Eifel Volcanic Field II

Recently Nathan took you on a comfortable journey into the Eifel volcanic field. But what is the origin of this intraplate volcanism and where will the journey go?

About 400 million years ago during the Devonian, the Age of Fish, when only plants and insects roamed the land, Laurussia and Gondwana converged into the supercontinent of Pangaea forming the European Variscan Belt. It includes vast mountain ranges stretching from Portugal to Turkey. The Rhenish Massif in central Europe is one of the outcrops of this period, others are the Massif Central in France or the Bohemian Massif in Czech Republic and Poland.

The Rhenish Massif is mainly made of highly folded sedimentary metamorphic rocks, mostly slates, hence the name “Rheinisches Schiefergebirge” or “Rhenish Slate Range”.

Geological map of the Rhenish Massif. Author Jo Weber (Wikimedia Commons)

When the Age of the Mammals dawned and Africa started to collide with Eurasia, a whole lot of volcanic activity started north of the rising Alps. This belt was termed European Cenozoic Volcanic Province by Meyer and Foulger. In the Alpine forelands extensional rift systems developed with the Rhine graben as a prominent feature. Volcanic activity of that period can be found in France (Massif Central), Germany (High Eifel, Westerwald, Vogelsberg, Rhön), The Czech Republic (Eger graben) and Poland (Lower Silesia).

Figure 1 from Meyer and Foulger http://www.mantleplumes.org/Europe.html

The ductile and tough shale and slate bedrock of the Rhenish Massif presumably was incompatible with extensional rifting. Instead the region acted as a hinge between shear rifting along the Upper Rhine Graben and extensional rifting at the Lower Rhine Basin (Illies et al. 1981).

Tectonic situation in central Europe (from this thesis, modified from Illies and Fuchs, 1983)

The Eifel volcanic field is situated west of the Rhine river near Koblenz in the center of the Rhenish Massif. Fluvial deposits prove that this area was uplifted up to 300 m since the Pliocene epoch 5 million years ago and that the uplift had accelerated during the last 800,000 years with maximal elevation around the Eifel volcanic field. Since then the Rhine river and its tributaries were forced to cut deep valleys through the Rhenish Massif, flowing past Hunsrück and Taunus, Eifel and Westerwald, Ardennes and Süder Uplands.

Uplift in the Rhenish Massif, from Meyer and Stets (2002)

The most recent volcanic activity in the West and East Eifel volcanic fields coincides with this uplift which amounts to 0.35 mm per year on average. The dome building may be a combination of widespread uplift of the so-called Rhenish Shield due to horizontal deformation from Alpine orogeny (Illies et al., 1979 and 1981; Meyer and Stets, 2002) and more locally by uplift due to the Eifel mantle plume (Schmincke, 2007).

To study the deep structures of the Eifel region the Eifel Plume project temporarily deployed a large network of seismic stations in 1997. A shear wave velocity model suggested a 100 km wide low-velocity structure extending down at least 400 km into the upper mantle which could indicate an area of increased temperature and partial melting. It remains debated whether this anomaly caused the Eifel volcanism. Other volcanic areas of the European Cenozoic Volcanic Province lack clear evidence of deep mantle plumes and the spacial distribution and timing of eruptive phases is not consistent with movement of the European plate over a fixed hot spot.

Alternative models could be a magma source derived from previous Alpine subduction or local decompression melting from passive rifting caused by tectonic deformation of the crust. Notably, the Mohorovičić discontinuity (Moho) is only 30 km deep below the Eifel while under the Alps it goes down to about 50 km which could give rise to some mantle turbulence and convection.

South-North section of the Moho beneath Europe between 6 and 9° longitude. Depth is highly exaggerated (Image by chryphia). Data from www.seismo.helsinki.fi/mohomap/

There is an overwhelming amount of literature about the recent quaternary activity of the 300+ volcanoes in the Eifel, sadly most of it paywalled or even without online access, because published in books or exotic German journals. So the following is taken from secondary literature. The eruptive history was e.g. summarized by Schmincke in Mantle Plumes (2007), Schmitt et al. (2010) (see Fig. 1 here for a map of geological map of the East Eifel volcanic field) and is nicely illustrated in this German blog post.

