Debunking Caldera Myths

Post by cbus20122:

Before this post gets read, I would like to note that I am not a scientist or geologist. If any information is inaccurate in this post, I would like to encourage the more scientifically inclined to correct me and inform readers if there are any inaccuracies!

Caldera Volcanoes.. The Mythological Beast of Volcanology

Image Wikimedia Commons : Aniakchak Caldera – Alaska

If you’ve ever paid attention to volcanoes, there is a good chance you’re familiar with what a caldera is. For those who are new to the terminology, a caldera is a collapse structure that forms when the magma chamber below a volcano empties, leaving the overlying rock to subside into the ground. Calderas are to volcanoes what an atom bomb is to explosives. They’re the largest, most destructive, and rarest variety around, and because of that, they’re incredibly interesting.

Caldera forming eruptions are interesting and notable to scientists and casual observers alike since they’re both rare, and incredibly powerful. In fact, some caldera-forming eruptions can be so powerful, that they’ve been associated with global climate change, and small-scale extinction events. Due to their potentially cataclysmic nature, there is a lot of misinformation and doom & gloom in the press and media.
Chances are, you’ve heard the title “supervolcano”. The term “supervolcano” was coined by the media to describe the largest caldera-forming eruptions on earth. Ever since the inception of the term, it’s been used to describe any massive volcanic eruption, the likes which haven’t been seen in the modern era. So what are some common myths about calderas and supervolcanoes? Read the guide below!

Debunking Myths Associated With Calderas

Myth – There Are Only 6-7 Supervolcanoes on Earth

Somewhere along the line, the media decided that there were less than 10 supervolcanoes on earth. This myth is a bit difficult to dispel, because there is no real cutoff between “supervolcano” and “really large caldera” as it’s not a true scientific term.

Campi Flegrei in Italy is frequently described as a supervolcano, yet it’s not even 1/10 the size of Lake Toba. If we were to assume that Campi Flegrei is a proper supervolcano, then that means there are over 100 known supervolcanoes on our planet, and it would be on the lower end of the size spectrum. If we’re defining “supervolcano” by capability of producing a VEI – 8 eruption, then it’s true that there are only a few volcanic systems with this capability.

Myth – All Calderas form from explosive eruptions

While more calderas form as a result of a violent eruption, some caldera systems form from a gradual subsistence. Hawaiian volcanoes have calderas that formed slowly following the gradual effusion of basaltic magma, which caused a gradual drop in the size of the magma chamber. Subsistence calderas form most often in mafic shield volcanoes, which are common in oceanic hotspots such as the Galapagos, or the Hawaiian Islands.

Myth – Volcanoes that have had a violent caldera forming eruption are extremely violent by nature

Caldera forming eruptions are more of a cyclical process then they are indicative of a Volcano’s overall nature. Even extremely violent and active volcanoes such as Krakatoa show that they’ll stay active with small-scale eruptions post-collapse. A caldera-forming event typically happens only after a volcanic system has been “plugged” up for a long enough time, allowing pressure to build and magma to evolve to a degree that it can erupt in a dramatic fashion. For some volcanoes, this takes a very long time, others like Krakatoa can recharge much quicker. Some caldera volcanoes will create multiple massive caldera-forming eruptions. Others will only go massive one time, then they’ll sprout several smaller volcanoes after the initial caldera collapse event.

It’s also important to note that there are different varieties of explosive calderas. Caldera volcanoes formed from andesitic arc-volcanism behave in a much different fashion than Caldera volcanoes that form from basaltic rift-oriented volcanism, which typically erupt effusive basalt eruptions, but can create massive rhyolitic eruptions on rare occasion. These caldera systems are usually indicative of a large heat source (basaltic magma) transforming country rock into Rhyolite (the most explosive variety of magma) which later erupts after being disturbed by a fresh injection of basaltic magma.

