Eldgja: Eruption dating

The previous post described what we think we know about the Eldgja eruption. Our knowledge about one of the largest eruptions in Iceland is somewhat limited, surprisingly so given that Iceland was already well populated. One of the few things which seems secure is the date. Eldgja is believed to date to 934 AD, continuing for up to 7 years. But what is the evidence for this proven fact? Is it correct?



Let’s look at how we were able to get such an accurate date on such a poorly observed eruption. There are three strands: Greenland ice cores, written records from Iceland and elsewhere, and volcanic winters.

An example tephra layer in the NGRIP ice core. This layer is just over 28,000 yr old, long before Eldgja

An example tephra layer in the NGRIP ice core. This layer is just over 28,000 yr old, long before Eldgja

Ice cores Eldgja left a strong signal in the Greenland ice cores. The sulphate deposition was phenomenal, larger even than Laki. The ice cores are dated just like tree rings, by counting annual layers. The most accurate date comes from the GISP2 core, where the sulphate peak is dated to 934+-2 AD. The Greenland GRIP ice core shows it too, and here it has been dated to 938+-4. Ash found within high sulphate layer in the core has been shown to come from Eldgja: evidence doesn’t come much better than this. The GRIP core shows elevated sulphate over an 8 year period, 933-941 and the authors (Zielinski et al) argue for an extended period of eruptions. But this should be taken with care: a large spike may pollute the annual snow layers just below it, and this would show up as several years of slightly elevated sulphate before the main eruption occurred.

Book of Settlement Attempts have also been made to obtain dates from historical records. The problem is that these records don’t give exact dates, and don’t actually mention Eldgja. There is one indirect reference to Eldgja in the Book of Settlement:

Gnup went to Iceland on account of his own and his brother’s manslaughters and settled land between Kuda-fleet and Isles-river, and all Swans Haunts (Alftaver); there was a great standing-water then and swan catches thereon. Molda Gnup sold from his landtake to many men, and it became thickly peopled, until the earth-fire (i.e. lava) flowed adown there; when they fled westward to Head-Brink (Hofda-brekka), and made there tent-dwellings in the place which now is called Tent-field (Tjaldavöllr). But Vermund, the son of Sigmund Kleykir, would not allow them abidance there, so they went to Horse-garth (Hrossgard) and made a house there and sat there over the winter and quarrels and manslaughters befell there among them. But in the following spring Molda-Gnup and his companions went west to Grind-wick and took up their abode there. They had a scanty store of livestock. By then the sons of Mould (Molda) Gnup, Bjorn and Gnup, Thorstein Hrungnir and Thord Leg-wielder, were of ripe age. (part 4 ch. 12).

Book of Settlement. Photo from http://valkyrja.com/260815.html

Book of Settlement. Photo from Valkyrja.com

It gives a fascinating insight in Viking life! It was a world born of violence, especially so during the bleak and idle winters. Molda-Gnup is believed to have been born around 885 AD and his brother Vermund 890 AD, both in Norway. They came to Iceland for fear of revenge for murders they had committed (there seems to be a bit of a pattern here):

There was a man named Hrolf the Hewing, he dwelt at Nordmæri [Norway], at a place named Mould-Town (Molda-tún); his sons were Vermund and Mould-Gnup; they were men great at manslaughters and smiths in iron.

Their move to Iceland would have been around 910-920 AD. Taking time to have children and growing them to a ‘ripe age’ (presumably ripe for marriage) may have taken another 20 year, so this puts the eruption in the region 930-940, in agreement with the ice record. The sequence of events suggests a spring or summer eruption. Molda-Gnup is mentioned several times in the Book of Settlement: he was an ancestor of a wealthy and powerful family. One such text reads:

There was a man named Avang, an Irishman by race, he first settled in Botn (= Bottom). The wood was at that time so abundant there that he built from it a seagoing ship, and put in her cargo at the place which is now called Hladhamar. His son was Thorleif, the father of Thurid, the wedded wife of Thormod, who was the son of Thjoster at Alftanes, and of his wife Idunn, the daughter of Molda-Gnup. (part 1 ch 14).

Avang came to Iceland one generation before Molda-Gnup (perhaps around 880), when the country was still well forested. The statement that Idunn and Thjoster lived at Alftanes is interesting, as this area was overrun by Eldgja lava. Molda-Gnup had time for at least three grown-up children , and for his daughter (the oldest?) to have her own family before the eruption. This adds a few years to the estimates of when Eldgja occured, perhaps around 935 at the earliest.

There is one other oblique reference to the eruption in the Book of Settlement (part 4 ch 12):

Hrafn Haven-Key was a great Viking, he came to Iceland to settled land between Holm’s-river and Isle’s-river and dwelt at Din-Shaws (Dynskogar). He foretold a volcanic eruption, and moved his dwelling to Low-isle (Lágey): his son was Aslak ‘orgodi’ and from him the Lowislanders are descended.

This suggests there were precursors to the eruption. A significant earthquake is most likely, doing enough damage (but also leaving enough time) to make people move away.

Non-Icelandic records Strothers has identified one possible reference to Eldgja in European records, by the 10th century Saxon chronicler Widukind of Corvey:

Indeed before the death of King Henry many prodigies occurred, such as: The brightness of the Sun outdoors in a cloudless sky appeared almost nil, but it streamed indoors, red as blood, through the windows of houses. Likewise for the mountain where the overlord of the states was buried, according to report, because the mountain erupted flames in many places.

