The (ash) history of Iceland, in my backyard – Part II

In part I we found the main bands of a excavation here in SW Iceland:

- dark band of Veidivötn 1477
- double white layer band of Hekla from 1341 (or Öraefajökull 1362) and Hekla 1104
- dark band of Vatnaöldur 870 (called the “Settlement Ash”).
- thick white layer of Hekla 3 (around 1000 BC), one of largest eruptions in Iceland.

But there are many minor layers besides the obvious ones. We will get to them now.

FIGURE 1, Below the 870 settlement ash layer, there is one unknown grey and well visible band. There is also a possible eruption of Grimsvötn and Hekla, and then we find the major Hekla 3 band. Below that, we find a thick dark band, probably from Katla, around 2200 BC, and before that we probably have the ash from the Grimsnes eruptions (dating 3500 BC). Photograph by Irpsit, all rights belonging to him, used by permission.

An unknown metalic grey layer

Above the Hekla 3 layer, there is an unknown layer. It has a strange shiny metalic gray color. This is an unknown eruption estimated between 500 AC and 500 BC. Many eruptions happened at that time, not only from Hekla and Katla, but also an eruption at Torfajökull at 150 AC, Hengill at 80 BC (which is only 20km to the west), and also in Vatnafjöll. I don’t think this ash came from Katla or Hekla, unless they erupted a different ash. This type of shiny metalic ash color is notoriously different from every other ash I have seen in Iceland. But I have seen similar shiny lava rocks in Iceland, in a few places, but I can’t remember where. Until then I cannot make a guess about the identity of this layer.

There also seems to be a brown band between the Hekla 3 and the unknown grey layer. It is probably an undated eruption of Grimsvötn, which usually has this color of ash (around 100-500 BC). There is also a light colored band (just above Hekla 3), and that is probably an eruption of Hekla (around 500-1000 BC).

Below the Hekla 3 layer, there are several bands, shown in Fig.2. We first find a thin band of orange material (at 53cm), then a very large band of dark ash (starting at 56cm), and then another broad band of orange material at around 65cm. There is a thin white layer between both broad bands of dark and orange material (not visible in Fig.2).

FIGURE 2. Below Hekla 3, there is a lot of bands to be found, upon close look. But especially notorious is one dark band around 2000 BC (56 cm deep), possibly from Katla (its something really thick), and also a double band around 5000 BC (75cm deep). Photograph by Irpsit, all rights belonging to him, used by permission.

Around 1500-2000 BC: Torfajökull and Katla?

The first thin orange band is estimated at around 1500 BC. The most likely candidate is the eruption of Torfajökull around 1200 BC, because it tend to erupt such colorful rhyolite ash. The broad grey band is estimated around 2200 BC. Most likely it was one strong eruption from Katla (tephra N4 or N2). What is was, it was big, because this is a thick layer. However around this time, we also had records of an eruptions at Langjökull, dated as ~2050 BC, which was actually nearby, only 40km north (it’s closer than Hekla), in a small shield volcano named Lambahraun. If the eruption started explosively, then its ash might have reached here, but officially there is no known ash from the Langjökull volcanoes and I also don’t expect that even a nearby shield volcano would deposit such a major amount of ash. So we stay with Katla.

Hekla 4 and Grimsnes eruptions (2300 to 4200 BC)

The thin white band is probably the eruption of Hekla4, around 2300 BC, which was a very large eruption. The broad orange material is almost likely from nearby Grimsnes volcano, that erupted several times circa 3500 to 4200 BC. I am actually inside Grimsnes volcanic region; its monogenic cones are just 5-8 km away. During the Grimsnes eruptions, there was some local ash fall. The volcano is just composed of crater rows, with one major explosion crater and the other cones being a deep red. It’s no wonder that the layers from Grimsnes eruptions are of a similar color.

FIGURE 3. Grimsnes volcano, located only 5km away. Its the smallest active volcanic system in Iceland, producing crater rows every few thousand years. It produces plenty of iron-red rock material. In the picture, we see Seydisholar volcanic cone, with Búrfell pleistocene volcano in the background (this is another Búrfell; and behind it lies Hengill to one side and Langjökull to the other) Photograph by Irpsit, all rights belonging to him, used by permission.