Myth – Supervolcanoes Are Formed By Hotspots

The largest caldera systems in the world all have a few things in common, yet being hot spot volcanoes is not a similar trait they share. In fact, Yellowstone is the only supervolcano that is known to be formed in association with a mantle plume (hot spot), whereas most other supervolcanoes are located in subduction arcs. What they do have in common is extremely hot and shallow heat sources, typically produced by continental rifting. Rifting occurs when land pulls apart due to largely tectonic reasons. Rifting lowers underlying pressure and thins the surface, which in turn pulls magma and hot rock closer to the surface. Eventually, these large shallow heat sources melt and evolve country rock (often granite) into our familiar friend Rhyolite. If you accumulate enough Rhyolite, let it evolve for a long enough time, then set it off with a fresh injection of magma, you have the ingredients for a massive eruption.

For Yellowstone, the heat source comes from the mantle plume, instead of a rift-oriented heat source (although it’s likely some rifting is occurring as well).

Google Earth Overlays For Caldera Systems – Calderas Outlined in Green or Red (screenshots)

Ecuador has quite a few massive caldera systems, with the Chacana caldera being the largest

Of the 11 large calderas in Kamchatka, the smallest is still 10 square KM..

Cbus20122

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!

Urban volcanism!

The ironically named Mount Eden, near downtown Auckland.

Most people in the world agree on one thing: it is safer to live far from a volcano then it is living right on top of it. Living next too, or on top of a volcano is like sleeping in a cave with a friendly bear. Sure, it has it’s advantages, you stay nice and warm, you don’t have to worry about other predators, a good part of the year it is nice and quiet, but still….. you know that some day he will grab you and eat you. The inhabitants (some more permanent than others) of Herculanum, Pompeï, Heimaey and the Hawaiian Royal Gardens have found out the hard way.

New Zealand is, apart from being stunningly beautiful, one of the least populated countries in the World. When Western settlers arrived they could have chosen any location to go and build large cities. For some reason however, the inhabitants found it neccesary to build their largest city directly on top of a volcanic field with about 50 scoria cones, maars and tuff rings dotting the landscape. I suppose the knowledge of volcanism was not as developed back then as it is today, but nevertheless it is quite unfortunate.

Photograph by Mollivan Jon. Mount Taranaki.

New Zealand is dominated by subduction volcanism, with famous Mount Taranaki (or Egmont) as one of the most visually stunning stratovolcanoes in the world from both the ground and above, and with the infamous Taupo Volcanic Zone, best known for being one of the worlds “super” volcanoes. At 250 km from Auckland this is already quite a hazard on itself.

The Auckland Volcanic Field is a monogenetic volcanic field, meaning that an eruptive episode only happens once through a vent. Each eruptive episode generates a new vent somewhere within the volcanic field as opposed to “normal” volcanism where a volcanic vent has succesive eruptive episodes causing a volcano to build up and blow up occasionaly. The Auckland Volcanic Field produces basaltic scoria cones, maars and tuff rings (with the exception of the island of Rangitoto which erupted several times). All three are caused by the same type of magma, basaltic magma in this case, but the location the surface penetration, the eruptive flowrate and the total volume of the basalt determine the type of surface expression. The volcanic field has been active for about 150.000 (0.15M) years now. Older volcanic fields are found towards the south; South Auckland (1.5-0.5M), Ngatutura (1.8-1.5M) and Okete (1.8-2.7M).

The source of the basalt is not quite clear however. Basalt is normally not associated with subduction volcanism. Petrology and earthquake data have practically ruled out the possibility of the lava having an origin in melt generated by the subducting Pacific Plate. The Auckland volcanic field also sits some 200 km behind the active volcanic front of the Taupo Volcanic Zone. Furthermore, there is no evidence that the subducted Pacific plate reaches all the way to the Auckland volcanic Field.

Basalt is usually associated with mid-oceanic ridges/spreading centers or hotspot volcanism. Again, petrology has not been able to find much evidence for hotspot volcanism either. Additionaly, the propagation of the volcanic fields is directy opposite to the relative motion of the plate; the oldest volcanic field should have been in the north and the youngest in the south if a hotspot or mantle plume was involved. It is possible that the complex geology with major plates subducting, twisting and turning in the area is causing localised decompressional melting , leading to magma migration upwards right below the city of Auckland. There is some extention ongoing in the area, so this seems like a plausible explanation.