King Henry of Saxony died in July 936. The text is difficult to understand but as there are no ‘flaming mountains’ in Saxony, Strothers interprets this as a reference to Iceland. The ‘overlord of the states’ was a title of the leader of the Icelandic Parliament (the Althingi), and the first leader Úlfljótr, disappeared from the records in 934, at the start of his three-year term. He would have traveled to the parliament through the area covered by Eldgja lava. Does Widukind imply that he died in or shortly before this eruption? If so, the eruption can be dated to 934 AD or shortly after, in spring as the Althingi convened in mid-June. But one has to be careful. ‘Flames’ was often used in the old records to describe northern lights, in which case this may have nothing to do with Iceland, or volcanoes. There are some anachronisms: the ‘states’ did not come into existence until 964, and the actual title of the Althingi leader was ‘Law speaker’. Úlfljótr wrote the first law (parts of which survive in the Books) but there is no evidence he was the leader. Histories can be open to wishful interpretations, and here the evidence that Widukind talked about Iceland is not that strong.

Elsewhere in Iceland To complicate things further, there was another eruption, almost simultaneous with Eldgja, which left tephra over much of north-east Iceland. It is dated to 938+-6. We know little about it apart from the general location. These hardy Vikings just didn’t think large eruptions as worth noting. Such were the pre-VolcanoCafe days.

Date and duration


Based on the ice core dates and these historical records, Eldgja is normally said to have started in 934. The duration would have been at least 2 years, based on 8 episodes lasting 2-4 months each (the latter is derived from the structure of two lava lobes). The strong sulphur peak covers 3 years, which is one year longer than Laki. This gives a duration of 2 years (the extra year is needed for the sulphate to drop out of the atmosphere). However, the fact that the ice cores show two sulphur peaks dated 7 years apart is often taken as evidence that Eldgja lasted for 6-8 years.

There are some holes in these arguments. Can we do better?

Re-dating Eldgja

Let’s return to those ice cores. They show two events, separated by about 7 years, with the first one much stronger than the second. Ash found in the first event clearly identifies it with Katla, and thus with Eldgja. But the dates derived from the GISP2 and the NEEM ice cores for this event differ by 4 or 5 year.

Sulphate deposits in the Iceland ice cores. From Stigl et al.

Sulphate deposits in the Iceland ice cores, showing that the NEEM and NGRIP cores give an earlier date than the GISP2 core. From Stigl et al.

Michael Sigl, in a Nature article last year, has tried to fix this dating problem. In the 7th and 10th century, twice there was a strong jump in 14C in the atmosphere, which left a record in trees that were growing at the time. The tree rings give precise dates for these jumps, which may have been due to major solar flares. Such flares would also have produced 10Be. Sigl managed to identify the beryllium isotope in the ice cores, but the date differed from those given by tree rings by 5 years. He attributes this to an error in the established time line for the ice cores. People had miscounted. Shifting the ice core dates to the ones from the tree records changes the time line for these cores: the date for the large sulphate spike in 934 now moved to 939 AD, for both ice cores.

This is an intriguing proposal. There are some lose ends: they don’t state why or when the error or miscounting in the ice core dating occurred (dates more recent than 1200 seem fine), and in the revised time line the sulphate spike around 1104 AD has moved to 1108 and no longer aligns with the 1104 Hekla eruption it was assigned to. The peak in 77 AD has moved to 88 AD and can no longer be attributed to Vesuvius. But in light of the other evidence above, the case for Eldgja to have started in 939 has certainly strengthened.

Remember the dimming of the Sun described by Widukind? There are two similar records for this. The Spanish historian Juan de Mariana (17th century) states that there was a darkening of the Sun on two days of different months of the same year around this time, 19 July and 15 October. The first is easily identified as the total solar eclipse of 19 July 939, when the path of totality was near Madrid and Lisbon. For the second darkening, 15 Oct 939, the light of the Sun changed to a pale colour. The Irish Annals of Astronomy, 939, mention The colour of blood on the sun from the break of day until the middle of the next day.

Is the revised chronology correct? The authors make a good case (although the evidence for dimming of the Sun is more dubious because of the inconsistent date of Widukind, plus the fact that major peat fires can have similar effects). There are two concerns. First, the GISP2 core which gives the later data has a larger uncertainty of +-4 yr, so it does not disagree with a 934 date. Second, the re-dating is based on tree ring records. If there is an error in these, the rug is pulled from underneath their method. That is not entirely impossible. If a year is so cold that the selected trees (which are normally from the edge of their distribution, to maximise the effect of climate on the growth rings) do not grow at all, that year will be missed and all older dates shift forward. The main risk for this comes from the extreme climate event of 1258, caused by Rinjani. This may be less likely but more evidence is needed.

Volcano weather

John S. Dykes

John S. Dykes

The final piece of evidence is from the weather. The limited records from this period sadly do not include daily weather reports. Only extreme weather events would have been described. The winter of 933/934 was cold: the Black Sea and Bosporus were reportedly frozen (presumably only along the edges, not fully!). But an even colder winter had happened in 928/929 when the Thames was frozen for 13 weeks, so this was not completely exceptional. After that, no extreme weather is reported until 939/940, when a severe winter happened, listed for instance in the Annals of Ulster. This cold weather affected much of Europe and Asia and lasted three years. Famine followed in Germany and France, first in 940 and again in 942. Records of Chinese weather (Fei and Zhou, 2006, Fig. 2) tell us that the winters of the years following AD 934 were not unusually cold. But extreme cold came in mid 939 with snow in July. The following winter was the coldest in the 30 years between 923 and 954, and the next two winters again were cold. To make things worse, this was followed by two years of exceptional drought. In a recent paper the same authors hesitantly suggest that this was caused by Eldgja, although that presumably started 8 years earlier. In the revised dates, the cold and drought follows Eldgja much more closely.

