When glaciers start running 1

What is a jökulhlaup?

At the moment (December 30, 2012), water height and conductivity are again on the rise in Múlakvísl, an outlet river of Mýrdalsjökull glacier, the one covering the famous Katla volcano in the south of Iceland.

Plots from Icelandic Metorological Office (vatnafar)

This could be a sign for again another jökulhlaup in this region, Mýrdalssandur, where the Ringroad bridge was destroyed last year – or maybe not.

But what is a jökulhlaup?

Jökulhlaup is an Icelandic word which comes from „hlaup(ið)“ which means „(the) run“ and „jökull“ meaning „glacier“ – one of many Icelandic composita, as in Icelandic language words are set together like in puzzles. English words describing the phenomenon are “glacier run” or “glacial outlet flood”. (There is also a German expression for it: “Gletscherlauf”.)

For a long time, it has been sort of a puzzle, what is happening to glaciers and their lakes as well as under the many Icelandic glacier caps where volcanoes lie in wait for their next awakening. Because jökulhlaup often happen without any (observable) eruption.

Glacier outlet floods can have a lot of different origins.

There can be some which have nothing to do with volcanic activity, like the famous Lituya Bay run. http://en.wikipedia.org/wiki/Lituya_Bay In this case, a landslide in 1958 caused an enormous tsunami. Dam failures by earthquake or material break down can also be behind this, as was the case sometimes in the Alps.

Some floods can also be initiated by surging glaciers (http://en.wikipedia.org/wiki/Surge_%28glacier%29 ), which may or may not stand in a complicated and not yet clear relationship to volcanic activity.

And now we are approaching the really interesting cases.

There are also some different kinds of jökulhlaup, the real ones, which are in some way or other caused be volcanic activity.

Some of you were a bit disappointed by the last glacier run from a region with two famous ice cauldrons in the southwest of Vatnajökull, which is called the Skaftárkatlar (Skaftá cauldrons) in August 2012 http://www.vedur.is/um-vi/frettir/nr/2520 . It was a really small one, of about 185 m3/sec.

As is to be seen in this film https://contour.com/stories/skaftarkatlar-30-08-2012 the glacier runs normally come from sort of an indentation in the big ice caps that cover in the whole 11% of Iceland. And the runs from this region are produced very regularly, in a rhythm of 1-2 years, mostly in the spring or summer, so that the people down in Kirkjubaejarklaustur almost set their clock in connection to them.

They are caused by a high temperature area, and intrusion under the ice of Vatnajökull, which melts more or less the same amount of ice in the same time span. The water is collected in a bowl shaped lake under the ice and when there is enough of it, breakes the ice barrier in front and runs down through a system of tubes in the ice, which is built up and widened by the water pressure and heat of the water. These are – as we have seen this summer again – small glacier runs, which build up rather fast to a certain maximum and then slowly fade out again, “(…) there are, however, other types of Skaftá floods which have no distinct discharge peak but maintain a strong, stable current for up to 2 weeks before terminating.” (H. Björnsson, 2010, 2).

A similar kind of smaller, but volcanically induced hlaups is also found at the Grímsvötn volcanic system. We saw two of them in the last months ( see: http://www.vedur.is/um-vi/frettir/nr/2583 and http://icelandreview.com/icelandreview/daily_news/?cat_id=28304&ew_0_a_id=369682 .

Björnsson, 2002, p.4: 2 drawings, fig.3

In former times, travellers were always very fearful about traversing the sander plains in front of the big glacier caps in Iceland. And the guides, local men, farmers and eg. Postmen, praised like heroes. The most famous one of these was Hannes Jónsson of Núpsstaðir (1880 – 1968) whose life could be subject of a post of its own.

But, if these floods were so small and predictable, why would that be so?

There is still another kind of glacier run to be reckoned with, and that is the one normally connected with the notion: It is the cataclysmic outlet flood, having its origin in a big volcanic eruption under one of the icecaps, by which an enormous quantity of melting water is produced and after a longer or shorter period of time depending on intensity of eruption and last but not least on water pressure and decline of slope rushes down to the sander planes below taking with it a mixture of sediments, gases and icebergs.