In summary, there seem to have been at least four main eruptive phases:

700,000 to 450,000 years before present: the main bulk of monogenetic volcanoes, small cinder cones and short lava flows erupted in the West Eifel and late some in the East Eifel. Their lava contained leucite (potassium rich) basalts, poor in SiO₂, indicating an upper mantle source.

The West Eifel then fell dormant for several hundred thousand years.

430,000 to 360,000 years before present: In the East Eifel the Rieden complex (“Riedener Kessel”) west of the Laacher See had its most productive episode sputtering out several cubic km of lava in larger cinder cones and kilometer long phonolithic lava flows out of a 4 km diameter caldera system.

215,000 to 190,000 years before present: In the East Eifel the Wehr volcano (“Wehrer Kessel”, a 2 km diameter depression) west of the Laacher See and many large scoria cones in the Neuwieder tectonic basin erupted several cubic km of dense rock equivalent. The lava was highly differentiated phonolitic and rich in SiO₂, indicating that country rock had been partially melted. During this time the first Maars were blasted out of the West Eifel volcanic field.

100,000 to 10,000 years before present: the West Eifel field was peppered with Maars still erupting the original lava, the last one to be the Ulmener Maar. Simultaneously, a new kind of lava, basanites, poor in potassium, hence leucite free, presumably from the asthenosphere, created large cinder cones and lava flows sometimes right next to the Maars (e.g. Meerfelder Maar next to the Mosenberg).

In the East Eifel only the Laacher See erupted 12,900 years ago, without doubt the most powerful eruption of all time in the Eifel probably equalling the total output of the West Eifel volcanic field. The Laacher See erupted more than 6 cubic km of magma within days, with an at least 25 km high eruptive column spreading tephra from Italy to Sweden. The magma is thought to have differentiated over several thousand, possibly tens of thousands of years, showing zonation from mafic to evolved phonolite and carbonatite. Pyroclastic flows temporarily built a dam in the Rhine river which eventually broke unleashing torrential floods, illustrated here (in German). Finally the emptied magma chamber collapsed leaving this recreational lake.

The “Loch Lochy” of Germany, the Laacher See. Image by USEBlackbird (Wikimedia Commons)

So the Eifel volcanism occurred in tens to hundred thousand years periods intermitted by hundred thousand years of dormancy. There was a general trend of eruptions starting in the NW progressing to the SE. Eruptions became increasingly voluminous and explosive with time and there was a shift of lava from an upper mantle source to partially melted crust.

Today the Eifel volcanism is dormant. As already featured in Nathan´s post abundant CO₂ emission is a sign that the Eifel volcanic field is not extinct. But also seismically the region is active. Earthquakes during the past 36 years are almost exclusively confined to the upper 15 km. There is no indication of magmatic origin so far. The highest earthquake density is east of the Laacher See and west of the Neuwieder basin along the Ochtendunger fault zone on a NW to SE axis, aligned to the general tectonic setting in the Rhenish Massif.

Recent earthquakes (Sep 2012 to Jan 2013, green, enlarged) and earthquakes dating back 36 years recorded by the seismic station Bensberg, University of Cologne. Image by chryphia.

And here a 3D plot:

Since 1975 up until January 2013 over 1180 local earthquakes were reported by the seismic station Bensberg (University of Cologne) with some increased frequency in the last years.

Earthquake data from the seismic station Bensberg from 1975 to 2013 (between 5.21 and 5.472° lat and 7.25 and 7.65° lon, as in 3D plot). Image by chryphia

Helium and other noble gases that are found in high concentrations around the Laacher See are indicators of the volcanic origin of the Mofettas. Helium isotope 4 (⁴He) is naturally formed in earth´s crust. Another rare Helium isotope, Helium 3 (³He), is produced by fission and bombardement with high-energy cosmic rays, so what we find on earth was created before our solar system formed. In the atmosphere it escapes into space. Looking at the ³He to ⁴He ratio in volcanic gases relative to the ratio in earth´s atmosphere (Ra) gives a clue about the source of the magma. If it´s of deep origin, it still should contain relatively high ³He. The ³He/⁴He ratio measured from Mofettas from the Laacher See is 5.5 Ra, indicating an upper mantle source, but it is less than measured at mid oceanic ridges (8 Ra), thus there is mixing with ⁴He from the crust.

So there we are today. Was this the end of it for the next 100,000 years? As long as the Brubbel squirts and the earth rumbles occasionally we can´t be sure of it. Maybe the ants will tell us one day.