The Pacific plate and the Australian plate in a complicated geological setup

This image shows the subdution margin, the strike-slip faults to the southwest and extention(volcanic back-arc) to the northwest of the subduction margin.

Monogenetic volcanic fields are very interesting and highly unpredictable. The eruptions are not very large or extremely violent, but they can occur pretty much anywhere within the field at any time. With a large city with hundreds of thousands of inhabitants spanning the field, this is exactly what you don’t want. Paricutin in Mexico is the most famous example of this type of volcanism. One day you are happily working your crops, the next day you have to flee from your land because a volcano decided to take over your land. Bad luck, deal with it. Any new eruption within the Auckland Volcanic field will have as much compassion with buildings, streets, highways, parks and emergency shelters as Paricutin had with the crops that were growing there. This is what makes Auckland a relatively dangerous place to live in because it is not clear how much warning time there will be and how accurately the location of an eruption can be predicted with modern equipment.

The reason why new volcanoes pop up at random has to do with the generation of the magma. It is important that the generation occurs very slow. Slow enough to be unable to build a plumbing system that would efficiently conduct the magma to surface. Every new, hot, fresh slug of magma finds it’s own path to the surface, erupts and that’s it. The conduit cools and is no longer usable for the next slug of magma that arrives several decades or hundreds of years later below a slightly different part of the volcanic field. There is not enough magma flowing into one area to create a magma chamber in which the magma can evolve and produce more silicic types of magma.

Ridiculous in Los Angeles, not so ridiculous in Auckland. Bring out Tommy Lee Jones!

We have all seen the Hollywood movie “Volcano” and no doubt that many Los Angeles citizens have had a very good laugh at it (the La Brea tar pits are the surface expression of a leaking oilfield through a fault, it has nothing to do with volcanism whatsoever), but for the citizens of Auckland, those images are not even very far from the truth. The past gives an excellent example of what can happen. The next eruption in the field will most likely follow this scenario:

1 – Magma is forced upward through weak points in the crust.

2 – Either the magma contacts ground-water, or reduced pressure near the surface causes gases to bubble out of solution. The result is a phraetic or steam-blast eruption. The heaviest material is thrown out horizontally to form a tuff ring. Lighter material is blasted vertically to form an eruptive column. After a few days, weeks or months, the volcano falls quiet. Several of Auckland’s volcanos became extinct at this point.

3 – Additional magma may rise in the conduit. If enough magma is supplied, fire fountaining starts through one or more vents. Small lava flows may be produced, which do not escape the tuff ring. Sometimes the eruptions build scoria cones.

4- If fire fountaining continues beyond this point, the scoria cones can coalesce to rise and bury the tuff ring. Lava flows can also fill the surrounding valleys.

5 – Sometimes the outflow of lava is so great that it undermines the cone, which collapses into the flow and is carried away, leaving a horseshoe-shaped breached crater. If lava flows for long enough, nearby valleys are totally filled in and the lava floods the entire area with a large sheet.

Isn’t that just wonderful right in your own neighbourhood?

Map showing the city of Auckland and the eruptive centers.Pick your favourite spot to build your house.

The big question that remains is then: When is the next eruption going to be? Well, you will have to chop off one of the arms of a geologist to get a clear answer on that, but there are usually several hundred to several thousand years between eruptions in this field. The last one was about 600 years ago, so it might be a while before it is “overdue”, but it might be soon as well.

El Nathan

The Kerguelen Hypervolcano™

Below the Clouds Stair-case by Swedish architects at Stockholm-based TAF Architect Office.

OK, so what in Gódabunga’s name do Swedish stairs and volcanoes have in common! Apart from the fact they can do you a real mischief if you fall down, a staircase in Swedish is trappa and this gives the name to the extensive flood basalt flows of the Traps volcanic provinces from the stair-like appearance of the flows!