Lómagnúpur and the Skeiðarársandur with outlet glacier Skeiðarárjökull in the background. Image from Wikimedia Commons.

In case of an eruption in the Grímsvötn region – Gjálp 1996 was a fissure system which is part of it – , the subglacial lake in the caldera below the mountain Grímsfjall fills, and in filling takes on the form of a sort of subglacial balloon. When a certain limit is reached, the water pressure is high enough and the ice barrier in front of it is broken, and the water rushes downhill. At the beginning through tunnels in the ice, but if the water discharge surmounts a certain limit, the water lifts first the barrier and then the whole glacier and rushes downhill in sort of a big subglacial wave with the glacier riding onto it. And down in the valley, the water spills out in a lot of bigger and smaller outlets at the snout of the glacier and more or less covers the plain in front. (Björnsson, 2010) The jökulhlaup connected to the Gjálp eruption in 1996 produced no less than 55.000 m3/sec at its peak covering all of Skeiðarársandur. (M.T. Gudmundsson, etal. (2004).

Wikimedia Commons

Drangajökull, Langjökull, Hofsjökull, Vatnajökull, Eyjafjallajökull and Mýrdalsjökull

When I started researching this, I was surprised to find out that not only had the “usual suspects” Vatnajökull and Mýrdalsjökull produced bigger and smaller jökulhlaup running in all directions, but that smaller ones were known from virtually all 5 big Icelandic ice caps, i.e. also Drangajökull (most probably not volcanic, because it is situated outside of the active rift zone), Hofsjökull and Langjökull. (O. Sigurdsson, 2005) And two years ago, we could observe how the outlet glacier Gígjökull from Eyjafjallajökull covered a small glacier lake with debris during some jökulhlaup. http://en.vedur.is/hydrology/articles/nr/2097

The phenomenon is of course not limited to Iceland, but is possible in all parts of the world with glacier covered volcanoes like Alaska or the Andes in South America.

And there have also been famous floods in the distant past, i.e. the Pleistocene, like the Lake Missoula floods which carved out the Columbia River Gorge in the USA, the Altai floods in Siberia, even one in Germany (Münsterland) and then there is the case of Jökulsárgljúfur in Iceland. Some of them are thought to have involved such an enormous quantity of water that the one of 1996 in Iceland is really dwarfed by comparison. But we’ll speak more about these in later posts.

Disclaimer: I am no specialist, just an interested layman.
IngeB

Literature:

⁃ Subglacial lakes and jökulhlaups in Iceland, by H. Björnsson (2002)
⁃ Understanding jökulhlaup, from tale to theory, by H. Björnsson (2010)
⁃ Jökulhlaupannáll 1989-2004, by O. Sigurdsson, etal. (2005) (in Icelandic)
⁃ The 1996 eruption at Gjálp, Vatnajökull ice cap, Iceland: efficiency
of heat transfer, ice deformation and subglacial water pressure, by Magnús T. Gudmundsson, Freysteinn Sigmundsson, etal. (2004)

Katla and volcanic weekend Riddles!

Today commenter Islander brought to my attention some news that the Icelandic volcano Katla seems to be picking up activity. But what is actually all the hoopla about really?

Katla is as most of you know one of the larger volcanoes on Iceland, but not the largest. The largest volcano on Iceland is the far less known Bárdarbunga volcano. Katla has been dormant since 1918, with the possible exceptions of small hydro magmatic events during 1955, 1999 and 2011. The hydro magmatic events are though not entirely proven to even have happened.

Katla has a 10 by 14km caldera that seems to have been formed during a minimum of two large VEI-6 eruptions. The volcano is covered by Myrdalsjökul glacier. During eruptions the volcano emits copious Jökulhlaups (water discharged as the volcano melts the glacier) and they are the single greatest threat from this volcano. Normally Katla has eruptions that range between VEI-3 to VEI-4, with larger eruptions happening after longer times of dormancy. Katla has had two large flood basalts after the ice age ended, the 5 500BC Hólmsá Fires and the Eldgjá eruption, both of them where following NW rifts trending roughly towards the larger volcano of Grimsvötn.