Hekla 5 and Botnahraun/Laki, or Holmsá fires eruptions, or Thjorsáhhraun (5000-6000 BC)?

At around 75cm deep (estimated at 5000 BC) we find what looks like a double layer: white material above and a deep dark brown below. It is easy to assign this white material to Hekla5 (another large Hekla eruption at 5050 BC). The brown material underneath is unknown, but likely Grimsvötn. Possibly the Botnahraun/Laki eruption. Alternatively it might also correspond to another big eruption at this time: the Holmsá fires, another Eldgjá-like fissure that opened to the east of Katla. And still it might also be the Thjorsáhraun lava from Bardarbunga/Veidivötn, around 6600 BC. That lava actually travelled some 200km from Veidivötn towards the southwest, passing only cross 5km east from this location. That is the largest lava field on Earth since the ice age.

And now we get even older in time… Seydisholar 7000 BC

Below this point, it starts to get complicate to assign the identity of any layer because of a mud deposit underneath. There is some orange material just above it, which I assume it might have been the eruption of Seydisholar at 7000 BC, from the nearest Grimsnes crater row. That was the largest eruption of the Grimsnes system, with an estimated VEI4. And I am just a few kms from it.

Saksunarvatn ash 8000 BC?

At some points, there is a strange thick dark brown band around this depth, at around 70-80cm (see Fig. 4), which could have been the famous Saksunarvatn ash layer (Grimsvötn, 8000 BC): the largest eruption in Iceland in the Holocene. This ash is widespread recorded in northern Europe, and is used as an important marker dating the beginning of the Boreal period (end of the Young Dryas glaciation). Both the double layer (the 5000-6000 BC, referred before) and this deep dark brown layer, seem to ondulate, with one sometimes appearing over the other, and then exchanging positions. Their age is therefore highly uncertain.

Figure 4. Overall of our soil profile, with major bands identified. We cannot go before 8000 BC, as mud was deposited. Photograph by Irpsit, all rights belonging to him, used by permission.

The tale of a river bed, nearer sea levels, and also the ice age

Below 90cm we mostly find mud. This might have been a time when glaciers were over this region. At the glaciation peak, the ice sheet must have been at least 400m thick here, because of the nearby tuya Ingolfsfjall. However the peak glaciation must have been short, because we find much more shield volcanoes at this region than tuyas. About 5 km north, there is a large moraine, from where most of the time the large glacier terminated. For most of the ice age I was just at outside of the glacier.

The mud might been also caused by nearby Hvitá river (which drains the now distant Langjökull). The excavation is just next to a waterfall-like valley, thay I know it was the path of Hvitá river now 2km east.  Therefore it might been subject to much soil erosion and river deposits sometime before 8000 BC.

In early post-glacial time, the sea level was higher and the coast was actually nearby. There is actually evidende of a coast just 5km south (in the nearby shield volcano Hestfjall). The sea must have been pretty close and again this location was subject to much erosion. Because of all these reasons, we possibly do not have the record for the famous Vedde Ash (Katla 10.000 BC), which is one of the two largest eruptions in Iceland in recent millenia; the other was the Grimsvötn Saksunarvatn ash (both VEI6). In one spot, I did see some white material around 90cm deep, but I am unsure if this was it.

Ancient Lava (from Lyndhalsheidi)

Finally, on the bottom of the excavation, around 1.5m deep, there is a bedrock of lava rock (visible at some spots at lesser depths, such as in Fig.2). They are eroded and rounded (probably by the last glaciation). This is lava from the shield volcano Lyndhalsheidi that is just 8km northwest. Its lava actually flowed where I now stand, but that eruption was on the interglacial before the last glacial, so it was a long ago. However the glaciation continuously exposed and eroded that ancient bedrock.

Layers near the surface

There is no significant ash layer since the 1477 Veidivötn ash. However we can see sometimes faint layers from recent eruptions. One black layer around 5cm is probably the VEI5 Katla 1918. One faint white layer at 8cm is probably Hekla 1845 eruption. And a faint dark layer around 12cm is probably Laki 1783 (but it could have been the eruptions of Katla in 1755 or 1721; not visible in Fig.5).

