How the Composition of the Crust May Affect Volcanism

 

A TAS-diagram; total alkali versus silica (TAS) diagram. Another way to look at it is to replace the Na2O + K2O on the y-axis with “feldspathoid component” and the SiO2 on the x-axis with silica or “quartz”. This relationship is a major point of the present article. (After LeBas et al., 1986, Fig. 1)

A TAS-diagram; total alkali versus silica (TAS) diagram. Another way to look at it is to replace the Na2O + K2O on the y-axis with “feldspathoid component” and the SiO2 on the x-axis with silica or “quartz”. This relationship is a major point of the present article. (After LeBas et al., 1986, Fig. 1)

Before I start on the subject of this post, we would like to thank everyone for their input and support of VC recently! It is good to see so many of our contributors, old and new, and some of the input is truly staggering in its quality. Thank you! Also, we will let the the Scientific Project post run for a while longer before we follow it up, so please, there is still time to search and post your contributions! The more, the better! And now onto today’s post which is speculative:

In his latest installment on Mt Cameroon, Carl remarks that “The ensuing volcanism is highly programmatic and follows a pattern where the volcanoes are born through large scale basalt eruptions creating layers between 50 and 600 meters thick. After that comes a period of trachytic lava with minor rhyolitic ignimbrites, after that comes a large caldera event… …There is no good explanation to why the basaltic eruptions during a fairly short time switch to highly explosive volcanism.”

Ol Doinyo Lengai, the unique volcano in Tanzania that erupts carbonatite magma (uncredited image on questhorizontanzania.com)

Ol Doinyo Lengai, the unique volcano in Tanzania that erupts carbonatite magma (uncredited image on questhorizontanzania.com)

The same might be said of volcanism everywhere. How come there is such a wide variety with the eruptions of some volcanoes being so very explosive, the lavas they erupt being of such greatly varied composition that one, Ol Doinyo Lengai, even erupts carbonatites? After all, isn’t all volcanism driven by basalt from the Earth’s mantle and basalt is basalt is basalt, remarkably uniform in its chemistry and temperature as it leaves the mantle for its journey towards the surface?

One answer that is given is that if magma sits long enough in a magma chamber, it will begin to cool and as it does so, minerals begin to drop out of the solution as their solidification/crystallisation point is reached. This changes the composition of what remains from mafic to progressively acidic as more and more minerals crystallise out and the remaining solution moves from basaltic through andesitic then dacitic until what remains is rhyolitic. But this takes so many thousands, if not tens or even hundreds of thousands, of years for the very large bodies of magma required to explain some of the larger eruptions known. It does not explain how some volcanoes very quickly turn from mainly effusive or strombolian basaltic eruptions to explosive eruptions of large quantities of evolved magmas. And it does not explain how the thrachytes, phonolites or various other varieties of evolved magmas, not to mention carbonatites, are produced. The answer must be that the chemical composition of the crust plays a major role in the formation of evolved magmas, one not yet fully recognised.

Dodecahedral (12-sided) crystal of the feldspathoid mineral Leucite associated with the Italian volcano Roccamonfina (WikiMedia Commons)

Dodecahedral (12-sided) crystal of the feldspathoid mineral Leucite associated with the Italian volcano Roccamonfina (WikiMedia Commons)

Some years ago when I was doing research for the Roccamonfina article, I came across an interesting piece of information. Roccamonfina is a large, dormant or extinct stratovolcano with an associated caldera about 6 x 7 km wide located about 60 km NNW of Naples. During the major part of Roccamonfina’s “life”, it erupted magmas rich in the feldspathoid mineral leucite. Only towards the end did they become leucite-poor. In one of the four then-available papers on this volcano, there was some pertinent information on the underlying geology. Because Springer Publishing have now appropriated the rights and demand $39.95 / €34.95 / £29.95 for each of them – an unmitigated curse on that greedy, moneygrubbing obstacle to scientific progress – they are no longer available to us, nor can I point out exactly where I obtained my information, a basic tenet of science.

The salient information was that underlying the the top 10 km of crust, there is an equally thick layer of sediments underlying all the Campanian region of Italy. Interestingly, in the vicinity of the Roccamonfina volcano that layer has been depleted and is no more than 5 km thick. Another aspect of Roman volcanism, Pliocene to recent, is that it is relatively rich in potassium.