Earlier today (2012-09-29) the geologist Kristin Vogfjörð appeared on Icelandic Radio reporting that during the 24^th of September a set of deep earthquakes had happened under Katla and that is considered as a sign of an increase of the risk for an eruption. She carefully avoided making any predictions.

So what happened then? Late on the 23^rd 6 earthquakes ranging between 0 and 0.5M happened at depths ranging from 30 to 20 kilometers. On the 24^th two more earthquakes occurred at 26 kilometers depth at the same location, those two where 0.4 and 0.7M respectively. The area is in the eastern part of the caldera.

Normally I have a deep respect for Icelandic geologists, but this is just so much hot air blowing. First of all the earthquakes are so minute that the amount of in-fluxed magma can comfortably be counted in wheelbarrows. It is like taking a piss in the ocean.

There was no associated tremor pointing towards magmatic movement of any significant scale. There is also absolutely no visible motion on the GPS stations surrounding Katla. Even at worst and these earthquakes where just precursors of a later influx we are still talking about years before an eruption taking place, and one should remember that Katla has had no less than two significant influxes of magma during the last 13 years without erupting. This so called “news” will only bring out the Katla fearmongerers, without anything having happened really.

http://strokkur.raunvis.hi.is/~sigrun/KATLA11.html

---

Katla with trap formation in mid-ground. (behind the lake)

Image Copyright Eggert Norddahl

---

The weekend volcanic Riddles

GeoLurking posted a riddle yesterday. I am reposting it as the official volcanic riddle of the week so it will not get lost.

A Bald Hiker disappeared, the other ran away.

Who or what am I talking about? The answer should be one volcano, and one volcano-related name. Two points to be had.

Return of the Evil One’s Riddle

This time Alan has outdone himself to create a brain-wrecking geological riddle for us. I must admit that it is beyond my limited capacity.

Ek jou gesonde-of-tuin se gesig-gesentreerde kubieke “, maar my hoppers sou wees van geen nut in ‘n stope

What am I? What do I look like? What is the mining jargon? 3 points to be had.

Have a nice weekend everybody!

CARL

The Dead Zone

Updated 13 Sep 2012, see end of article.

In logic, an assumption is a proposition that is taken for granted, as if it were true based upon presupposition without preponderance of the facts. (Wikipedia)

Back around May of this year, Carl asked me to do a series of simulations using KWare’s Heat3D, a program written by Ken Woheltz and the Reagents of the University of California under the sponsorship of the US Governement. It’s a cool little program that allows you to run heat simulations of magma intrusions into rock of varying characteristics. I had been prompted to write an article about one of the more perplexing areas in Iceland (well, to me it is). Not feeling that I was up to the task, I offered to do the supporting graphics if Carl could find someone to write the meat and potatoes of the article. I killed off a weekend working up the plots, but two of the catch points that we ran into were; “What temperature of the intruding magma should we use?” and “What exactly is the geothermal gradient of the region?”

With those two uncertainties, and the bedlam of real life, the post never made it to the forum. Things happen.

Before I go on, I must warn each and every reader here that I am not a seismologist, geologist, or bona-fide expert in the field. I read a lot, have been “studying” geology and physics in some shape form or fashion for about 37 years. I am just an amateur like many of you, so there is ample room for error.

With that out of the way… now we discuss

First, “The Dead Zone” is not an actual named place. It’s just a colloquialism specific to VolcanoCafe. It’s that region of Iceland between Katla/Torfajökull and Bárðarbunga/Grímsvötn. I refer to it as “The Dead Zone” due to the seeming low number of quakes. Historically, and pre-historically, the region is quite active with fissure eruptions. Irpsit and others can give you more definitive dates and names about the area, but I am limited to what I can cobble together from various sources. There are many other features here, but the main ones that I can find data on are Veidivötn, Vatnaoldur, Skaftar, Eldgja and Trollagigar. (spelling as listed in GVP and may be missing some of the diacritical marks) Veidivötn, Vatnaoldur, and Trollagigar are part of the Bárðarbunga system, Eldgja belongs to Katla, and Skaftar belongs to Grímsvötn. (As parts of the parent volcanoes fissure swarms). As you can see from the overview plot, there just are not very many quakes in this region. (Ignore the dot dashed blue line, that was part of the original plot set and is not used here)