FIGURE 5. Ash layers from recent eruptions in Iceland. There are not as continuous as the bigger ones, they only appear here and there. One can also see that the main white layer are actually two distinct white bands. The ash of 1477 is also double but because it started as a first eruption of rhyolite pink ash from Torfajökull, followed by basaltic brown ash of Veidivötn later. Photograph by Irpsit, all rights belonging to him, used by permission.

ICELANDIC ASH RECORDED IN GREENLAND

FIGURE 6. Again, an overview of the soil profile, but now using the initial picture form part I, and color enhanced. It’s exciting to contemplate the history of 10.000 years of eruptions in such a small soil wall. Photograph by Irpsit, all rights belonging to him, used by permission.

To finish today I read some papers that described which ash layers appeared in Greenland ice cores. We find there the 1362 Öraefajokull, 1104 Hekla, 870 Vatnaöldur ash, Hekla 3 and Hekla 4 eruptions, the very large ash layers of Saksunarvatn/ Grimsvötn (8000 BC) and Vedde / Katla (10000 BC), followed by many ash layers from Katla, Hekla or Grimsvötn, and finally other two very large ash layers: one from Tindfjallajökull Thorsmörk ignimbrite, 53.000 years ago (that was a VEI6+, and possibly even a VEI7). The volcano is still dormant now and right next to Katla and Eyjafjallajökull); the other big eruption was 300.000 years ago, and hypothesized to be from Krafla or Hofsjökull.

After this lenghty post, please feel free to call me a big ash hole.

IRPSIT

Editors note: Do click on the images, then you will see all of the details since they are rather large.

Update by Spica:
Here is the link to part I of the story.

What’s going on at Katla? Part 2

Part 2, A view of Katla

Fig. 1. Katla from Háfell looking NNW (RUV webcam capture)

So what really is going on at Katla? Well, we’re not really there yet. In this instalment, I will summarise what I have learnt from reading various scientific or otherwise papers and articles and my current understanding of it. At certain points I will supplement this with what I believe to be or could be the explanation, but when I do, I will say so. Again, I emphasise that I am not an expert in any way.

Katla is a relatively young volcano which like so many Icelandic volcanoes formed when Iceland was covered by ice. Hence it is a tuya, steep-sided with a broad, flat top. Like other large Icelandic volcanoes, it has a very large summit crater described as a caldera, but one that did not come about as a result of the collapse of the volcanic edifice into an emptied and very large magma chamber as happened at Mount Mazama a.k.a. Crater Lake in Oregon, at Krakatoa or at Long Valley.

Fig. 2. Herðubreið, a subglacially formed tuya with steep sides and a flat top. Post-glaciation, erosion has
made the sides less steep and a small post-glacial cone makes the top appear less flat than it once was. The
similarity to Katla, once you allow for the vast differences in size, is obvious. (extremeiceland.is)

One of the keys to understand what goes on at Katla is to have an idea of what lies beneath the up to 700 meters thick glacier that covers her crater/caldera. In schematic representations of Katla, a magma chamber at the very shallow depth of three to five kilometres is often displayed. From reading descriptions of other volcanoes that have suffered caldera collapse or looking up a general definition of ”caldera”, it is easy to assume that Katla too must have a magma chamber that spans the entire width of the “caldera” and which, “once-upon-a- time” collapsed to for the present-day caldera. Nothing could be further from the truth, but alas, there is no direct information available that accurately describes what Katla’s magmatic system, the true volcano, looks like. We have to fill this gap ourselves.

The first thing to do is to look at what she has done in the past. If we look up her “Eruptive History” on the Smithsonian Global Volcanism Program website, we find that Katla is listed as having had 27 eruptions during the period Iceland has been settled by humans, some eleven centuries and counting. Of these, only the larger eruptions seem to have been registered prior to the middle of the 20th Century. Thus the 27 eruptions are divided as follows: Two VEI 0 (1955 and 1999), three VEI 3, fourteen VEI 4 (including the AD 934 “Eldgjá fissure eruption”) and four VEI 5 with a further four not assigned a VEI number. Of the four unassigned eruptions, one is listed as “subglacial, lava flows” and three “subglacial, explosive”. Please take note of the dearth of smaller eruptions, VEI 0 – 2, as this is important and something we’ll return to later.