A similar point has been made regarding the Nile. Its water level is sensitive to major eruptions, which tend to affect the rainfall in its source region. Climate models have confirmed there is a relation. The year after Laki was extreme, with the lowest flow rate over 400 year. A lesser low point happened after Katmai. The flow around 934 was normal, bit it was extremely low during and after 939. Luke Oman (2006) has used this to argue already a decade ago that Eldgja may have been misdated.

Nile flow after major northern eruptions. From Oman et al. 2006

Nile flow after major northern eruptions. From Oman et al. 2006

While 934 was cold only in Europe, the period 939-942 was cold across the northern hemisphere. This is similar to what happened after Laki. Laki has been blamed for hastening the French revolution. How about Eldgja? What is the evidence for political upheaval? Actually, surprisingly limited. In China, the Later Jin Dynasty collapsed shortly after and this was clearly infuenced by climate. The Abbasid rulers in Baghdad crumbled and the Buyid Dynasty took over, but this process had been on-going for a decade or more. The Viking wars continued across Europe and the Middle East but did not change much. There may have been a fortuitous element: Eldgja happened at the start of the Medieval Warm Period, when European climate was benign. Laki happened during the Little Ice Age and this may have worsened its effects.

As an aside, the Icelandic fires affect climate much more than may be expected from there volume. They emit much more sulphur than normal volcanoes, and this has climatic damage beyond their lava size. The distribution is not world-wide because Iceland is so far north. Icelandic sulphate stays mainly at mid-to-high northern latitudes. But this makes things worse because with a much smaller area to affect, it is much less diluted. The concentration may be 5 times or more higher than that of an equivalent tropical eruption. Iceland may be small, but it packs a punch.

From all this, it seems plausible, but not fully proven, that the Eldgja eruption started in spring 939 AD, five years later than generally accepted. It lasted perhaps two years (not 6 or 8 years: this came from attempts to reconcile the different dates from different ice cores), and caused a volcanic winter which affected Europe, China and Africa for three years.

Next time We now know a plausible date. There are some questions that still need to be discussed: what actually caused the fissure eruption? Where did the magma come from? Why did the eruption stop? And when will the next one be? Come back for part 3 of the Eldgja saga!

updated to add the bit about the Nile

Part III

Albert Zijlstra

335 thoughts on “Eldgja: Eruption dating

  1. There is a swarm going on beneath Myrdalsjökul.! It will be interesting to see how long-lifed this one will be. Around 70 quakes since Tuesday. Only thing that makes me wonder – most of the activity is right at the surface!

    • This is very shallow activity and the earthquakes are not that large. Not a single earthquakes has been deeper than 0.5km.
      I would say that this batch is just regular earthquakes caused by melting and the repetitive hlaups that have ran in the last few weeks (I think this is the fifth hlaup in 5 weeks).

      The intriguing question is what is causing all this melt making the water for the hlaups. Seasonal melting could have caused 1 or 2 of the hlaups, but not all of them.

      • Similar sort of activity over near Baaardy too following the dike towards Askja. . It has been extra warm this year so yes. Ice melt is greater. However I still think Katla is worth watching. Less pressure from the ice could let her breath a sigh of relief. I know how I feel when my constricting fashion wear has been removed.

        (Looks both ways and over her shoulder for Geoloco or whatever his name is now)

        • PS Can someone please explain the repetitive, almost single stroke signals, that are occurring on the God. drumplot please. They are most obvious at about 11.40.

      • Current swqarm very close to yesterday shallow 3+ quake. Could that one and the curren swarm be entirely a waterboiling thing?

        • It could be “a water boiling thing”, it can also be caused by unloading of water and ice on top. And this year has been warm so there has been more meltwater than usual and then there has also been several jökulhlaups. It is possible that as much as 2 to 5 meters of water has been removed out of the system, and that is a lot of weight disappearing.

  2. IMO just issued a statement.
    Around 50 quakes have been measured in Katla today and activoty started to increase around noon. This is the biggest swarm in Katla this year and frankly since 2011.
    Conductivity in Múlakvísl is 190uS which is abnormally high during this time of the year.

    • The conductivity is high due to the ongoing series of Jökulhlaups from Katla and is not a sign of any volcanic activity.
      This is not the largest sequence of earthquakes if you count magnitude as a factor. That swarm was a couple of weeks ago and consisted of one M4.5 and one M4.6 earthquake.
      Also, due to the shallow nature this is so far a non-volcanic earthquake swarm, it is caused by unloading of water from Katla, both in the form of unusually large summer meltwater discharge and the sequence of jökulhlaups. About 2 to 5 meters of ice has melted this year so the unloading is quite large.

      • There was a flyover on the 27th and no real change in the ice was visble. According to imo.

        You’d sort of expect that much melted ice, to cause some cauldrons to sink beyond the normal levels, wouldn’t you?

        • They are monitoring Katla very closely. I guess if the icecap is melting due to temperature of the surrounding air it will be pretty uniform and melting from the surface rather than from deep geothermal heat so the cauldrons will maybe be wider at the surface rather than forming deep crevasses from melt below. I am not an expert hydrologist so this is just a guess.The melt water from the surface will of course find it’s way down via the crevasses as and collect beneath the ice an exit in rushing torrents along with any geothermal water. It may be heated again by the heat from inside too hence the raised conductivity.Again a guess.

        • No I would not expect that.
          A) The Ice on top is uniformly melting top down.
          B) A cauldron forms either via intense hydrothermal activity, a small eruption, or by a small water filled “cauldron” emptying out.

          In this case we have a caldera with liquid water at the bottom that can empty out uniformly, there would be no cauldron forming at all, just like as it doesn’t form cauldrons at Grimsvötn.