Another interesting observation is that so many Arc volcanoes, that is volcanoes overlying a subduction zone, are characterised as bimodal. They mainly erupt basalt or basaltic andesite and even if not all follow the same pattern slavishly, as an eruption progresses, the erupted material becomes more basaltic in composition. Typically, their eruptions are in the VEI 2 to low VEI 3 region, but occasionally they have much larger ones, often erupting more evolved magmas such as dacite. Eventually, most of them seem to have caldera-forming VEI 6 to VEI 7, ingimbrite eruptions of dacitic or even rhyolitic magmas. Now why is this?

Schematic of the Earth’s upper crust and mantle showing the location of the Benioff Zone which is where as inferred from the location of very deep earthquakes a subducting continental plate slides dow into the mantle (Wikimedia Commons)

Schematic of the Earth’s upper crust and mantle showing the location of the Benioff Zone which is where as inferred from the location of very deep earthquakes a subducting continental plate slides dow into the mantle (Wikimedia Commons)

Let us start at great depth, at the Benioff Zone some 150 km or more down where the subducting plate is reabsorbed! Here, all water carried down with the kilometres or even tens of kilometres thick sediment layers “evaporates”. In effect, the subducting slab is being dehydrated and the superheated water begins to rise upwards. As it does so, it enters into solution with the basalt of the mantle. Not only does it rise upwards being of lesser density due to the water content. At such temperatures and pressures, water acts as a flux, a catalyst if you will, that greatly enhances the speed of dissolution of already formed minerals back to their basic constituents.

As it rises, it will eventually rise above the mantle proper into the Astenosphere, the very top layer of the Upper Mantle, which is semi-solid. As the ~1,400 degrees C hot “glob of juvenile magma” eats its way through the Astenosphere, it also reheats and dissolves the astenospheric minerals. As they are basaltic in nature, more basaltic magma is formed but the temperature is somewhat lowered. The next resistance to its progress is the underside of a continental craton. This usually consists of old oceanic crust, again basaltic, so yet more basalt is formed by remobilising and the temperature again drops slightly as it eats its way through the approximately 1,100 to 700C hot old oceanic crust.

Once through the old oceanic crust, the juvenile magma quickly finds a path through the overlaying sedimentary layer and the topmost layer containing the most recent deposits. it does so quickly due to two factors, the chemical composition of the overlying layers and that being of progressively lower temperature as we approach the surface, they become progressively more brittle as well. Eruptions at this stage are basaltic, very large and usually form a broad shield that can be several tens to hundred kilometers in extent. Initially, because of the resistance it encountered, there was a lot of pressure built up which is why so much material was erupted initially. Once over, the conduit through the uppermost layer quickly solidifies which means the blob has to begin to build up more pressure for a new cycle to begin. It is now the fun begins.

Schematic showing the crust, divided into an upper layer and a sedimentary layer overlying the ancient ocenic crust, and the Upper mantle with the semi-solid Astensophere overlying the semi-liquid mantle. (Author)

Schematic showing the crust, divided into an upper layer and a sedimentary layer overlying the ancient ocenic crust, and the Upper mantle with the semi-solid Astensophere overlying the semi-liquid mantle. (Author)

The sedimentary layer is composed of weathered minerals, primarily quartz and feldspar clays with a large amount of water intermixed. At depths of several kilometers, pressure and temperatures are such that it metamorphoses into sandstone – there is a slight surface remelting which fuses the individual grains together. (Yes Dr Behncke, I remember. Etna is known to have erupted sandstone!) Left long enough, deep enough so that it is hot enough, this will eventually metamorphose into gneisses. But if you emplace a body of basaltic magma, usually well in excess of 1,000 degrees Celcius, plus the abundant presence of water acting as a flux, this quickly remobilises – melts, dissolves – the sedimentary minerals. And quartz and feldspar are the building blocks of evolved magmas such as dacite and rhyolite.

At first, they enter solution with the basaltic magma left behind. Because quartz (rho 2.65) and feldspar (rho 2.55 – 2.76) are lighter than basalt (rho 2.8 – 3.0 with the higher value for juvenile material), the resulting basaltic-andesitic mixture is lighter and thus rests on top. If there is an eruption now, the lightest magmas will thus erupt initially and once they have cleared the vent, what follows becomes progressively more basaltic in nature. This is evident from a plethora of volcanic eruptions. At this stage, an andesitic stratovolcanic edifice is built over the initial shield.