And just in case: a list of webcams

chryphia

Many thanks to Nathan for discussion and support!

Did you notice the erupting Supervolcano?

This idylic scene is from Lake Tondano situated within the 20 by 30km Tondano Caldera.

Some volcanoes just can’t catch a break. Imagine for a little while that you are a bona fidé supervolcano. You are the largest of your type on the planet, you are highly active, and by gosh you have shown what you are capable of. In a perfect world your 20 by 30 caldera explosion should have put the world into awe, and the 1 000 cubic kilometer of DRE you ejected in the form of pumicious tuff covers an entire sub-continent. Yepp, you really did reach the small highly exclusive club of VEI-8 volcanoes. You smirk at your little sibling Monte Sommas antics with Vesuvius. Your Vesuvius event left a 3.5 by 5 km God honest caldera on its own. To top it off you have a huge underground reservoir of liquid acid that would seriously alter the planets weather if you felt like discharging it. You are also perfectly located to have a maximum kill ratio. So, you wake up and stretch your arms and start a double eruption from two different sub-volcanoes just to celebrate the new day. You have your largest eruption in recorded history. Then you look around to see the fearful faces of the residents as they offer up motorcycles in your name, you expect volcanologists doing somersaults as they play lip banjo, and literally thousands of blog pages glorifying your power and shear awesomeness. What do you find? Yawning people and a cockerel trying to wake up a pig sty. You find that for being an erupting supervolcano you are a massive PR failure. One single small earthquake at Yellowstone outperforms you in publicity.

Tondano

Compund satellite image/map of the Tondano area courtesy of JPL.

The quarternary volcano of Tondano in northern Sulawesi (Indonesia) had its massive caldera event about 2.5 to 2 million years ago. Technically it is a somma type volcano, complete with the remnants of Pangalombian, a former stratovolcano that disappeared in a Vesuvian VEI-7 total caldera event.

Parts of the Pangalombian caldera were later covered by the now dormant Tompaso volcano that ejected large amounts of basaltic andesites in a long series of VEI-6 eruptions.

Todays Tondano is known for having acidic maar eruptions inside the caldera, a couple of mud volcano events during recorded history and no less than 4 active volcanoes, Lokon-Empung, Mahawu, Sempu and Soputan. Quite often Lokon-Empung and Soputan have tandem eruptions.

Lokon-Empung

Lokon-Empung is a double coned strato-volcano located at the northern rim of Tondano. Lokon is a flat topped probably dormant volcano that no longer exhibits a crater on top and Empung is a historically active volcano that last erupted 1775. From 1829 onwards the site of no less than 25 eruptions has been Tompaluan, a smaller double crater situated in the saddle pass between Empung and Lokon. It has erupted since 2011 in tandem eruption with Soputan. The tandem eruption before that occurred on the 13^th of May in 2000.

The current ongoing eruption is slowly working its way to becoming a VEI-3 eruption. But it has so far mainly been consisting of small explosive ash eruptions so it takes time to reach that level.

Soputan

This small stratovolcano is located on the southern rim of the Tondano caldera. It is part of trending line of ring dyke vents that formed in consequent eruptions ending with the formation of Soputan stratovolcano. It normally erupts from either the flanking vent of Aeseput or through the unusually large summit crater that pretty much has the same width as the top of the stratovolcano. This is of course a sign of a very young volcano with a highly potent vent system.

The current eruption consists of ejections of small to moderate explosive ash plumes. The ash columns according to the Darwin VAAC have been up towards 12.1km, with several slightly smaller columns reaching 9km height during the last few weeks. Smaller explosive ash plumes have been pretty much ongoing for the last 3 months now. This eruption is quickly ramping up to becoming a VEI-4, and is as such the largest sub-aerial eruption since Grimsvötn 2011 and that is without even counting in Lokon-Empung into the picture.

The system

As any volcano capable of a large caldera event Tondano has a large and intricate internal plumbing. It is believed that there is a very large reformed magma chamber at depth. As pressure increases in that magma chamber when new hot magma arrives it is believed that the magma either goes up into the caldera as emplacements, and that those sometimes cause maar explosions or reheats the very active thermal fields contained within the caldera. Or that the magma is pushed up into smaller sub-chambers under the active rim volcanoes. When that happens eruptions normally follows very rapidly. A sign of the rim volcanoes being systemically interconnected somehow is that Soputan and Lokon-Empung on many occasions has had eruption interspaced with mere hours.