          • Gotcha, I’ve been silly enough (for some reason) to think that all the melting happened in one’ish place, where all the quakes happen.

          • It has actually meandered about between almost all known hydrothermal fields in Katla.

      • Another M3 and a M2.2 at 5km depth. Doesn’t show any signs of letting up yet.

        • Wait until they are manually confirmed before making any conclusions. The M3 is now changed to 0.1km depth. So far the deepest earthquake in Katla is 1km.

    • From the same article “The earthquakes are not located under any of the icecauldrons, but are located 1½ km north of katli (which I assume is one of the known cauldrons), which is to the southeast of the center of the caldera”

      • Hi,

        1)A lot of graph shows this “jump”. Artefacts?
        source imo

        2) from a non-volcanologic non-alcoholic guy : regarding activity katla: could be related to release of gaz ?

        • Jökulhlaup is the answer. One started on the 27th. The fifth in about the same amount of weeks.

  3. Septermber temperature has been pretty high in Iceland, and also precipitation.
    Katla was already experiencing increased seismicity, especially deep quakes.

    Now, after uncommon melting, Katla shows much more seismic unrest, due to the more dramatic release of ice cap pressure.

    Question is: is this unusual melting going to lead to an eruption. So far a lot of shallow quakes.

    I wouldn´t be surprised by a sudden run-up to an eruption that last just a few days.

    Volcanoes are also chaotic systems I would assume, so a tiny bounce might be just enough to push Katla into an eruption.

    • Remember that there has been no deeper earthquakes at Katla since the two large ones.
      If there would be an eruption we would see a very intense earthquake swarm first with earthuakes ranging from 0 to 10km first.

  4. Also the quakes at Hekla continue almost daily. Almost once every day!
    I have never seen this type of activity in Hekla…

    But Katla activity is also unusual. The only comparable high level I have seen was back in 2011, during the supposed yet unconfirmed subglacial small eruption. Now, because we have STILL no tremor, no eruption is ongoing.

    Five jokulhlaups so far and no tremor.
    Also no GPS shifts yet. This makes me think that a large eruption is not yet to be expected. But maybe Katla erupts without GPS changes…

    • This is not unusual. This has been normal for decades. There is just a bit more of it this year, probably due to the unusual warmth this year.

      I do though agree on Hekla, that one has me a bit more confused than usual 🙂

  5. This orange glow on the right is that the reflection of the sun?

    • Yes. An eruption would look like a very large nuclear bomb going off. Unless you see that it is a cloud or the sun.

  6. Pinned answer to all things KATLA so that people do not need to ask the same thing again and again!

    1. There is currently no signs of an eruption being imminent.
    2. The tremor visible since the 27th is due to an ongoing jökulhlaup.
    3. The current earthquake swarm is normal for the season, it occurs at a normal spot, and at a normal depth and they are so far of normal size. Nor is this swarm unusually large.
    4. The earthquakes are caused by loss of weight. The weight loss is caused by 1 meter of ice having melted away due to the unusually warm summer. There has also been a series of minor Jökulhlaups from Katla in the last two months. This means that the average loss of ice this year is between 2 and 5 meters for the entire Caldera. The loss of ice is uniform all over the caldera.
    5. This swarm contains no earthquakes deeper than 1km with the bulk of them being at 0.1km depth. This is consistent with glacial rebound earthquakes and inconsistent with a volcano closing in on an eruption.
    6. All earthquakes in the swarm are of tectonic type with no fluid component.
    7. An eruption would look like a very large nuclear bomb going off. Unless you see that it is a cloud, the sun, sheep, a car, a farmer, a helicopter, or a flying sheep. To cut it short, unless you are screaming “Holy Crap” it is not an eruption.

    What would we expect IF Katla would erupt in the near future.
    An intense earthquake swarm with about 100 earthquakes an hour with depths ranging from 0 to 10km depth with magnitudes up to, but not limited to, M5. There would also be different earthquake signatures indicating movement of magma and harmonic tremor as magma moves up the conduits. We are basically talking about activity that is 100 to 100 000 times more intense than we are seeing now.


      • I just wanted a few standard answers collected at one spot so that everyone could discuss more in detail instead of us getting stuck in the same question 🙂

        • Carl
          Thanks for this comment
          “-The current earthquake swarm is normal for the season, it occurs at a normal spot, and at a normal depth and they are so far of normal size. …..
          4. The earthquakes are caused by loss of weight. The weight loss is caused by 1 meter of ice having melted away due to the unusually warm summer. ,,,,,”

          Then rise the question: Why we don’t see the same phenomena for Vatnajökull (Bárðarbunga,Grimsvötn, etc,…) or in Alaska. What is the difference

          • I will not answer for Alaska since there are as far as I know few glacier filled calderas there.
            The difference is most likely that there is a different distribution of the magma chambers between Grimsvötn and Katla, and Bárdarbunga is not geothermally active and has no sub-glacial lake like Katla and Grimsvötn.
            Also, the Myrdalsjökull is far smaller than Vatnajökull, so there will be a different pattern to the weight distribution changes.

    • I wonder if Katla would go off in a similar fashion as Grimsvotn in 2011


      It went very quiet over the entire region before it blew up so fast. There was no real run up of activity over a long period of time, I could see Katla just going off as well.

      Carl, Do you see any connection to Godabunga, it sometimes looks like there is a feed or track of quakes from that area to the middle of Katla?

      • One has to remember that Grimsvötn erupts often, so all conduits are constantly opening, this makes it a rather quiet volcano for being so powerful. Katla would almost certainly be noisier, I would expect something more on the order of what Bárdarbunga produced before all hell broke loose.