From this information, it is clear that Katla cannot have a single, caldera-sized magma chamber because such a chamber would contain several tens to even hundreds of cubic kilometers of magma, which in turn would have led to far larger eruptions. None have occurred. Since VEI 5 is assigned to eruptions that eject between 1 and 9 cubic kilometres of Dense Rock Equivalent (DRE) explosively, and Katla’s VEI 5 eruptions are remarkably consistent at between 1.2 and 1.5 cubic kilometres, anything much larger than some 3 – 4 cu km is rather out of the question. A caveat – given the area covered by the crater/caldera, there could be more than one such chamber responsible for her eruptions, in which case it would be fair to ask the question if Katla really is a single volcano or if not a description of her being several volcanoes rolled into one would be more accurate.

If we look at her eruptive history prior to Iceland being settled, deduced by tephrochronology – ash layers deposited being identified by their physical properties, such as chemical composition and grain size, as belonging to Katla and from the size, distribution and time derived for each individual layer of tephra, an eruption responsible for it is inferred – we find that there have been a multitude of eruptions, but only a few of which have been assigned a VEI number. Interestingly in every such case a VEI 3 or 4 has been deduced. Anything much larger must have left such extensive deposits that such a huge eruption cannot have escaped detection, hence we can conclude that no explosive eruptions larger than a small VEI 5 have ever occurred at Katla.

There have been two exceptions to the rule that Katla’s eruptions normally are in the VEI 4 range volume-wise. Both originate on her NE flank, outside the crater/caldera. Around 5550 BC, Katla was the source of the 5 cubic kilometres “Hólmsá Fires eruption” lava flow. In 934 AD, the four times larger “Eldgjá eruption” spewed forth some 18 cu km of lava and five cu km of tephra, or ash. Even if the total volume erupted in 934 AD, about 22 cu km DRE, is on the order of 50 times greater (25 to 200 times), a lowly “VEI 4?” has been assigned.

As the underlying causes and processes that drive “regional fissure eruptions” are vastly different and as they happen very rarely, seemingly with a time interval measured in several millennia in the same-ish location, fissure or rift eruptions should be considered separately – even if the visual appearance of the Katla crater/caldera suggests that a fissure eruption has at some point in the distant past intersected it. They are mentioned here because an article such as this cannot fail to do so, nor can it fail to give a reason why they are not included in the discussion.

Earlier I mentioned the apparent absence of small eruptions from her eruptive record with only two “possible subglacial eruptions” in 1955 and 1999 listed, to which can now be added the equally suspected or “possible” July 2011 subglacial eruption. As I write this, it seems that there may have been yet another, very minor hlaup. That such eruptions were not noted in earlier days is not surprising as the very small hlaups they resulted in were local nuisances rather than regional catastrophes of a major Katla jökulhlaup and would not have been seen as important enough to be recorded, even had they been observed. But how frequent could this type of small eruption be?

Fig 3. Seljansfoss Waterfall during the 2010 Eyjafjallajökull eruption (Binaural Waves Blogspot). Notice
evidence of several minor eruptions on the mountainside above the waterfall.

We know from the 2010 Eyjafjallajökull eruption that it was preceded by two fissure eruptions at Fimmvörduhals that intersected each other. If we look at the topography and geography of Eyjafjallajökull, we can see many areas of monogenetic cones. This indicates that eruptions of the Fimmvörduhals type greatly outnumber eruptions at the main vent. At Askja, a similarly sized volcano albeit glacier-free and with a slightly smaller summit crater/ caldera, there have been six small eruptions since the great eruption of 1875 and many prior.

Of the 24 eruptions (not counting the AD 934 Eldgjá fissure eruption) listed before it was realised that there were smaller eruptions that would only show as minor jökulhlaups, 20 are listed as VEI 3 or higher and three of the four not assigned a VEI number are listed as (subglacial and) explosive. At least 17 of the 23 explosive eruptions have been assigned a VEI of 4 or 5. The eruptive record of Katla thus indicates that in order to break through the up to 700 meters thick Mýrdalsjökull glacier, an eruption would need to be at least as powerful as to merit a designation of VEI 3. Thus – the reason for the dearth of smaller eruptions observed is that they are not energetic enough to break through thick glaciers such as Vatnajökull or Mýrdalsjökull to be visually obvious and the minor hlaups resulting have been much too insignificant to have been considered as a result of an eruption that never was seen.