As any Somma volcano the Tondano caldera is highly intricate and complex, and still it is surprisingly badly researched. The only good material is an Icelandic funded study on the possibility for hydrothermal energy plants in the region. Yepp, the Icelanders are going international with their knowhow.

Why it won’t happen

Image by Andreas / AFP – Getty Images. This image shows how relatively close the volcano is to villages, the height of the ash column and at the same time that the base of the ash column is equally wide to the width of the top of the volcano of Soputan.

For those who dream dark dreams about enormously destructive eruptions Tondano is a bad bet. Why? Tondano has it all really, large magmatic influx, steady inflation, a large central chamber, active volcanism. Pretty much everything that it should need for a VEI-8 eruption. Except for 3 small things, it does not any longer have the amount of water necessary to drive an eruption like that. As many of you know water is a large part of large caldera events. When Tondano went massively caldera it was situated pretty much at ocean level, so as the final large eruption (probably a large VEI-7) happened and the top of the caldera slumped inwards the ocean roared in and what is probably the largest steam explosion happened. Think of it as hundreds of Krakatau eruptions happening at the same time, and you have the picture. As time has passed the land has been lifted due to tectonic uplift.

Second thing is that the magma before the massive caldera event was highly crystallized rhyolites. After the eruption the magmas have been predominantly alkali basalt-andesites.

And the third reason is that Tondano is very well vented as long as the rim volcanoes are connected to the central magma chamber. As soon as the pressure gets above a certain level the magma squirts into the sub-chambers and the volcano on top erupts.

To put it simply, Tondano is a champagne bottle with 5 bottlenecks. The cork is well fastened on top of the actual central chamber, so it cannot erupt that way. Then it has one volcano with the cork slammed back fairly well (Sempu), but that is not fully dormant. One that has the cork put lightly back on (Mahawu) and two bottlenecks that haven’t seen a cork for hundreds of years.

Basically, the pressure is almost constantly being released by Lokon-Empung and Soputan, and if that is not enough Mahawu erupts too. Last time Mahawu erupted with another of the volcanoes it was Lokon-Empung in 1958. Currently even if pressure got really high the only thing that would happen is that all 4 volcanoes would go off.

The only risk for anything really interesting happening would be if one or two of the vents got blocked off. Even then no caldera event would happen, but the likelihood of a Vesuvius event would increase a lot. Currently the candidates for that is either Lokon-Empung or Soputan. Soputan seems to have a very wide bore caliber vent so it could probably release the pressure without exploding from the face of the planet. But Lokon-Empung has evolved quite a lot more, and as it has grown older the vent has narrowed down considerably. If Lokon-Empung was subjected to high pressure it would probably not be able to handle the stress and subsequently go off with a VEI-6+. This is though not likely at current geological timescale.

The only real risk is that a magmatic emplacement will happen in, or around the large reservoir of sulphuric acid (water with a ph of 2). I think anybody can imagine how un-nice a maar event, or even worse, a phreatic explosion, would be if it happened to cubic kilometers of liquid acid. First of all it would make northern Sulawesi uninhabitable and kill off large portions of all life there. And a phreatic explosion would severely hamper the world weather for quite some time. Not a nice thought is it, an acid caldera event. I would decidedly not want to be around if that happens.

Tondano today

Lokon-Empung belching out a 3km ash column.

For being a highly active volcanic region with at best medium risk of fatalities the volcano is surprisingly badly monitored and highly under-studied. Almost all I have written is from one study alone, and that was produced by Orkustöfnun as a part of the geothermal engineering program. Interestingly that report predates the recent article in Nature about a new tectonic plate forming next to Sulawesi. You can clearly see the rift fault in one of the maps in the PDF. Nature seem to have done a bad background check on their paper before publication.

In reality if we look beyond the doom and gloom prophecies of a large caldera event volcano the risk is the bad monitoring. The area is heavily populated and an unexpected VEI-4 eruption at a flanking vent, or lahars, or pyroclastic flows will kill people, potentially a lot of people.