        Godabunga is a conundrum to say the least. It has a separate feeding system from depth from Katla and a separate magma reservoir. But, the on the other hand it may have had a trickle dyke towards Fimmvörduhals. If that is true the same could be true in the opposite direction too. I guess we will not have a clear answer until after either Katla or Godabunga erupts.

        • I expected actually something more like Eyjafjallajokull in late 2009 and early 2010. Bardarbunga was a different sort of event altogether, it was a rifting event, so much more noisy. Katla should be moderately noisy (like Eyja in 2010), as noisy as it is now, but with deep quakes instead of shallow ones, pointing to a great influx of new magma into its shallow chamber, which will also see GPS changes. Though the chamber is probably already quite full. But the absense of deep quakes and GPS shifts makes me think no eruption is going to happen yet.

          But like Grimsvotn showed, things could change quickly, and a new influx of magma could provoke an eruption in short notice. Also in Bardarbunga, this was relatively quick (within two weeks). And in Eyja the pre-eruption noise lasted about 3 months (all deep quakes but less than M3 usually).

          Hekla…. is a oddball. This makes me think everything on Hekla is different now. Perhaps because in the big scale in Iceland everything changed after the rifting event and also things were already different in the SISZ after the two quake series in 2000 and 2008. Nothing like this happened since 1947 until these last few years, and I think this is why we see a different behavior of Hekla. I think she will probably return to her ~50 year cycle “pattern” of eruptions.

        • Also interesting. Today several quakes at the Holuhraun dike region. Some of them deep.

          I dont discard the possibility that Holuhraun erupts again. In fact if my shield volcano theory is right, Holuhraun will erupt again for a quite some years, on and off, in several eruptions. But my theory is dead wrong, then Holuhraun will never erupt again and the magma will just freeze itself in the dike. I guess we dont really know how shield volcanoes erupt in Iceland….

          • I have been watching the dike between Baardy and Askja too Irpsit. As you say deep quakes and magma in intruding along it I think.
            Carl mentioned there was not lake beneath Baaardy a bit further up there^^^^.

            Does anyone remember at the beginning of the eruption when we were waiting for a Jökulhlaupto appear? It never did! Where did that water go? Vatnajokull is a huge icecap and there have been one or two floods from beneath this year because of meltwater. Is there a lake under there somewhere?

            As always in Iceland it’s wise to watch and wait before jumping to any predictions. The rifting process changes the ball play. It must open potentially new systems. Narrowing some older fault lines and also making some new thus the magma below is spread or squeezed making differently behaving pathways to the surface. Added to this the changes in pressure under the glacial “Lids” it is not surprising we are seeing these changes in the form of “No action”, “Major action” or ” Hidden action”. All of which make prediction of an eruption even more difficult than in a nicely behaving volcano like those without Ice Caps or huge rifts between tectonic plates. The other places in the world where this occurs such as in Africa have no Ice to confound the volcanologists (expert or amateur) . Hekla has no icecap and is situated right next to a spreading zone so any increase in movement from the MAR will affect her system without the added confusion caused by ice making her also difficult to find any “pattern” in eruption modus operandi.
            I just imagine those filled magma chambers like inflated balloons, under pressure and being squeezed every which way. One day the varying pressures will cause them to pop! This is just my simplistic take on the situation at present.
            For the start of Friday, here’s some ” relevant ” entertainment to start the weekend. Like the MAR, Freddie calls the shots. The crowd follow the expected musical phrases (harmonic tremours! ) but now and again Freddie changes the expected routine and the crowd gets confused. Have a wonderful weekend.
            I am off in our newly acquired VW T5 converted camper and will be spending time if it’s raining trying to work out what is causing the “error” display on the Solar panel monitor whilst we are cheating with an electrical hook up. Off now to test the leisure battery and to prepare some pre cooked camping food.
            I feel back to my normal ruminating self at last, so time for coffee # 2


          • That was the best summary of Icelandic volcanoes and their intricacies I have seen in a good long time!

          • Diana – enjoy your weekend of leisure, we’ll keep an eye on things whilst you’re away. Glad you’re back to your ruminating old self 🙂

        • Looks like Katla is boiling over this morning. Someone forgott her on the stove?
          The depth is shallow. Does it refer to the sea surface level or actual ground surface level? I have seen different statements on that issue here at VC..

          • It is the ocean surface, there are two ways depth can be given, as the average height of all involved stations used in the solution, or recalculated as ocean surface. In this case it is ocean surface.

    • I think some of the earthquakes may be a bit deeper than the 0.1km that is listed. Obviously they are not deep, but it is hard to measure that kind of depth. At Bardarbunga, a lot of the quake activity at this depth went away and resurfaced at 3-4 km depth, when new seismometers were installed. Something similar may be happening here. Also, I think the system can’t distinguish depths below the altitude of the seismometers (close to sea level, in Iceland) from those above it. Hence the lack of ‘negative’ depths.

      A loss of 1 meter of ice would be immaterial. That is only the weight of 40cm of rock, and you don;t set of M3 quakes by digging a pond.

      Ice loss may affect the water floods: less ice would mean smaller but more frequent jokulhaups (it takes less water to lift the ice). The glacial melting over the past years may play a role here. The glacier is probably thinning at the moment even without any melt from below: glaciers worldwide show this. But the reports of smell does suggest hydrothermal melting is happening.

      I’ll take the point that the current swarm is tectonic and no eruption is imminent. But with Katla, predicting that an eruption will happen in the future is fairly safe. Whatever it does, there will always be an eruption following it. It must be the most predictable unpredictable volcano in the world.

      • I agree with what you are stating here Albert, I just wish to clarify things for other readers.