Fig. 4. Pits formed by melting from below in the Katla glacier, summer 2011. The glacier was still covered
with tephra from the Eyjafjallajökull eruption which made such features stand out unusually well.
(ModernSurvivalBlog, picture may originate with Icelandreview)

With the advent of aircraft, it was noted that there were pits in the glacier as if it had melted from below and the collapsed to form an ice crater. These pits are relatively numerous and vary in size. They have been explained as due to either strong hydrothermal activity or, in the case of the larger ones, as the result minor subglacial eruptions.

The obvious conclusion is that in the case of Katla, small eruptions of the Fimmvörduhals type far outnumber the bigger, recorded eruptions. This is vital for understanding how a volcano such as Katla is built and works.

Let us for a moment return to what I like to call “Katla’s defrosted twin”, Askja. Here we can see, side by side, the effects of the two types of eruption. In 1875 she had the big VEI 5 eruption, about four times as great as Katla’s historic VEI 5s, that would eventually form lake Öskjuvátn. Here we have a magma chamber where magma collected over time, partially re-melting and absorbing the chamber walls which together with fractionating led to the body of magma collected being far more silicic than the basalt injected into the chamber, which provided the heat or energy for the process. This went on for centuries, quite likely millennia as GVP lists the preceding very large eruption at Askja as having occurred about 11,000 years ago, until a final basaltic intrusion was energetic enough to unbalance the magma chamber and the big eruption of 1875 followed. Please note that both before and after, there have been many smaller, basaltic eruptions that have evidently bypassed the main magma chamber on their way to the surface, one of which caused the miniscule crater Vítí located immediately north of Lake Öskjuvátn.

Fig. 5. “Katla’s defrosted twin”, Askja. Aerial photograph inside and above the Askja caldera with Lake
Öskjuvatn and the miniscule crater Viti barely discernible on the near left-hand side of the lake. (uwmyvatn
blogspot)

This too is what I believe must have been happening and is going on at Katla. Sturkell and his co-authors in their 2009 paper “Katla And Eyjafjallajökull Volcanoes” note that the products of Katla’s eruptions are bimodal, comprising alkali basalt and mildly alkalic rhyolites “with intermediates very subordinate”. One, or possibly more magma chambers where magma collects, fractionates and grows more silicic, a process that takes hundreds if not thousands of years which is why more than one magma chamber seems to be required in order to account for the relatively frequent eruptions of Katla, until there eventually is an eruption of “mildly alcalic rhyolites”, accompanied by tens to hundreds of smaller, alkali-basaltic eruptions which due to their location under the ice in a watery environment, gouge out small craters and fill in the bigger ones with mostly small, broken fragments of lava, piles of pillow lava or even small lava flows or easily eroded cones. When a big eruption occurs, the glacier first closes the wound, then the crater gets back-filled with loose rubble which gets pasted over with more solid lava flows from later eruptions.

This process has been going on for as long as Katla has existed. Not only has this constant remodelling inside the crater/caldera left a kilometres-deep zone of clastic, i.e. broken or fragmented, rock mixed with water, it also in my opinion explains how the caldera was formed in the first place. This layer extends down to not much above the roof/-s of the magma chamber/-s. As freshly injected basalt from the mantle makes its way up, it will eventually encounter this water-rich zone and result in intense activity, hydrothermal at first, and if the intrusion continues, hydromagmatic. It is primarily this activity we see when we look at the tremor charts of the SIL-stations surrounding Katla, in particular the one located at Austmannsbunga, on the north-eastern crater/caldera wall.

In the next instalment, it is time to take a look at Katla’s neighbours Eyajafjallajökull and the Gódabunga “cryptodome” and try and separate their activity from that of Katla so that we can finally figure out what she may have been up to over the last few years and how likely an eruption in the near future could be.

HENRIK