eruption, which is at least a possibility. Given, as I look at the caldera dimensions, it’s somewhat comparable to the Kikai Akahoya caldera, so it may still be smaller than that.
Masaya and the entire Managua region are fascinating to me for a variety of reasons, mostly due to how it’s “different” from typical arc subduction volcanoes. Also a much higher risk region for so many different reasons.
As for the supposed caldera near Cleveland volcano, here is a short research summary on it. It was a volcanic news item around 2020. https://news.uaf.edu/wp-content/uploads/2020/12/IFM_Caldera_AGU_2020_July28.pdf
Masaya is a fascinating volcano. Really, Masaya and Taal make up their own unique class of volcanic systems.
1212 c for Masayas lava lake making it one of the hottest subduction basalt lavas ever known outside west mata. Masaya maybe also feed by its rifting related to the tectonic setting
Masaya is definitely a product at least in part of the rifting environment it exists within.
I am personally interested in Okmok and it’s history.
While in the Coast Guard, I came within 3 miles of the Caldera.
It is a fascinating island full of Reindeer, lots of grass and moss, snow, and lots and LOTS of rock and gravel!
Interesting experience.
How did the reindeer get there? Was it connected to the mainland at one point?
Do they exhibit any island dwarfism?
reindeer are not native to this part of the world. They must have been introduced
Correct. They were introduced by the native population many years ago. They MAY have just walked or even swam over there, on or along the ice edge, during the winter time, as well. Even though the currents around the islands can be quite strong!
Hector this is fascinating stuff.
Shishaldin and Westdahl may have not yet collapsed into calderas as high-Fe volcanoes due to Fisher which as you say has had one of the largest outputs and at least 2 caldera events, and is likely dominant in the area.
It is worth noting however that Roundtop reportedly had a large VEI5 rhyolite eruption about 10,000 years ago. Shishaldin itself is likely a somma-volcano also with an ancestral volcano having collapsed in the same spot – the most recent edifice is relatively young.
Thanks, Andy.
Excellent! this old russian area is very productive magmaticaly and some individual volcanoes there are much more productive than many “weak hotspots” on Earth. The Arc smd its neighburs shares some of the planets largest mafic /intermediate subduction zone complexes with Kamchatka and some Asian systems where many 100 s of km3 per complex sometimes nearly a 1000km3 is possible souch a monster is Klyuchevskaya complex ( which can have many daugther volcanic peaks ) can be possible in some cases. This is a edifice volume thats rare among many subduction zone stratovolcanoes who often struggle to reach beyond 200 km3. Ambrym and Tambora and some indonesian systems is also absolutey monsterious compared to many small subduction complexes like Colima.
Aleutians shares some of the geological local features of these that the area is quite young oceanic crust and because of that the litosphere there is quite thin its magmatic fluids and astenospheric materials are not welling up under a massive old continental platform like Andes or western US and subduction there is also fast alot more subduction magma is allowed to surface there without extreme massive evolution ( differentiation ) than in example Andes plateau that pushes our sillic mushes. Alaska is hot, thin and flexible proto – continetal crust perhaps similar to an archean protocontinetal arc
These special subduction areas are places where magma haves quite easy to reach the surface which is not always the case in souch a geological setting
The west Alaska crust area have not have time yet to cool down because intrusions and magmatic chamber formations are simply way too active keeping the geothermal gradient high at depth and the volcanoes happy
Hi Jesper. Indeed, some of the Aleutian volcanoes are large, though the arc as a whole is a bit average I think. There is likely a good number of subduction zone volcanoes that have erupted over 2,000 km3 in the last 1 million years (Kamchatka and Sunda, as you say, being some of the more notable places). Over 5,000 km3 is also achievable, and I think at least two complexes have such a high output.
Thank you, Héctor! What a thorough and fascinating article. Great read!
Thanks Clive.
Great article Hector!
Since the floor of the Pacific is driving WNW, wouldn’t that mean the crust is older (and cooler) at the Asia end of the Aleutians than than to the east?
Might this have an effect of the slope of the subducting plate, thus differences in the type of vulcanism such as melt/viscosity, eruption frequency, etc?
Thanks Craig. That’s right, it’s older at the Asia end. Although over the active arc, east of Buldir portion, the variation is not huge and stays within 50 to 60 million years old according to ocean crust age maps. The reason is that this ocean crust was generated at the spreading ridge between the Kula and Pacific plates. Locally at the eastern end of the arc there might be some younger ~45 million-year-old crust under the volcanoes, and the oldest does so near Seguam.