        I would say that the range of error on depth at this place is unusually small since there are so many instruments and that the quality of the instruments are first class. It is harder to measure depth than location, but in this case the rate of error would be 250 meters plus or minus if all instruments are working at the same time. This is though only true for earthquakes M1.5 and above, on the other hand all earthquakes smaller than that are pointless since there is to little energy involved.
        Pointless here carries the meaning that they are not significant when trying to deduce Katlas volcanic activity.

        Now, over to the water. 1 meter of water is 70 million tons of water, so in this case the loss of weight is between 140 and 350 million tons during this year. And if you remove that weight the fragile hydrothermal fields and surface reaching faultlines will most likely be affected (as is almost evidenced by the current activity).
        What is not affected (at least emediatly) is deeper features like the magma reservoir, or most importantly, the magma producing MOHO boundary. That will take time and will require more ice to be melted. Question here is when there is enough removed water/ice to do that, and how close to that we are now.

        I agree with you that this will most likely help to aid Katla to erupt in the future, and the future is anything between 1 week and 10 years away.

      • So this means the last swarms took place more likely inside or maybe under the 2km caldera lid and not on top of it?

          • imo: “Due to the unusually high level of unrest at the Katla volcano we rise the aviation colour code from green to yellow.”

          • Do note that the activity is shallow and contain no volcanogenic signatures.

          • I think that Katla is on the way to an eruption!
            I am increasingly confident.

            The shallow chamber already received several deep intrusions in previous months. Quakes are now above the top of the very shallow magma chamber, located around 2km below sea level. Magma is probably around 0.5km from the crustal surface.

            I think the next thing to wait for is a very rapid sequence of events. A stronger earthquake swarm starts, and sudden high tremor follows. This is just a couple of hours until a flood occurs and a few more hours until a dark ash cloud appears above the ice cap, but by then the tremor has been already ongoing since a few hours. Perhaps anywhere 2-5 hours. Time is short however to warn tourists and locals.

            In my opinion this build-up will not last longer than 4 weeks. If no eruption happens in October, then activity will die out.

            I think authorities in Iceland already expected more an eruption rather than not. This is reflected in their preparatory meetings and behavior.

    • That was a really odd one.
      Long period, ultra low frequency. In many ways it looks like a teleseism.
      There is though an initial high frequency signal that indicates local origin. My guess is magma moving at depth at a slow speed in a wide conduit, or some sort of sloshing in the magma reservoir. But, I will readily admit to this at best being an educated guess.

      • And after toying with things…
        It is a teleseism from a M5.1 in Honduras, and it is also a tornillo and a VT event, all of them almost at exactly the same time. That made it hard to say what it was. Most of the energy is the teleseism.

  7. Looks like a slight drop off of earthquakes in Katla running up to midnight, but it looks like its starting to pick back up again.

  8. There’s a useful map showing Katla seismicity from 2011. Noteworthy are the main areas of activity and how they align; how active the presumed Godabunga cryptodome has been; and the full size and extent of the ‘hot spot’ way outside the caldera on the south flank (reference the paper I posted recently):

    Also note the two recent M4+ quakes released pretty much about as much seismic energy as *everything else on that map put together* – size totally does matter!

  9. Another green star is this the start of something .


  10. There have been over 200 earthquakes in the last 24 hours in the Katla Caldera.

    Quoted :

    An Earthquake M3,1 occurred at 04:40 and another M3,7 at 04:41 within the Katla caldera. The larger earthquake was felt in Þórsmörk and had a few aftershocks. About 200 earthquakes have been detected in the Katla volcano in the last 24 hours and the activity is still ongoing There was an increas in activity between 04:30 and 05:00 that has now decreased. The other largest earthquakes reached magnitude M3.2 (at 02:43 tonight) and M3.0 (at 16:28 yesterday). This is the highest daily rate this year. All of the earthquakes are very shallow. The activity is just south of the center of the caldera.


  11. Could tidal strain combined with melting play a part in the swarm at Katla?
    Last night was “new moon”.

    And at Momotombo, the activity started roughly 14 days before last night (full moon). The activity went up again, as we got close to “new moon”

  12. The above average weight loss… IMO noticed no abnormal ice loss, and the temperatures were not that above normal at all. There were higher temperature abnormalities in the past years that did not cause such strong activity.

    Here are latest 30 and 90 day temperature anomalies.

    This year as a whole is different for Katla, than a few recent years. But weather factors are not much different if at all than in the past years.

  13. Have just checked and a lot of the other stations are showing red tips at the top of the spikes posted one as an example?

    • As the waveform travels the higher frequencies are dampened out. This means that over distance lower frequencies looks far larger than they actually are.
      It is like a normal stereo system, bas notes travel far better through walls compared to high notes.

  14. So, as the reference for giving the quake depth is ocean level and the medium hight of the caldera floor is guesstimated 800 meters (Katla caldera), then the quakes are actually occurring some 8-900 meters under the surface of the caldera lid? Not almost on the surface of the lid as I thought. Makes a difference in thinking here!! The caldera lid has bee said to be around 2 km thick. Quakes almost half way down….

    • It corresponds exactly with the assumed depth of the hydrothermal systems under Katla.

  15. And another Icelandic anniversary: yesterday was the 75th anniversary of the start of the regular convoys. of the Murmansk run. The PQ-1 convoy set off from Hvalfjörður in 1941, 29 September. There had been a trial run a month before, but this was the official start. Well over a thousand supply ships made it through over all the convoys, but over 100 allied ships were lost in the process.


  16. Less water in rivers and lakes close to Hekla.

    Very short version. When there’s little water in these places, an eruption is close, it was like that in 2000 and there are legends and tales about it going back to the 18th century.