Deccan Traps from: www.volcano.oregonstate.edu/vwdocs/volc_images/europe_west_asia/india/deccan.html

Kerguelen

A little known, but very extensive trap province exists in the southern Indian Ocean, some 4000km west of Australia and 1500km north of Antarctica – the Kerguelen Plateau that has developed over the Kerguelen mantle plume.

The Kerguelen Plateau – the second largest submarine plateau -  lies at approximately 1-2000 metres depth, in an abyssal depth of 3-4000 metres, and has three small island groups, Kerguelen, Heard Island and Mcdonald Island as surface expressions. The plateau extends north-westwards for c2200km covering an area of about 2.2m sq km.

Geologically, the plateau has had a colourful history, being classed as a ‘micro-continent’, it is a remnant of the break-up of the Gondwanaland super-continent and is located over the Kerguelen hot-spot. Deep water geological information is from the JOIDES ODP (ocean drilling programme) and seismic interpretation of oil prospecting data; the plateau is shown to be constructed on a general base of Cretaceous terrestrial and/or shallow water sediments – including coal horizons for at about 40m years. Volcanism began during the middle/late Cretaceous (c120m years ago) with emplacement of trachytes and basalts and continued on a large scale into the Miocene/Oligocene and continues up to the present on Mcdonald Island. Recovered ODP samples of felsic and metamorphic rock indicate the possible presence of a crystalline basement at least in part below the Cretaceous deposits. The total volume of the Kerguelen volcanic province is estimated to be in the order of 25million cu km giving an average of 0.2cu km/year. Submergence of the whole plateau was around 20m years ago.

The references below are superb!

http://www.ga.gov.au/energy/province-sedimentary-basin-geology/petroleum/offshore-southern-australia/kerguelen-plateau.html

http://petrology.oxfordjournals.org/content/43/7/1121.full.pdf

Kerguelen plateau, from Wikipedia: Kerguelen plateau topography.

The island groups involved here, are the tiny yellow dots near the north-west end on the elongate NW-SE pale blue area, Antarctica is the orange-red area at the bottom.

Kerguelen Island is the largest of the island groups surfacing above the Kerguelen Plateau; administered under the French Southern and Antarctic Terretories; covers an area of about 3400sq km and rises to 1850m at Mt Ross, the youngest volcanic expression of Plio/Pleistocene lavas – brown on the map below.

Simplified geological map of the Kerguelen Islands from Wikipedia.

The majority of the island is composed of flood basalts, in grey above, along with minor amounts of trachyte, pinkish, and the plutonic complexes (buff-grey) of Foch -north centre – and Rallier du Baty – sw bottom and the small Mt Crozier intrusion – northern of the two eastern promontories. Volcanism, related to the Kerguelen hotspot, began c40m years ago and continued until about 100,000 BP.

Heard & McDonald Islands

Heard Island and the stratovolcano Big Ben
(photo by A. J. Graff, Australian Antarctic Division)

The Heard and McDonald Islands (colloquially the HIMI) are administered by Australia and as such are home to Australia’s only active volcanoes.

Heard Island, apart from having the highest point on Australian territory at 2745m on Big Ben (9006 ft), has two main volcanoes in Big Ben, in part a 5-6km diameter, glacier covered caldera and the smaller Mt Dixon, plus small scoria cones. Big Ben, approximately 18km in diameter, is mainly of basalt/trachytic composition.

Heard Island shows 3 distinct stages of development, the oldest being the deposition of Miocene limestones 40-50my ago being found over much of the Kerguelen Plateau. These carbonates were followed around 9my ago, by 300-350m of volcaniclastic sediments and pillow lavas of the Drygalski Formation. A period of peneplanation of the Drygalski deposits preceeded the present volcanism, starting about 1my ago.