Now, why is the Dead Zone dead? Because it is really… really hot. Much more than you would think. When an eruption is completed, magma sits and cools after the eruption is over with. This cooling rate depends on the thermal conductivity of the surrounding rock. For Basalt, the heat capacity is 840 J/kg K. (this is what I used in the simulations), Granite, for comparison is 790 J/kg K. This is in part due to its lower density. How it works… in order to raise the temperature of one kilogram of the material by one Kelvin (same as one degree C), you need 840 Joules of energy (for Basalt). Since we are talking about heat capacity, Water is 4185.5 J/kg K and Ice (at 0°C) is 2090 J/kg , so you can see how water or ice can drastically affect what is going on. This is one of those “gotchas” that can throw this whole scenario off. This area has a high water table and that can seriously affect how accurate the simulations are. Keep that in mind as I continue.

Anyway… when a dike intrudes into rock, whether it erupts or not, it starts loosing heat at a rate that can be calculated (provided you have the skill, or a program written by someone with the skill). Heat3D runs through the iterations of how heat migrates into the surrounding rock.
Here is how a single intrusion works out over a few years.

In my original set of graphics, I used a temperature of 1600°C magma due to the runniness of the flows and how far they traveled. My original guess was 1100°C based on a statement that I had seen in a paper, and much discussion occurred between Carl and myself about what would be the sane value to use.

“Time constraints on the origin of large volume basalts derived from O-isotope and trace element mineral zoning and U-series disequilibria in the Laki and Grímsvötn volcanic system” Binderman et al (2006) places the temp in the 1120–1140 °C range based on a “Mg in glass” geothermometer. (calculating diffusion and formation rates vs temp and pressure). Another reference (that I can’t locate at this moment) implies a temperature of 1200°C at 250MPa for one of the clast minerals. 250 MPa is in the 10 km depth range. Still uncertain of what temp to use, I went with the program default of 1250°C.

I used a 10 meter dike width based off of the average of three known dike sizes contained in “Geodetic GPS measurements in south Iceland: Strain accumulation and partitioning in a propagating ridge system” LaFemina et al (2005). This produces a really crappy 95% confidence range of 0.5 to 10.2 meters. (three samples is horrendous, but it’s all I had) Since the size of the plot grid has a direct play in how long the simulations take to run, I used 10 meters in order to get the simulations done in one evening.

Okay… now the actual run. As noted, this is not the original, and for brevity, I focused on only one system, Veidivötn. In case you didn’t know it, Veidivötn is probably the most lively fissure system in the region. It’s responsible for many of the Tungnaárhraun tephra layers. (THc. THd, THe…) GVP places an event there at the following dates: -6650, -4800, -4600, -4550, -4400, -4200, -1200, 150. For each eruption, I placed a 10 meter wide dike and ran the program out until the next intrusion date, which was then added and the process repeated. Another “gotcha” that you should be aware of, the eruptions did not necessarily occur in the same part of the fissure. This simulation assumes that they did. In effect, this skews the region towards being hotter than it might really be (and don’t forget the possible effect of the water that I mentioned previously)
So… here is the final product for what conditions may be like under the Veidivötn fissure. The temperature scale from the previous plot applies here.

Pretty gnarly eh? This is the crux of why I think that you won’t really see many small quakes in this region. Each one of those fissure lines has a heat structure similar to this. The crust is for the most part, plastic and yields to any stress that comes along… until it arrives too quickly for it to give. Then you have the larger quakes and potentially an opening of the fissure if the conditions are right… such as a nearby parent volcano being at or near erupting and having a ready supply of magma to flow down the rift and open it the rest of the way up. Structurally, there isn’t really much there to hold the two sides together. Plate shifts can do it (tectonic), or a parent volcano.