But for example, Great Sitkin and Emmons Lake get crust about the same age of 55 Ma, while the arc is much more productive around the second one. So if anything is related to the ocean crust age it would maybe be the rifting at Seguam (due to older heavier lithosphere doing rollback), which I hadn’t realized earlier. The large variations in the type of volcanism and eruption rate I think, are unlikely to be related to the subduction rate or ocean crust age, which instead vary very little over the arc.
Thank you Hector for the additional thoughts!
I had not considered the residual compositional changes in the subducting plate following the demise of the Kula plate which finished it’s subduction around 48Ma ago. https://en.wikipedia.org/wiki/Kula_plate
That the Kula plate was compositionally different (it was a separate entity not unlike the the Farallon Plate here on the west coast of NA) than the Pacific and NA plates, the timing would suggest the peak subduction period probably occurred around 60Ma, and correlates (roughly) to the emergence of the Aleutian arc around the same time (according to your estimates). Initially, I would think that the early volcanism along the arc would be erupting Kula material before transitioning to Pacific plate material once the Kula plate got shoved further north under the Bering Sea?
I wouldn’t mind seeing a breakdown of Arctic tectonics. Not just around the Bering sea area (possibly Bering plate) but into the Arctic Ocean itself. There’s a lot of volcanism that we have little current knowledge of such as the North Alaskan volcanics (Espenberg, Imuruk Lake etc.) as well as the Siberian offshore islands like Kotelny and beyond.
Thanks for your in-depth-essay about the Aleutes Chain!
The Aleutes host the currently longest continuously erupting volcano of USA: Great Sitkin. It does it slowly with the speed of a lava dome eruption, but “lava flows have filled most of the summit crater and advanced into valleys below.” https://avo.alaska.edu/volcano/great-sitkin/activity
Thanks, volcanophil. This is an interesting animation: https://avo.alaska.edu/image/view/195241
During the 20th century Great Sitkin only did small explosive eruptions up to VEI2. The 2021 beginning eruption is different with a much higher volume and a purely effusive eruption. The eruption has the speed and behaviour of a glacier.
Did the magma degass during the 20th century that has been erupted since 2021?
The slows Andesite or Basaltic lava in this case behaves like some Rhyolite lava flows.
Great article, the only question i can think of is whether we should expect “half arcs” (where if there is somewhere else a similar plate is diagonally subducting) to get half of a similar pattern in terms of chemistry?
There are all sorts of patterns for each arc I imagine. There indeed are probably some where the higher productivity and more fluid lavas erupt near one end (Kamchatka-Kuriles, maybe).
Hello everyone! Excellent article!
I’ve been following Loto Island on FIRMS for several months now, and today it looks like there’s quite a barbecue going on. It seems there are phreatomagmatic explosions occurring around the island, particularly along the beaches.
Could these explosions be contributing to the formation of a caldera ring fault around the island?
You can see the FIRMS images here:
https://firms.modaps.eosdis.nasa.gov/map/#d:24hrs;@141.331,24.782,13.340z
There is already a caldera ring-fault that the volcano uses to erupt. Ring-faults aren’t formed by repeated small eruptions like the ones happening at Ioto. Using FIRMS isn’t wholly reliable as the hot sand and small meaningless hydro-thermal explosions can send signals.
Pyrocumulonimbus clouds:
.webp
https://www.bbc.com/news/articles/c2350xnl4j5o
A volcanic ash plume is usually driven upwards by the heat like a balloon. So it’s a subtype of the Pyrocumulonimbus cloud, and we know that Plinian Plumes can produce nice lightnings.
Breaking: another Shindo 7 earthquake in Kumamoto Prefecture. Appears to be on the Hinagu fault zone, slightly southwest of the 2016 earthquakes.
Reported as M6.8, but shallow with damage.
It and its aftershocks are awfully close to Aso-san.
USGS does not report the depths as anything other than 10K which is the default, right?
This was genuinely shallow. That’s why it maxed out the Shindo scale. I assume it’s related to the 2016 sequence.
5.2 Magnitude quake at Bardabunga with a few fore and after shocks, the one we have been waiting for?
Very early with not much increasing seismic activity beforehand so this was somewhat unexpected to me. Then again the swarm last month was not the typical m5 sequence and we are sitting on nearly a km³ of added magma so surprises can always happen.
The last large quake, the one with the swarm, was only M4.8. Converted to linear scale, that’s a factor 4 less released seismic moment than the current M5.2. It’s not very surprising with a proportionally shorter time before the next one.
The lack of a gradually increasing seismicity is a bit surprising though.
IMO on the M5.2:
Magnitude 5.2 earthquake in Bárðarbunga (RÚV, 28 Jul)
Bárðarbunga is still the classic volcano with the last eruption in Iceland 2014-15. Will it be the next one? I guess that this is uncertain, but possible. Bárðarbunga does currently more visible activity than the others.