A thought

When a volcano of this size erupts and the world’s volcanologists, volcano-bloggers, and generally the large number of volcano aficionados yawn and continue to look at other less interesting volcanoes that is not even erupting, then something is a bit wrong. I happily admit that it took me almost a week before I actually got around researching the volcano. Then my jaw dropped and I started doing somersaults while playing lip banjo. It is just the sad truth that there are more well known supervolcanoes in the predominantly white western world that steal all the attention.

While we sit and moan about there being no interesting eruption we did not even reflect as we read that two more volcanoes in Indonesia erupted simultaneously 30km from each other. The only comments about it was that people rode their motorcycles inside an ash cloud to get to and from work (Lokon-Empung), and that a rooster cackled at a video of Soputan barfing up a 9km ash column. Then we went back to looking at out Katlas, Heklas, and the rest of the non-erupting volcanoes. Indonesian volcanoes could do with a good PR-Agency.

CARL

http://www.os.is/gogn/unu-gtp-report/UNU-GTP-2010-03.pdf

Eruptions at Tongariro & Whaakari (White Island) and 1 million viewers!

Image by IGNS Ltd.

As most of you know 2012 had up until a couple of days ago been rather free from significant eruptions, but that has now changed. As the ash and smoke starts to clear we now know that the explosions at both Whaakari and Tongariro was not the main events.

Tongariro

Image by Lurking showing ash column height and ash spread radius. This plot was also made at the same time as Lurking became the 1 millionth viewer. Quite fitting really.

The eruption that happened during last night was mainly driven by water pushed past the steam flash point. That in turn caused a large steam driven explosion that hurled incandescent stones out of no less than 3 new vents in the mountain close to the Te Mari craters. The steam also lofted ash and steam up to a height of 6 000 meters (20 000 feet, or FLA 200 as the VAAC terminology goes).

Photograph by Diana Booth. Rare image of an ash and steam cloud taken from below as it rises into the heavens after an explosive phase ends.

The steam explosion was caused by rising magma hitting the permanent water table, also, the magma from Tongariro contains a lot of water, and that most likely decompressed into a steam explosion.

The event was rather short in duration. According to the seismograph plots the actual explosion was about 1 minute long, and the main eruptive phase was about 20 minutes long. After that there was mainly steam being ejected. The steam phase lasted for about 20 hours when a second smaller steam driven ash explosion occurred.

Image by Geonet.

Risks at Tongariro

This is most likely not the main event, this is just a pre-cursor activity as magma rises. It is quite normal for andesitic subduction volcanoes to have an initial phase of steam driven ash explosions like this. This phase can last for a day or two up to a few weeks before the real eruption starts.

Quite often the size of the steam explosions are indicative of what will come during the main event, and a steam driven ash explosion that lofts up material to 6000 meters height is telling us that there can be something rather large in the making. My best guess is that this will be around a VEI-3 eruption.

Earlier today I read an interview with a local woman living close to the volcano. I was taken rather aback when I read that she felt safe where she was living. She was telling about seeing ash and steam rolling down the side of the volcano into the valley she lived in. Apparently she and other locals think this is as bad as it gets.  This is rather ignorant since the main dangers are lahars and the even worse pyroclastic flows running down the mountain into the valleys.

I hope that the valleys will be evacuated in time. One should though not forget that the eruption can change pace rapidly, and that it is better to be safe than sorry. Dead is a rather permanent position in life.

http://www.stuff.co.nz/national/7426862/First-Tongariro-eruption-in-over-100-years

Whaakari (White Island)

Image by Geonet. Moonlighting volcano at its best! Beginning of the nightly steam explosion at Whaakari (White Island) back lighted by the wonderful moonligh.

Whaakari is also a member of the TVZ (Taupo Volcanic Zone). It is a very large volcano built up by no less than 78 cubic kilometers of material. It is a complex volcano containing multiple vents and craters. A few days ago the Crater Lake went from being a small mud pool into being a sizeable lake as the water level rose 6 meters over night due to increase in hydrothermal pressure. A day later (also at night) a steam driven explosion hurled up ash and mud covering the new crater, the same area that killed eleven sulphur miners during the end of the mining epoch at Whaakari.

Image by Geonet. The man activity was on the fourth of August, but the level of tremor is still above normal, a probable sign of rising magma in the system causing steam explosions during its progress.