Lake Taupo as imaged by Landsat (WikiMedia Commons)

Lake Taupo as imaged by Landsat (WikiMedia Commons)

But with each eruption, the sedimentary layer is progressively being depleted which leaves room for more juvenile magma to enter. The larger the “cavity” becomes, the greater – exponentially greater – the surface area becomes and the larger the surface area, the more material is remobilised. A self-perpetuating magma chamber has been born, one that depending on the chemical composition of the sedimentary layer produces different magmas. The greater the content of feldspars in relation to that of quartz, the more trachytic the magma produced. At one point, this magma reservoir will have grown so large that it produces large quantities of evolved magmas, geologically speaking, quickly. There is too much of the stuff produced for it to mix with the basalt to form andesites. In time, you will have very large dacitic to rhyolitic ignimbrite forming eruptions.

As the stratovolcanic edifice grows, more and more of the sedimentary layer is depleted and the top crust layer begins to sag under the weight. Because of the sag and because topside, rocks are not ductible at all, concentric cracks begin to appear at the perimeter. An eruption now will be so great that the top collapses, pushing out even more magma. You have a caldera-forming ignimbrite eruption. After such an eruption, immediately subsequent eruptions will be relatively low on sedimentary contributed quartz and feldspars such as observed at Roccamonfina. Then one of two things will happen. If there remains enough sedimentary material, the cycle will begin anew. A new stratovolcanic edifice or dome complex will be built centrally followed by further ignimbrite forming eruptions. But if the sedimentary layer has been exhausted, the central volcano will seemingly become extinct. The self-sustaining chemical reaction will move to where there is sufficient available sedimentary deposits, at the edges of the old magma reservoir. Around the edges of the surface caldera, new stratovolcanic edifices will be built or in some cases, to the side of the caldera.

This, to my mind, is why Iceland and Hawaii, poor in sedimentary deposits, have so little explosive volcanism. This is why areas around the Ring of Fire such as Italy, Indonesia and the Philippines, which posses rich sedimentary deposits, have these very large andesitic stratovolcanoes and the associated huge, caldera-forming ignimbrite eruptions. This is also why the Taupo Volcanic zone has been able to produce no less than six very large eruptions, two of which were VEI 8, in the past 280,000 years and I am pretty certain that if a detailed analysis of the bedrock underlying Ol Doinya Lengai was undertaken, a deep layer of carbonatite crust would be found.

Henrik

 

105 thoughts on “How the Composition of the Crust May Affect Volcanism

  1. Thank you for the informative article. I am only just now beginning to seriously delve into petrology and the whole process of magma evolution and your article made a few concepts more clear for me.

    • Thank you! But I wonder if your teachers will thank me for the weird ideas I may have put in your head! 😉

      • For me that seems too straight forward to be a “weird” idea, it just seems like common sense (but perhaps that’s because I struggle to think inside the box), so perhaps it’s just actually true – great post 🙂

  2. Very interesting, Henrik 🙂

    Don’t know about the lavas of Roccamonfina, but not sure that all overriding lithosphere has a sedimentary layer below the upper crust. Looks like the subduction zone(s) round the Med have an interesting, and possibly complex, history.

    • Afaik, there are three areas with extensive granitic shields; Scandinavia, Canada and parts of Africa. The rest of the world is made up of sedimentary deposits but no two are exactly alike which I think is why you have all those differences – and similarities – in volcanism.

      • From the subduction zones I have looked at (not many), the sedimentary layer overrides the crust. I would guess that the formation of the lithosphere around the west of Italy is more complex – which would make sense as, in addition to the collison zone between African and European plates, there is the break of the Adriatic plate.

      • Hell, speaking of granite shields, I live on a granitic/tonalitic “shield” in NE Slovenia. 😀 Pohorje hill. Everywhere you look around, especially in the forests, you can see tons and tons of (iron rich) felsic igneous rocks. Mostly 20-15milion years old rhyolite/dacite.

        • And just down the coast near Montenegro, is a region of “super subduction” where several subdution episodes are layered together in the mountain stratigraphic record.

      • I would also put in Iceland as a decidedly sedimentary free zone. But then on the other hand you mentioned it as relatively free of large scare explosive free volcanism.

    • Memo to self: don’t put ice cream out to soften before reading a VC article, unless you want it melted when you get back to it 😉

  3. Great article and thank you Henrik. It’s so refreshing to read your heartfelt anger against publishers blocking and pay-walling science with such ridiculous fees. As a librarian of 30 years, I am incensed by the traditional publishing world’s inability to progress, rather than inhibit, research. Burn the lot of them. Meanwhile, your article was very informative thank you.