Satellite image from July 2000, showing an active two kilometre long (and 50-90 metre wide) lava flow trending south-west from the summit of Big Ben.
Photo: Thermal Alert Team, University of Hawai'i

The McDonald Island group lies about 27 miles west of Heard Island and is home to the second of Australia’s most recently active volcanoes and the whole total about 1sq mile in area, rising to 212m at Maxwell Hill. McDonald Island burst into action in 1992 after a 75,000year sleep and has been sporadically active since in late 1995-early1996, 2000-2001 and lastly in 2005 from Samarang Hill. The effect these eruptions had on the island was to almost double the size and increase the height by about100m!

The island is composed mainly of interbedded ,viscous phonolitic tuffs and lavas; phonolite being named after the resounding ‘ring’ when struck, is tough, pale coloured with a high felsic content of predominant feldspathoids over feldspar and is characteristic where a mantle plume is overlain by a thick continental crust.

2004 satellite image of McDonald Island showing island's extent in 1980 (striped).

ALAN C

Tungnafellsjökull – Tectonic Earthquakes

Photograph by our own Jamie. Tungnafellsjökull seen from Sprengisandur area. Notice that the Jökull is almost gone from Tungnafellsjökull, soon to become known as Tungnafjöll only.

There has been an earthquake swarm at the northern end of the Tungnafellsjökull during the evening and throughout the night. The swarm is still ongoing. There has been a lot of speculation out there in the blogosphere about it being volcanic in nature. It is not, it is purely tectonic.

As some of you know Iceland is divided by the Mid Atlantic Rift (MAR). The MAR in turn is divided in Iceland into two separate active seismic zones, the Eastern and the Western Icelandic Seismic Zone. Lately it has been the EISZ that has been most active of the two. But the WISZ is not in any way dead or dormant. Both of them are driven by the spreading of the MAR. From the WISZ the North American Plate is spread, and from the EISZ the Eurasian Plate is spread. In between them are two micro-plates that have formed by volcanism caused by the rifting.

The map is showing the Icelandic Volcanic Zones, where the MAR runs up into Iceland, where the MAR leaves Iceland and the more important volcanic features. The Icelandic Seismic Zones are corresponding to the volcanic zone.

Along both the WISZ and EISZ are lines of volcanoes spread, it is where the spreading causes magma to pour up and fill the spaces created by the spreading.

If you look at the map you see that WISZ runs from Hengill, up to Langjökull (2 known volcanoes), via Hofsjökull (at least one volcano), onwards through Tungnafellsjökull, and then ending up at the triple-junction at Bárdarbunga.

During the last few years the area of Tungnafellsjökull has been inactive, but there is ample evidence of it having been tectonically active, something that can be found in the Sprungur (tectonic faults) found in the area. The dormancy is likely due to the area having been locked at depth, probably by old magma that has solidified the area.

Various versions of tectonic faulting. Tungnafellsjökull is suffering from strike-slip faulting.

Lately the area has been subject to an uplift not seen in Iceland since de-glaciation after the last Ice age. This is due to the melting and diminishing of the glaciers of Tungnafellsjökull (almost gone) and Vatnajökull. This uplift process has accelerated during the last decade. It is now up to 3 cm year in the area according to Sigrún Hreinsdottir (source, private email). Yes, the famed inflation of Hamarinn is not happening, it is a combination of Grimsvötn motion and isostatic rebound.

This motion might have started to release the seismic lock at Tungnafellsjökull. If that is so, there is a risk that the swarm of earthquakes is just precursor quakes for a large earthquake.

This map shows the features discussed in this text in relation to the Bárdarbunga triple-junction and the hotspots location.

What makes this interpretation the more likely one is that there is no discernible evidence of any harmonic tremoring during the earthquakes. This makes it into tectonic seismicity, not magmatic seismicity.

If there would be a large earthquake that tears the rift-lock, then magmatic movements could start in the area, but not before that. Worst case scenario here is not a volcanic eruption; it is a 6M earthquake as the slip-lock disintegrates over a large area.

Another thing that I want to point out, the earthquakes are all of low probability and some of them are as I write this due to change after revision, and some of them will be removed due to being false representations of earthquakes, so called Ghosts. And as I wrote this IMO has started to revision the earthquakes, right now there is at least one at 3M.

CARL