During the 2014-15 eruption the caldera had many quakes during subsidence. In January 2015 there were some Magnitude 4.5 quakes. Do we now see the counter-movement to this movement?
If the caldera of Bárðarbunga was free of ice, could we see the rising caldera floor over time?
Theres been multiple studies talking about these earthquakes being the reverse of the collapse quakes. Bardarbunga’s caldera floor seems to be rising 2-3m per year so i wouldnt know a reason why u shouldnt be able to see it if it were ice-free.
Kilauea is building up very fast now towards the next episode. Based on the tilt plots, it could begin within the next day
Inflation is very steep now, after a period of three days with soft deflation:
The “forecast window for the start of episode 52 fountains” is now “July 29 through August 2”. https://www.usgs.gov/volcanoes/kilauea/volcano-updates
The past 24 hours or so it’s recharged at an amazing 4.1 µrad/day, or about 17.3 m³/s. Don’t recall that’s occurred previously during this eruption as part of a D/I or other similar events. Could result in E52 already by the evening HST, assuming it continues.
The first overflow already happened
Popcorn time. Episode 52 of the Kilauea soap opera
Today’s best viewing is from channel V1
You do wonder whether HVO will commercialise it and sell the viewing rights to FIFA
Once north vent stopped smothering south vent in ejecta, the south vent started flaming.
According to HVO the volume of E52 was 4.7 million cubic meters. E52 alone had 62% more volume than the September 2024 eruption in Napau crater with 2.9 million cubic meters, although that eruption lasted for five days, while E52 lasted for appr. 7 hours.
Born too early to see the Aleutians go wild, born too late to see the Aleutians go wild. Born at the right time to study the past and ponder the future.
Héctor:
I was working with another researcher on Aleutian Arc volcanoes and their precursors. At one time, we were very concerned about 6 possible volcano eruptions becoming active. This was in the time period 2006-2010.
What fascinated me was the earthquake sequences connected with the Kasatoshi Volcano. My colleague and I were aware that this volcano was primed for a major eruption (which occurred August 7, 2008) and we were very concerned about an unexpected and violent eruption occurring.
I watched as magma moved very quickly from the Aleutian trench to the volcano, in a matter of just few hours, as evidenced by the earthquake sequences. It was very obvious that magma was moving quickly here. Finally a magnitude 5+ quake occurred which was the final announcement of this eruption. Thankfully the two scientists were successfully evacuated less than 30 minutes to this major event.
This movement of magma from the ocean trenches to surface volcanoes needs to be more carefully examined and recognized.
Thank you for your post.
Was the formation of the Aleutes Arc related to the development of the Pacific Plate? During the Cretacous Era the Pacific Plate substituted three older oceanic plates in the Pacific/Panthalassa Ocean: Farallon, Phoenix, and Izanagi plate.
Etna is starting its second paroxysm of the day, lots of ash and very rapid build-up both times. Voragine again.
There is a nice pattern to the changes to the tilt at Kilauea
There is a slow increase in the average tilt. But the range between top and bottom is slowly decreasing. At this rate, the range will be zero in March 2027, and will be at the same level as at which time the episodes would have ended. At the time, the tilt will be around what it was in early January 2025, after the December 2024 eruption and at the start of the episodes.
This assumed everything continues smoothly. Volcanoes tend to spring surprises at some point.
Does this development precede the creation of a lava lake like 2008 (Overlook Crater)? 2008 the lava lake was born after 26 years without no summit eruptions. This time it may happen after an episodic summit eruption. There is already subterranean magma below the two vents. 2008 the lava lake began with a collapse event in March 2008.
A sort of convecting lava lake is already present at the south vent of the eruption, albeit it’s small.
I think it is unpredictable. The December 2024 eruption emptied quite a bit of the shallow magma. Since that time, the episodes at first kept the situation (or at least the tilt) stable, but with further significant decline in late 2025 and a return to slow net inflation in Spring 2026. I wouldn’t read too much into it. Tilt is more sensitive than the GPS but also less informative. So many things can affect it. What is clear is that major refilling of the magma reservoir is not possible at the moment because of the open vent. The episodic nature seems a feature of a late stage of an effusive eruption (also seen in Reykjanes). There is no reason to expect a major summit eruption in the near future. Chad may be right that it will end with a rift break out, but I expect that to be a minor event.
But Pu’u’O’o lasted 30 years. Could the summit become stuck in the current state for decades?