    But as the farmer says, it’s not completely reliable, as there have been periods with little water when no eruption have happened, but it has something to do with the underground moving. He’s however not going to lose any sleep over it. If Hekla wants to erupt it will.


  17. Colima having a great webcam moment!

    /Users/BillBeach/Desktop/Screen Shot 2016-09-30 at 7.13.14 AM.png

  18. Aviation Code raised at Katla

    Specialist remark
    An intense seismic swarm is ongoing since yesterday morning (29 September) at Katla volcano. An intense pulse, the largest one if compared with the previous activity, started today at 12:02 (30 September) with several earthquakes around magnitude 3 or larger. No seismic tremor has been detected until now in the caldera. Due to the unusually high level of unrest at the Katla volcano we rise the aviation colour code from green to yellow.
    Written by a specialist at 30 Sep 12:34 GMT

    • No seismic tremor yet, as Carl also pointed out. But the earthquakes are increasing, while most tectonic events would see it decreasing over time. Something is giving way and it is progressively getting worse. Whether this ends with a bang depends on whether the something finds a connection to the overpressurised magma. If not, this will eventually die down again. If yes, things could go quickly. Either way, something to learn from.

      Eldgja part III is scheduled for later today. Katla is kindly requested to postpone blowing up until Eldgja is finished.

  19. Friday
    30.09.2016 12:10:55 63.619 -19.129 0.0 km 3.2 99.0 4.5 km NNW of Hábunga
    30.09.2016 12:09:18 63.623 -19.105 0.0 km 3.6 99.0 4.8 km N of Hábunga
    30.09.2016 12:07:37 63.620 -19.111 0.0 km 3.6 99.0 4.4 km N of Hábunga

    30.09.2016 12:02:36 63.625 -19.112 0.1 km 2.9 99.0 5.1 km N of Hábunga

  20. Katle just had a triple green star and Aviation has been elevated toYellow(Source:IMO)

  21. Rising the colour code is just a precaution. There are no signs of unrest except the ongoing swarm of very shallow earthquakes.
    The yellow allert does not mean that there is an eruption around the corner, just that the IMO are thinking that there may be signs coming in the future.

  22. Trying to be honest about Katla and its data.

    Earthquakes in Katla are notoriously known for being difficult to locate, and be accessed correctly. Also the automatic detection confuses depth when it is near sea level, because the crust surface of Katla is around 500-700 meters altitude, plus some 400-700 meters of thick ice cap above it.

    Earthquakes are happening – this is a fact – but we are not fully sure about the depth of those earthquakes. They are mostly shown as 0.1km depth. They could well be happening at about 2km below the caldera top (just around halfway in between the top of its shallow chamber and the top of the ice cap). That would be about 500 meters below sea level (which is nothing compared to the 0.1km depth value). That is actually 1km below the crust surface, because magma has then a further 500-700 meters of rock crust to break.

    GPS changes. Katla shows seasonal inflation and deflation, of a great degree. So perhaps this makes the GPS measurements worthless. Because we would never see anything sudden shifting before a major eruption. In fact this happened in Bardarbunga, which was showing slow inflation over many years but nothing significan happened prior to the start of the many earthquakes in 16th August 2014. Then yes two weeks of GPS shifts as the dike migrated northwards, but again this was a rifting event, so this will not be seen in Katla.

    Geothermal release. In my opinion this is a big show that magma is indeed near surface. The big question is whether the pressure is enough to result in a large eruption or not.

    Is the shallow magma chamber full or not? We do not know, it could have been after 98 years past since the last big eruption.

    In my opinnion Katla could erupt suddently. With much earthquake noise a couple of weeks before the start of the eruption and big tremor a few hours before. If magma is already pushing in big way at 500 meters below sea level, then an eruption could be just days away. Just my two cents.

    Perhaps after all we do not see any deep quakes or GPS shifts. We already had months of continuous deep earthquake at Katla, something that did not happen in 2011.

    Disclaimer: a small eruption, up to VEI2, would never break through the ice, but it would just cause tremor and a large flood, like in 2011. A VEI3-5 eruption would break through the glacier, and would be explosive, initially with fine ash, then slightly milder, lasting perhaps about a month. This is the typical modus operandi of Katla.

    So, the concerns: 1) that the ash disrupts aviation in Europe, 2) ash would cover a significant part of Iceland too, 3) Vik, a village of 400, near Katla, would have to be evacuated, because pumice bombs are likely to land there, 5) tourists hiking in the region are much less of a worry than during summertime, but some may still hike around and near Katla, especially near Takgil camping ground (but these are mostly Icelanders rather than foreigners), and….

    6) perhaps the biggest concern is the expected huge glacial flood that should follow, with a notice time of just 1-2h before it reaches the farms and ring road, and this flood always is the largest of its kind in the world.

    • I find the increase in conductivity to the north side a little puzzling since most of the floods in the past have gone southward when the activity is in the current location.

      • FLoods have happened to the north and west of Katla. I hiked in the area and its clear. How often, this I dont know.

  23. The glacial floods from Katla, when it erupts, are larger than the combined flow of the Amazon river and Mississipi river, at their ocean discharge!

    This is simply astonishing.

    Amazon river has a discharge of 200.000m3/s
    Mississipi river has a discharge of 50.000 to 80.000m3/s, depending if it is in flood mode.
    Katla has more than both of them combined!

    Eyfjallajokull in 2010 reached only 3000 m3/s, which is 100 times smaller!
    About the average flow of the Danube river.

    Grimsvotn in 1996 reached 50.000 m3/s. Which was as large as the Mississipi river flow.

    This puts things into perspective!