  4. And if you remember “Bob”… There was some conjecture among the geologic authorities about what exactly was in the “Restolingas,” Trachyte or Rhyolite. The issue may have been over analysis of the black stripes vs the whole rock analysis.

    Photo by J.A. Rodriguez Losada → http://www.elmundo.es/elmundo/2011/11/03/ciencia/1320338180.html

    Notice that it looks a lot like chocolate swirled into vanilla. The white part is from mobilized sedimentary material underneath the island that was originally deposited when this area was still just part of a Jurassic era rift basin as the Atlantic Opened up. The lithospheric pressure gradient in that region is only capable of metamorphing the sediment into Phyllite, but with a magma conduit passing through it, the material can incorporate some of the material into the mix. The black part is the more juvenile basaltic magma.

    Essentially, that photograph is a snapshot of the process. Many samples of it were evident in the “floaters” that Bob coughed up. The issue at the time was over how much of a threat the El Hierro crisis was. Rhyolite → typically gassy and prone to explosive events = bad. Trachyte = less gassy = less bad. (Rhyolite has a higher silica content and doesn’t release it’s dissolved gas load peaceably.)

  5. Good educational post, and this only scratches the basics (in the best way). Some volcanoes do the opposite of what is common here, where they start out explosive, and then become more shield-like as they evolve, although that’s a bit less common.

    On another note, Cotopaxi’s seismicity has started to change a bit from the high tremor long period events that were seen over the last week, and also in mid-june.

    As of today at least, there have been more proper tornillos – quakes that start with a big quake, and taper off as magma pushes into the space vacated by the break. I interpret this as magma starting to move, albeit somewhat slowly. The frequency of these quakes isn’t high enough to say an eruption would be happening soon, in most scenarios like this prior to a true eruption, you would start to see frequent tornillos every few minutes, which is not what we’re seeing now.

    I wouldn’t be surprised if this reverted back to that odd intense tremor that we’ve seen a few times here in the next few days. But if the popper style quakes we’re seeing today become much more frequent over a period of a few days, then it’s time to take some notice and prepare for an eruption (in my opinion).

    • You haven’t happened to have seen a quake list of the local seismos have you? It would be handy to plot the depth over time of the smaller quakes. That might yield a clue as to when it might get hot.

      BTW, those are starting to look like poppers.

      • I haven’t found anywhere to find that information, I’m guessing it’s not available for public use like we have in Iceland or other areas.

  6. Bardarbunga GPS was leveled off for a while, but looking closer it seems to go slightly back down again. If the trend were to continue and intensify, it would be an interesting case, if the caldera would be subsiding and there would be no eruption anywhere. If the same principle as Holuhraun would apply, the magma should go somwhere, which given the increased number of quakes SE of Bardy, could cross the intersection of Grimsvotn fissure swarm and go towards SW. That would be interesting.

    The fact is, that given the number and mag. of the quakes at Bardy, the caldera is far from balanced/at equlibrium.

      • I agree about the cooling. I think it is still hot down there and shall take a very long time to cool.

  7. Speaking of Fog….

    OT and messing with Carl.

    First, listen to Train Kept a Rollin by Aerosmith.

    Now listen to Honey Hush by Foghat.

    Notice anything odd? The main rhythm line and chord progression seems to be quite similar. Square that with the original version of each song:

    Train Kept a Rollin TINY BRADSHAW

    Honey Hush – Big Joe Turner

    Did the two songs share a common musician at some point in time in their transition to a more modern cover of them?

    Yep. Johnny Burnette

    While not as readily apparent, they share a similar style in his renditions of them.
    Johnny Burnette Train Kept A Rollin’

    Johnny Burnette Honey Hush

    And, just for the sake of mentioning it, I must say that the Aerosmith version of it in Jackson MS was phenomenal when I saw them in concert there. The most spectacular bit of their light show was simply two triangular mirror columns with flood lights shining into them. As they rotated the horizontal mirror columns large rectangular beams of light swept down over the crowd in the smoke filled auditorium. An amazing effect with so little technology needed to pull it off.

    • You needed to have me thinking early in the morning didn’t you? 🙂
      *putters back for more coffee*

    • {grin} This is one of my favorite music topics. I brought it up a few years ago and one of my clan-mates discovered the Johnny Burnette linkage.