I imagine a next step towards an open, convecting lava lake in which the episodes continue to occur. The Kilauea Iki eruption 1959 behaved sometimes like this. https://www.nps.gov/havo/learn/nature/kilauea-iki.htm
?maxwidth=1300&maxheight=1300&autorotate=false&format=webp
The increase in average inclination, plus the lowering of the caldera floor recorded a few episodes ago, indicates that a lava basin already exists several meters below the surface, and is quite large. I believe the north and south craters are currently the vents for this lava basin; when sufficient pressure builds up, the current episodes we’re counting occur.
I believe the current pressure isn’t sufficient to breach the surface, but the increase in average inclination could be a clue. A few episodes ago, after the eruption, a lowering of the caldera floor was recorded; this, in my opinion, indicates depressurization. I believe that when the average inclination increases to a certain level, something will happen; we’ll likely see a collapse of a portion of the caldera floor accompanied by massive degassing, and we’ll probably actually see this open-air lava lake.
This is just speculation.
Etna feeling jealous of the coverage Kilauea gets!
It seems there was a shallow earthquake at Solfatara. 4.5
https://earthquake.usgs.gov/earthquakes/eventpage/us6000th99/executive
There are reports coming in of damage. Depth is reported as 2.6 km
oh
M3.8 aftershock slightly southwest of the original quake.
8th M4.0+ in the vicinity of the inflating area around Solfatara within the past 18 months?
That was an ugly, bad one with “extensive damage”. https://www.euronews.com/my-europe/2026/07/31/strongest-earthquake-in-years-strikes-volcanic-area-near-naples-injuring-four
70 years before the Monte Nuovo eruption 1538 earthquake activity increased significantly and accelerated in the years towards the eruption. So it can still be a long hard way with many damaging earthquakes, until an eruption occures. But the next eruption may be different to Monte Nuovo and unfold faster. Monte Nuovo was a phreatomagmatic eruption with (wet) magma getting to the surface to build the “New Mountain”. A phreatic eruption that only ejects cold tephra (crushed rock) may occur with fewer warning signs and time.
Alaska was home of the most recent Maar Eruption in the world: https://volcano.si.edu/volcano.cfm?vn=312131
Lago d’Averno shows that Campi Flegrei can do similar eruptions.
Report on the swarm at Solfatara
https://www.theguardian.com/world/2026/aug/01/scores-injured-as-earthquake-hits-near-naples-in-seismic-swarm
here is INGV update. (giggle translate works)
https://ingvterremoti.com/2026/08/01/sciame-sismico-ai-campi-flegrei-comunicato-di-aggiornamento-del-1-agosto-2026/
I have 10 000 s of VC comments Is there a more effective way to search this?
I needs serious help … please any group sorting / search instead of just manual ? cannot find tools ti handle commentaries when I logs in in wordpress either .. : (
You can try a specific search VolcanoCafe on a search engine, like this:
campi flegrei site:volcanocafe.org
I just stuck in campi flegrei as an example. The syntax is the same with other search engines like Google. This probably isn’t what you want, but may give you some ideas though.
Unfortunately neither Bing nor Google seem to categorize VC comments though, as I tried to find one of your Io comments just now by forcing the search engines with an exact quote. Didn’t work.
The DI events of Kilauea’s cycles show some regular behaviour: The first deformation begins, if inflation reachs near the level of 0 microrad. We see this in the first July cycle (E51) as well as in the second July cycle (E52) and the current August cycle towards E53.
I assume that inflation accumulates magmatic pressure (N/m²) that at a certain point breaks a barrier to a magma cave somewhere. The same happens at present around every 5 microrads. It looks as every 5 microrads there is a chance for a DI event. This will likely continue as long as the magma cave preserves its structure.
It seems true for the past few episodes, but not further back. I am wondering whether it is related to the shape of the conduit, perhaps hitting a bifurcation at this point. This structure may be changing quite quickly
Is there a growing intrusion that builds a future magma chamber? This magma chamber may become the base for future eruptions in Halema’uma’u.
During the 19th century the lava lake in Halema’uma’u had a large diameter and area compared to the tiny Overlook lava lake 2008-2018. A broad lava lake like this needs a shallow source, that isn’t as deep as the present source of Kilauea’s episodes. Maybe the shallow source is in slow formation now with the DI events as indicators.
It has a long way to go before the tilt is as high as it was before the start of this eruption, now 20 months ago. The GPS (UWEV) has been more stable over this time. I think the eruption is fed by fairly shallow lava already, which initially became depleted but over the past few months has recovered a little.
An M5 earthquake recorded at the Suez Canal
https://www.youtube.com/watch?v=1b4tQJS6bjk
Fuego
New post is up! The need for observatories
https://www.volcanocafe.org/observing-volcanoes/