---

From “IAVCEI General Assembly 2008 Conference Field Excursions, Excursion 1: Historical Flood Lava Eruptions The 1783-84 Laki and 934-40 Eldgjá events” August 14-17 2008

“In 1783 the people of south Iceland had enjoyed a favourable spring and were looking forward to summer. However, their destiny was about to change. Weak earthquakes in the Skaftártunga district in mid-May were the first sign of what was to come. The intensity of these earthquakes increased steadily and on 1 June they were strong enough to be felt across the region from Mýrdalur and Öræfi. The earthquake activity escalated up until 8 June when a dark volcanic cloud spread over the district, blanketing the ground with ash (Figure 18a). The Great Laki eruption had begun.”

I’ve worked out the distances to Mýrdalur and Öræfi from the Laki site and applied an Mw to MMI estimate based on a few real world quakes from the USGS catalog in order to get a feel for how the power drops off over distance. Based on the MMI levels at which a quake becomes detectable by an unaided person, the quakes leading into the Laki event were in the Mag 4.5 to 5.0 range.

It’s a bit of a reach, but extending the formulas from “New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement” Wells and Coppersmith (1994) down to Mag 4.5, you get the following numbers.

Mag 4.5 – Surface rupture length 0.5 km, Subsurface rupture length – 1.3 km, Downdip rupture width – 1.7km.
Mag 5.0 – Surface rupture length 1.3 km, Subsurface rupture length – 2.7 km, Downdip rupture width – 2.9 km.

THESE ARE ESTIMATES

There is a bit of slop in the formulas, it is an attempt to get a working estimate of the physical manifestations that you would see from a quake. These particular formulas are only considered reliable for events down to Mag 5.2, but they do track well with no oddities in the curves. Below 5.2 the confidence in what the formula says drops off.

From that, it seems that the Mag 4.5 to 5.0 quakes are what is needed to open the system up. They have the right sort of features; the crust itself has likely healed very little from the previous events and should not take a lot of energy to re-open.

All this rumination and reading is one thing… but there is always something missing when you think and talk about these fissure eruptions. That’s the scale of the things. Since none of us were around, we just don’t know or have a frame of reference. All we have are eyewitness accounts. From some of those accounts, we know how long or how tall the fire curtain was, but that’s it. Just numbers in a book. Here, I have scaled an image of a generic fissure eruption and placed a few well known silhouettes in front of it so that you can see just how big these things are.

Enjoy.

GEOLURKING

GL Edit: The silhouetted buildings are;
Empire State Building – 443.2 m, Taipei 101 – 449.2 m, Burj Khalifa – 829.84 m, Sears Tower – 527 m, Petronas Towers – 451.9 m

“GVP” = Smithsonian Institution – Global Volcanism Program

UPDATE:

Irpsit says:
September 12, 2012 at 18:26

From what I know Laki eruption could be observed from almost anywhere in Iceland, in distance. The reports even speak that you could see the fountains from far away, but probably not everywhere in Iceland, as 1km high is not enough for that.

This put me on a search for two of the images that I made for the original article. I was able to pull them from Google archive of my mail.

They are not as stunning as the scaled image, but they are worth pondering. The ruddy maroon rectangle represents the Skaftar (Laki) fire curtain anchored to the surface, as seen from a couple of locations.

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.

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

This week I was lucky enough to have a recently dug square hole (10m per 10m, about 2 meter deep) some 200 meters from my house in Southwest Iceland.

Needless to say I spend the past bright summer evenings of Iceland inside this hole, which has nothing else but dirt and rocks. To us, volcano lovers, having such a hole in a volcanic land is like finding a mine of gold!

The soil shows many layers of colored material, which is nothing but the ash that has fallen from the many eruptions that happened in Icelandic history. This is a science called tephrachronology and it became my newest hobby.

Photograph and copyright belonging to Irpsit, used on explicit permission by Volcano Café. An excavation near home. And I stayed until late night to look at its strange layers.

When an eruption happens (if it’s the explosive type) the ash usually drifts according to local winds. In Iceland, the wind can blow from every direction depending on the kind of weather. This results in ash being deposited in a space-specific way for every different eruption.

A large eruption such as Askja in 1875 (VEI5) blew almost entirely to the northeast (so since I live to the southwest, I cannot find any Askja ash). In practice this means that the absense of a famous eruption does not mean it did not happen, just that the ash blew somewhere else. Likewise, a smaller eruption can deposit plentiful ash if the same wind keeps blowing in one direction (example of Eyjafjallajökull blowing southwards towards Europe in 2010).