      BTW, GeoLoco, you will appreciate the Johnny Burnette version of Train kept a rollin. It’s got “eye candy” 😀

    • I had read a recent “news” article stating as such. Though it is quite likely that the graben that is pressurizing the formation is the ultimate driving force, the well had not been properly lined and was highly vulnerable to a “kick.” → Formation fluid entering the well bore and displacing the drilling mud. As any entrained gases come out of solution and begin to bubble, that reduces the hydrostatic pressure even more and it gets away from the well crew. From the way it all reads, the well suffered a blow out at depth where the highly pressurized strata fed fluids to a lower pressure strata… then it made it’s way to the surface near the well site. If they had not properly cased the well as they were drilling it, then yeah, I could see it being caused by humans.

      Even if the quake that preceded the event (by a couple of days) were just the graben settling and raising the pressures, a properly drilled and cased bore would not have suffered a blow-out at depth and would have been much more resilient to “kick.”

      Is it a volcano? Yes. A mud volcano. They are all over the place. Just off the coast of California in the Santa Barbara channel there is an asphalt volcano. It’s been there for thousands of years. Matter of fact, once they started extracting oil from those formations the number of natural hydrocarbon seeps went down.

    • I was wondering how long it would be before the Lusi Mud volcano turned up. If it’s a volcano depends on how you define the word “volcano”. It is much like that old linguists question “What bird is a “birdy” bird”, which once the students have identified the salient criteria is invariably followed by “Then tell me, is the Ostrich a bird?” Since Lusi possesses several of these criteria except perhaps the critical “magma” and “superheated”, calling it a “mud volcano” is accurate.

      But Lusi is interesting from other points of view, that of the current article just being one. Take a look at this image and note the similarities between a (hypothetical) mud volcano and a real one:

      PS. The Geological Society of London is another organisation that paywalls science albeit being a non-profit one. If the government does not pay for science, someone else has to…

  8. So many aspects of bimodal volcanism explained in such a common sense manner. Well done.

    Retrieved from Limbo – And welcome, Erik! /Lugh

    • In fact, I’ll run the article by my 8 year old daughter, she’s got a scientific mind and will love this.

      • Please tell us how she finds it! That said, I hope I haven’t stated the glaringly obvious as there is nothing worse than to discover that you have been preaching to the already converted… 😮

        • I don’t know about that. My grandson was asking about the fracas in the stock market today and wound up understanding the tax liability between short term vs long term capital gains.

          (Sometimes I do drift off topic when asked a question … Eventually I wound up talking about Black Swans… and the innate human tendency to glaringly overestimate or underestimate risk and how that could play out in Cotopaxi. Even with a 90% survival rate, there could well be over 10,000 dead from Lahar… and if the mortality rate is as bad as the Amero tragedy, probably upwards of 69,000 dead.)

          At this point I killed the conversation by looking at coin flip probabilities. The odds of getting 5 heads in a row is about 3.125%, which is still much greater than most people expect. (For a fair coin, 50% chance of getting a heads or a tails. Five tries = 0.55, or 0.5 raised to the 5th power).

          Once their eyes gloss over, you’re done. His response to the Cotopaxi info, if they sound an alarm they had best flee. My retort was that if you aren’t not already headed for high ground when the alarm goes… you probably aren’t gonna make it.

          • And yes, the same thing applies to Ranier… and the pending Cascadia subduction zone quake with subsequent tsunami. When the alarm goes, your best defense is to already be gone.

            (not pessimistic, just a realist)

            Imagine the entire population along the Pacific Northwest coast trying to leave at the same time and you will get the idea. I evacuated for one of the Hurricanes here along the Gulf Coast. As I crossed the interstate, it was bumper to bumper as far as you could see. Luckily, I knew the back way out of the county and didn’t get stuck in that mess. Of course, by the time it was over with, I was somewhere in Louisiana and the storm had missed Pensacola completely and I was out $300 in gasoline costs. (I was paying for two cars, my daughter had followed). It could have been worse, some people got stuck on the interstate wherever they happened to be at when the storm made landfall and passed over them. I can think of better places to ride out a Hurricane.