White Island is well known for its high rate of eruptions. It normally erupt very complex lavas pointing to either a mixed heritage of basaltic and andesitic feeder sources, or a complex magmatic system with high fractioning of the magmas. This produces the famous “clean” and “dirty” andesites. The volcano is at best highly unpredictable and can erupt without giving any untoward signs beyond the normal high background level of activity. To go there during an eruptive phase is to be considered very dangerous.

Image by Global Volcanism Program taken by Richard Waitt, 1986 (U.S. Geological Survey). The current active area, photograph is from 1986.

The same goes for Whaakari as for Tongariro; this is most likely only a pre-cursor phase before the real activity starts. Historically Whaakari has slightly stronger eruptions than Tongariro with the norm being VEI-2 eruptions, but with an upwards trend in strength of the eruptions during the last 170 years with the norm now being medium sized VEI-3s. The last eruption was in 2001 and rated as a VEI-2. But the year before there was a short and brutal VEI-3. And it is fairly indicative of the volcano that it has an upwards trend as the volcanic system evolves. What makes this volcano more prone for larger eruptions than Tongariro is the large (almost limitless) access to water to drive the hydro magmatic processes going on down in the volcano. The currently active crater floor is only 13 meters above sea level.

1 million viewers!

Image by Spica.

It is rather insane that it took us this short time to have 1 million viewers. From the beginning this has been a rather nutty experience. As I was convinced by a few others to create this place I expected a couple of hundred views per day, and a few comments. I never expected to start with 5000 viewers on the first day… And it just continued like that. As I have said many times, this is a group efforts and during the last half a year (slightly more) had a tremendous amount of posts published by many of our members. Keep those lovely posts coming and we will soon pass 2 million!

Little known fact, this is also Swedens largest blog… How about that?

CARL

Confirmed eruption at Mt Tongariro

Source: Global Volcanism Program. Photo by Jim Cole, 1974 (University of Canterbury)

This post will most likely be updated fairly quickly as news come up and we get more information.

It seems like Mt Tongariros awaited eruption has started. The eruption seems to be generated out of the Te Mari Craters. Witnesses report an ash column that exceeds 6 000 meters with steady lightning. There are also reports of lava bombs or incandescent lava slabs being ejected from the volcanic vent located on the side of the mountain. That witnesses talk about a hole in the side of the mountain points towards a new crater in the Te Mari crater-area.

Tongariro is a part of the Taupo volcanic belt. It is one of the most prolific volcanoes in New Zeeland. The last eruption was in 1977. During the last 115 years it has erupted 49 times through the southern crater complex, Ngauruhoe, while the Te Mari crater has been dormant. The Eruption follows magmatic emplacements during 2006 and 2009 and increased activity during the last few weeks.

The Ngauruhoe eruptions have been moderately explosive with only 3 eruptions ranging VEI-3; the others have been predominantly VEI-2 eruptions with just a few being even smaller. 550 BC there was the last larger eruption, a VEI-5 out of Ngauruhoe crater. The last VEI-5 out of Te Mari crater was 9350 BC.

There is currently nothing pointing towards this eruption going to exceed a VEI-3 eruption. One should though note that eruptions from previously semi-dormant craters in a complex andesitic volcano can be livelier than the previous eruptions from a well used crater part.

Source: Global Volcaniism Program. Photo by Graham Hancocks, 1975 (New Zealand Geological Survey)

The amount of activity and height of initial ash column seems to point more towards a small VEI-3 than a VEI-2. So there is some cause for concern for those who live close by.

This post will be updated as soon as we get more news. For latest news we recommend that you follow the comment thread. Expect that there will be a call for evacuation of locals soon.

CARL

Update:
Radio New Zealand News ( pointed out by IngeB )
Again another page on Radio NZ News
Bay of Plenty Times

GeoNet NZ Tongariro Activity
GeoNet NZ Seismometers called Drums

.
Webcam Tongariro
Other webcams listed, all are in Tongariro National park
One can watch a diashow of the “Rivercam” here.

Volcanic advisory Tongariro

GeoNet informations on Tongariro

Skiing the pacific “ring of fire” and beyond
Tongariro Alpine Crossing.

Wikipedia Tongariro
Weekly Activity report Smithsonian
GVP Tongariro

Claude Grandpey on Tongario today!
And last but not least Erik Klemtti on Eruptions about this event.

Update by Spica

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.