      • Interesting that you pointed that out Irpsit.
        I have it written up somewhere to write about it, but then I forgot it.
        What have me rather stumped is what may have changed since back then. First of all, there is no lake and no apparent hydrothermal activity in Bárdarbunga nowadays. So something definitely has changed. I think a clue may be in that there nowadays are few and small intra-caldera eruptions from Bárdarbunga. And it also seems like the “plug” is so solid and thick that there can be no hydrothermal systems in it, and it also seems like the “plug” is also pretty good at being a tight fit against the walls of the caldera ring-fault so no magma leaks go up to meat water/ice.
        I also think that by now the caldera is to deep for an ephemeral sub-glacial lake to ever become large enough to reach over the rim. I think that during each rift eruption the plug has lowered and that over time the depth has just become to deep, just remember that the last eruption lowered the plug by about 50 meters.
        So, now it would take a series of medium sized (VEI-4ish) intra caldera eruptions to create a hydrothermal system powerful enough to create a permanent sub-glacial lake. And in Bárdarbungas case that lava lake would need to be very deep. The height from the caldera bottom to the rim is 700 meters, if you add ice-pressure it would still need to be 600 meters deep before it could cause a hlaup. And those figures are from before the last eruption.
        So as a conclusion, every rift eruption makes a hlaup even more improbable.

    • Irpsit That is astonishing! It just shows what killers these floods could be. It is the floods that have taken lives and livestock as well as the noxious gasses and choking ash. I always remember your description of cleaning up the ash after Eyaf, I think it was. I felt so sorry for all you Icelanders but admire the way you all “get on with it” and not wait for “someone” to come and bail you out!

  24. The top of Katla’s magma chamber is only 1.5 km below the surface. I know that shallow often means harmless phreatic activity, but I think this situation is different given the extremely shallow depth of Katla’s magma chamber, and the high level of fluid interaction above the magma chamber.

    I don’t know yet exactly what is happening, but this can’t be written off as standard glacial melting isostatic quakes. Way too much energy, too much sustained earthquake vigor, and other signatures that seem to indicate that this is more than just that.

    Also, the idea that we would need to see lots of earthquakes from depth is wrong in my opinion. In the previous year or so, we were getting a lot of deep quakes at Katla, so the injections were already there, and are likely “open” right now in some ways. In essence, given the shallow depth of the magma chamber, anything below 3km could realistically be classified as a quake that is potentially loading the magma chamber. I do not think using Bardarbunga as a model for what will happen at Katla is a great idea, or particularly accurate.

    • i agree!

      Diana: i think the cauldrons just remained there until an eruption starts.

      the ice cap is 700 meters thick at its thickest, so it can stand with partly collapsed cauldrons

      when an eruption starts, then all that ice will melt and vaporize.
      its violent, both in the glacial flood and initial blast.

      when i hiked around katla i see many lava bombs to the north, where eruption from floods is lowest. must be hell to be there when an eruption happens, and nonetheless locals and tourists are still camping and hiking near katla, from spots like thorsmork and thakgil.

      the police is today searching for them, trying to warn them. according to the news

  25. I say as program director Kristín Jónsdóttir in Iclandic Met Office says to Ruv (Icelands national radio and tv): Quite impossible to say now whether it’s end with a volcanic eruption, flooding or die out.

    • I agree fully with her sentiment, right now it looks more like a flooding or that it will die out, but if it continues an eruption becomes more likely for every day that passes.

  26. Thank you all for your kind comments. especially Iannamoon. . I just took a break and saw the new green stars. Carl and others more knowledgeable than I… Is it possible that the cauldrons could collapse and cause a big signal? With all the melt water the Cauldrons will be very unstable below in all the tunnels the water must create in it’s downward flow towards the exit of the glacier tongues (and there are a good few of those around Vatnajokull).
    There is no other signals that there is an eruption about to start. But that of course can change rapidly as we all know.
    At the start of the last eruption the local farmers smelled sulphur near the rivers and then there were a few Big quakes then Boom… and a massive flood with little warning. These days Iceland is better prepared and the locals and the emergency services all know what to do to save life. That is not to say that all will be Hunky Dory. I just know that Icelanders are brilliant at what they do and all do as is asked ( unlike the many idiots here who think that the forces of nature will not take them out if they dance on the edge of a raging sea)
    The Yellow alert is a precaution saying “something’s happening so watch for new alerts or stand downs”
    Having said all this it really is very fascinating and it gives us all something to think about this week end.
    I do forecast that if Katla is in pre-eruption mode she will wait until I am on the way to the camp site THEN she will blow !!!!!! Just like Baaardy and Eyaf did.. both times they went when I set off down south to visit my son so I missed the excitement!

    • Diana is once again spot on.
      If this swarm continues there will sooner or later be earthquakes that start to migrate downwards, and then my ears will perk like on a sad basset hound on a retreat in a swamp. As Diana says, things can change rapidly, but right now there are no signs of an immanent eruption.

  27. Katla’s webcam is shaking atm. Its either the wind or another earthquake.

  28. So Carl, do you think that maybe what Jon said could be true, that the reason there are no deep quakes is because the rocks could be so soft the rocks can’t break and therefore cause an earthquake?

    • Hogwash. There has been many previous earthquakes there at pretty much all depths possible. Rocks just do not grow soft over night. Just in the last few months there have been several earthquakes that proves my point.
      I am surprised that Jón has forgotten this, he normally keeps better track of these things than this.

  29. Magnitude 1.6 at 5 kilometer depth
    30.09.2016 14:49:03 63.654 -19.182 5.0 km 1.6 47.79 3.7 km ENE of Goðabunga

  30. Pingback: Elgja: Feeding the Fire | VolcanoCafe

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