          • That depends on the advance warning and not only your location! Let us for arguments sake say that you get less than 30 minutes because the local authorities rely on police cruisers to come round to alert everyone once a lahar has started. (And let us also assume that they sacrifice themselves in order to do this instead of fleeing for their own lives). Roads will be clogged instantly so you have to do it on foot. You’re smart so you run at once. You immediately realise that your possessions are lost so you don’t go home for your prized posessions. You’re single, so you don’t have a family to pick up. You’re healthy so you manage to run four to five km before the lahar strikes.

            Will that be enough to get you out of harm’s way?

          • Depends on how far it is to “high ground”… and is that high enough?

            … also on what is between you and high ground.

            … and on the human tendency to overthink the situation. Been there done that! Sometimes the best move is to rely on instinct and run like hell. Sometimes not.

            I like the answer that the WOPR computer in the movie “WarGames” came up with. “Sometimes the best move is not to play at all.” (referring to the similar ideal winning solution of tick-tack-toe and Global Thermonuclear War) So, my threat is from Hurricanes instead, at least I can see them coming and watch how they are developing.

            https://youtu.be/NHWjlCaIrQo

  9. Great mud volcano diagrams, very helpful. Now I want a mud feeding gryphon for a pet 🙂

    Have also read that heat and steam are present, other volcanoes as close as 15 km away, and lots of mud volcanos in that part of Java. Is it just the eruption started by drilling that makes Lusi an especially interesting mud volcano?

    • There are several reasons why the Lusi, or to give it’s correct name, the “Sidoarjo mud flow” is interesting. Mud volcanoes are plentiful worldwide, but Lusi is the only one where a connection to human activities can be conceivably be made (that it’s the cause is contested and has not yet been verified: http://www.livescience.com/38329-mud-eruption-caused-by-earthquake.html ). As such, it is *far* more interesting to journalists wanting to peddle “a story” but the main reason is that it has offered a unique opportunity for studies.

      From the point of this article, the mud layer apparently begins at about 2,834 m depth or just above that at which you’d expect the top of a magma chamber underneath a volcano to be located (3 – 5 km). Here you have the clays and water at an appropriate depth…

      PS. In Azerbaijan there are nearly 400 mud volcanoes along the shores of the Caspian Sea.

  10. Thank you Henrik. Indeed this has made me think again after my recent course, and my list of “Must follow that ups” is getting longer 😀

    • About a year or so ago, I ran across a paper detailing the risk assessment to the (then) proposed BTC pipeline. One of the main concerns were mud-volcanoes that were prone to form and erupt in flame during an eruptive episode. (Static electricity could be an ignition source, as well as Phosphine compounds which spontaneously ignite when exposed to air… and they are also byproducts of decomposition just like H2S and Natural gas.)

      GL Edit: Changed Dung to During. Flaming dung was not an evaluated threat.

  11. Nice article Henrik, it makes perfest sense to me. When you say there is probably a layer of carbonatites deep under Ol Doinya Lengai, you do mean sedimentary carbonates (limestone/dolomite/chalk) don’t you? Going with the sedimentary basin theory, I mean.
    Carbonatites are amazing – lava made of calcite! Who’d have thunk it?

    • Yes, spot on! Limestone, chalk, dolomite. Any large, crustal source of calcium carbonate as the likelihood of it coming from the mantle is “very slim”.

      • Sorry, that craton formed prior to sedimentation started on earth.
        That is what makes the fun. Instead it is a remnant of the initial carbonate layer as the earth formed. It is the same layer as all the worlds diamonds formed in and there will be loads of those in the pipe that is forming in Ol’Doynia Lenggai.

        • I stand corrected and amend the statement: Yes, any large, crustal source of calcium carbonate, be it part of the original bedrock as in this case or of metamorphic sediment origin.

        • Heard somewhere that the carbonate lavas came from the limestone sediments on an old subducting slab under the craton from a previous subduction zone.

  12. I have a question, brought up by these pictures… http://www.boston.com/bigpicture/2011/02/nyiragongo_crater_journey_to_t.html

    How does a lava lake like that form? Originally it is size X, It overflows a bit, and then slows down a bit, so a bit of a ledge develops. Just keep doing that for years. It looks like the level of the lava lake is visible on Picture 25. Just about level with overflow.

    So is that right? That seems like one heck of a ledge as such.

    rescued from the dungeon /Hobbes

    • It’s always amazing to me how huge these things are. You know it’s big, but until someone stands beside it, wow.