In one single spot, the ash from different volcanoes accumulates over time, giving a profile of layers, that correspond to a time orderly of eruptions of different volcanoes. Usually, famous eruptions such Vatnaöldur in 870 (when the settlers arrived) can be used as markers for less known eruptions. The identity of a volcano can be roughly identified by looking at its color. We know that few volcanoes in Iceland produce white tephra, only Hekla and the rarer eruptions of Öræfajökull and Askja. Grimsvötn often produces brown ash, while Katla or Eyjafjallajökull black ash.

But enough of introductions! Let’s go for the real thing.

Photograph and copyright belonging to Irpsit, used on explicit permission by Volcano Café. The history of many eruptions is recurded as different ash layers.

The walls from the hole reveal, at instant glansing, two bright WHITE layers (figure 1). At close inspection, the upper white layer (at 25cm) is actually a double of two light colored layers, while the lower at (49 cm) is a single thick layer. Obviously these layers seem to come from Hekla.

The Hekla 3 white layer
To confirm whether or not these are from Hekla, there is a scientific paper of a soil profile done very near to where I live, around Grimsnes volcano (just 5km from where I live). They found only one large white layer at 50cm which corresponds to the largest eruption of Hekla during Holocene, the Hekla 3 eruption (a VEI5+) of 1000 BC. This is probably our second and largest layer.

Picture taken from Wikimedia Commons. Hekla is the source of much white ash in Iceland (as observe from the deposits on its flanks).

So, imagine, an eruption that deposited a layer of about 4cm thick ash here. That is pretty astonishing considering that a normal Hekla eruption barely deposits ash here (I am about 50km from it). This euption resulted in a 18 year climate change in Europe, observed in tree rings. It should have been one big huge eruption.

Now, if we look at the top white double layer, that is surrounded up and down by two thick DARK bands. These are actually a pinkish brown. Both are about 3cm thick ash (impressive too), the lower band is especially large at some spots.
The two dark Bardarbunga ash bands
According to other studies (and to Inge B), and also my conclusion, these are both the Veidivotn ash (1477) and the Vatnaöldur ash (870 AC), known as Settlement Ash (because it happen around the arrival of the vikings to Iceland). At least the Vatnaöldur ash is widepread reported everywhere in Southwest Iceland. Furthermore both have orange colored deposits underneath (actually light pink in Veidivotn ash, and bright orange in Vatnaöldur ash) which is expected. Both eruptions started with rhyolite ash from Torfajokull followed by the greyish/brown color of Bardarbunga fissures. The Torfajokull ash in 1477 was erupted from Brennisteinsalda, which is a mountain very colorful but mostly pink and orange.

Brennisteinsalda is the volcanic cone that erupted some colorfull rhyolite in 1477 (within Torfajökull).

The “double” white band of Hekla 1104 and 1341
If these are correct (I don’t confirm they are), then there are 2 white tephra eruptions in between. It’s easy to ascribe one to Hekla in 1104 (the largest eruption of Hekla since settlement (and second largest of all volcanoes), a very destructive one, but the ash during that one, was reported to go mostly northwards). The other one could either be the eruptions of Hekla in 1300 or 1341 (both with heavy ash) or less likely the 1362 eruption of Öræfajökull, which was the largest eruption of all, since settlement! Yes, larger (in tephra and intensity) than all Katla eruptions, Laki, Veidivotn, Askja or Hekla. Few of you know that Öræfajökull is a mamoth volcano, the largest in Iceland (and tallest).

However, I do think that this more recent white layer, was most likely the 1341 eruption. In 1300 the ash blew mostly northwards resulting in a famine, but in 1341 it blew westwards, and quite far away (towards Akranes). In 1362, the ash of Öræfajökull blew mostly to the southeast, opposite of where I am (and I know little ash felt to the west, in Vík – information from Skaftafell national park).

There is so much I write in a second part. All the minor layers in between (that you only see in close-ups) and all the broad bands below Hekla 3. Until then, let’s us discuss what we have so far.

IRPSIT