    • I wonder what you and KarenZ has in common. But I hope you and her enjoys the complimentary coffee and cookies down in the antechamber. 🙂
      Bonum autem in carcere duxit. (Pliny the Middle-aged)

  13. Maybe because I am using a Hotmail e-mail? Would a Yahoo E-mail work? Or a Gmail?

    • Depends on what the heuristic SPAM detection routine of Akismet cues in on. If it resembles SPAM, the algorithm jumps all over it until taught otherwise. It could be a simple as the server for your email address generating craploads of other links if it is accessed or sent to… or if the domain is in a blackhole list somewhere as being associated with SPAM mailings.

      When I got tangled up in a firewall a few days ago, the only way I could access VC was by creating a secondary account off of my phone. I used my Gmail addy for that one and Akismet never batted an eye. So, your gmail addy might work. (I was posting as “The Guy with a torch”)

    • Once you’re “allowed” you should always be ok, it’s strange how it’s always you…

      • …and KarenZ who ends up in the Dungeons. Karenzs problem has been ongoing for a couple of years. I suspect that is due to something the Dirndl did to KarenZ.

        • Well, Im no expert, but such swarms have happened here before and are not uncommon. Especially now when the whole region is undergoing stress/strain changes due to the rifting. For now the swarm is rather low energetic and shallower, with the strongest quake at the time I write this, being an M1.8. Tho I would expect to see M2+ quakes also. Tungna has been quite active seismically since the onset of the Holuhraun events, and ever since then, so it was kinda like the frog in the hot water. 😉

          But all in all, its nothing I would lose sleep for. 🙂

          • Active a bit before last year, so far as I know. There was an article about the events there by Þórhildur Björnsdóttir and Páll Einarsson in the journal Jokull 63 (2013), p. 17ff. I am pretty sure it’s freely available online.

            The abstract suggests that the faults, fractures and sink holes are in part associated with Gjalp, and concludes thus: “The widespread evidence of recent fault movements and the small magnitude of the earthquakes suggests that the fault activity is related to magma movements rather than tectonic faulting.”

        • … and I don’t really need a reason to loose sleep. I have the circadian rhythm of a random number generator or an astable multivibrator.

        • The current revised number of quakes in the swarm is around 30. Tho judging by the tremor plots and the forbidden tremor plots and drums, that number should be quite higher after the final revision.

        • Did you end up in the Dungeons or can’t you see your own comments?
          I can see them at least out in the open.

          • Yeah, I can see them, but can’t edit them.

            Btw, this is my phone.

          • (Don’t elevate this log-in’s permissions. Hard to moderate off of a phone anyway)

          • I have left a comment in the Admin booth. I have tried to reset it all. Are you sure you are logged in? I had a similar problem before end in the end discovered that I was not logged on.

        • Using Mastin et al, the ejection rate comes up as a function of plume height.

          In excel formula representation,

          Ejection rate = POWER((A2/2),(1/0.241))

          Where cell A2 is the height above the edifice in meters.

          This was derived/refined by Mastin et al off of the Sparks equation where

          Ejection rate =POWER((A2/1.67),(1/0.259))

          Again, with A2 being plume height.

          The whole idea was to be able to derive some emperical data on the eruption with only distant or spotty information. I usually use the VAAC reports and convert the Flight level warning data to meters, then back out the height of the edifice. That’s what I then push through the equation.

          For something that tosses a 12 km high plume, you’re looking at about 1,927.6 m³/s.

          And remember, that one cubic meter of DRE is about 2700 kg. Air fall tephra is usually much less dense, and the amount of “ash” that gets dispersed is an order of magnitude or so greater.

          If you keep a good track of the plume height over time, you can (with effort) work out how fast a given eruption will cross the different VEI levels and even project at about what time it will reach a given level unless the rate changes. It requires you to do a bit of curve fitting so that you have a somewhat workable (valid) plume height for each second. Sum all of the ejections rates and you have the (ESTIMATED) total emitted ash over time.

          The caveat is that VAAC reports may over report the height since their focus is specifically on aircraft safety. That means that your estimates using this method probably err on the high side.

          Some quick runs through the formula:

    • Got it! Thanks fer the help!

      Evidently my log-in session had a brain fart and I was only partially “here”.

  14. I found this footage of Raung of some days before todays eruption.

    It does not seem to be too big. But it does not have to be terrible big if its close enough to important airfields.

  15. Just testing my new login on here and saying that the tremor on BREF at Cotopaxi seems to have increased today.

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