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!

Eifel Volcanic Field I

Few volcanic areas in the world are as easily accessible and in such a friendly environment as the Eifel Volcanic Field. Volcanophiles generally know of the existence of this volcanic field and that it lies somewhere in western Germany. Most ordinary people however have no idea that an active volcanic field with some 200+ volcanoes is located in western Germany, some 25 km from Belgium, 50 km from Luxemburg and 80 km from The Netherlands.

Lush, green, easily accessible and previously quite dangerous. The Shire might have been here before. Image by author.

The field is generally located around the towns and cities of Hillesheim, Gerolstein, Daun, Mayen and Koblenz. The world-famous racing circuit of the Nürburgring (Nordschleife) actually lies around and in between a few volcanic cones, of which the “Hohe Acht” is the most famous one because it’s also the highest point in the Eifel hills.

If you believe some of the more sensation oriented media, you will be led to believe that the Eifel is actually an inflated supervolcano, which has already shown its potential at Laacher See (one of 2 caldera features in the volcanic field) and is just waiting to end civilization as we know it anywhere between Scandinavia, the UK, Spain and the Balkan countries. We’ve all read those articles and wondered if 99% or 100% of it was made up on the spot.

In fact, volcanic activity in the Eifel Volcanic Field has been almost exclusively monogenetic, leaving scoria cones, tuff rings, lava flows and maars scattered over the hills since about 700.000 years ago. The Eifel is the type locality for “maars”, so activity like this all over the world is named after the volcanic lakes in these hills. On one occasion, a significantly larger (one of the most recent) eruption occurred, which left the Laacher See caldera. The good thing about all this activity is that there are a lot of volcanic features to visit and they are almost all very easy to reach. No mountaineering skills are needed and no supplies need to be carried because all this is in the middle of the civilized world.

Scoria cone hidden in the trees in the middle of a field. No guided tour needed here. Image by author.

Once you reach the crater area, you suddenly realize this is the real deal, even though the surroundings don’t look like it. Image by author

The “Vulkanmuseum” in Daun is worth a visit if you have some spare time. A lot of things are explained and a lot of good information is provided. This too can be said about most volcanic features in the Eifel. At many cones and maars you will find information signs with tons of useful information about the volcanic feature you are visiting.

Of special interest are the cold-water Geysers that are found in the Eifel. They are not driven by heat, but by CO2. The CO2 escaping from the magma sources below the Eifel dissolves into the groundwater at some places. Whenever the amount of dissolved CO2 reaches a critical point, bubbles start forming, lowering the hydrostatic pressure of the underlying water, triggering the formation of more bubbles etc etc. This chain reaction, when combined with a ‘conduit’ leading to surface, is what drives the geyser, until enough CO2 has been released to restore the stable situation again. The one in Wallenborn (Geyser Brubbel) is quiet for about 35 minutes and ejects cold water for about 2 minutes. Almost perfect for a visit! The one in Andernach is actually the world’s highest cold-water geyser. If you cannot go and visit them there, just buy a bottle of Gerolsteiner water to play Volcano at home. This world-famous mineral water is extracted from a drilled well and is naturally carbonated by the volcanic field. Shake the bottle firmly, open the cap and you have your own Eifel Coldwater Geyser at home.

If you happen to be a big beer fan, you might want to visit the Vulkanbrauerei (Volcanobrewery) in Mendig, close to the Laacher See. They produce and sell various beer specialties and have a very cool underground cellar (felsenkeller) open for visitors, that is cut out of columnar basalt, which they claim is the deepest beercellar in the world. It certainly sounds awesome to have a huge cellar to keep your beer cold, cut out of columnar basalt underneath/inside an old lava flow.

Already a nice place for a swinging chair. Once you know of the maar below and the scoria cones in the distance, it suddenly gets even better, especially if you took your beer from the Vulkanbrauerei. Image by author.

If you ever happen to be near the Eifel and have a spare day or so, I think this area is definitely worth a visit. Don’t expect any huge and spectacular volcanoes, dangerous trips or much live activity, because most features are quite hidden and somewhat influenced by erosion. The area is kind of like the old Petting zoo of the volcanic realm with some very cool animals in it.

A group of German Crater Deer having a good time inside an old volcanic crater. Image by author.

In part II, chryphia will give some more info on the Eifel Volcanic Field in a follow up post. Thanks to chryphia and Spica for helping out on this one!

El Nathan

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Name that Volcano Riddle ……

 1 volcano 1 point

 

During the 1960s ‘cold war’, a discovery in a cavern under this volcano raised the spectre of an impending rocket attack …. SOLVED

Talla 1 point for Shinmoedake – some scenes from the 1967 You Only Live Twice were filmed on locationn at this volcano! (Spectre’s rocket base!)

Kilgharrah

Volcanic Mishaps 2: Mono Lake, California.

Tufa Outcrops, Mono Lake.
Image from Wikimedia Commons.

We had 2 weeks in California; after a weekend in San Francisco and some chillaxin’ by the pool in Sacramento; we took the roadtrip of a lifetime. (many thanks Val x) We visited Lake Tahoe, Mono Lake, Yosemite, Mariposa and drove back to Sacramento via Route 49; the gold rush route…

Trees were more my thing in those days, I armed myself with Stuart and Sawyer’s Trees and Shrubs of California; (ISBN 0520221095) bought for $8 in a second hand bookstore in Berkeley and managed to tick off a fair few… Including this baby:

The Grizzly Giant, species: Sequoiadendron giganteum.
From Wikimedia commons.

Statistics: 63.7m high (somewhat truncated by a lightning strike, I guess…) Circumference at ground level: 29.5m, Diameter at 1.5 m from the ground: 7.8m, Estimated bole volume: 963m^3 and old enough to have lived through the action described below!!!

It wasn’t much of a mishap, more of an oversight… We were visiting because my girlfriend (at the time) had seen a picture; something like the one above, and had fallen in love with the desolate beauty of the place. So we went and looked around; we saw the tufa rock formed by accretion of materials at hydro thermal vents and exposed when Los Angeles began tapping Mono Lake’s tributaries; the lake itself is highly saline/ alkaline. We saw Black Point, formed under a much deeper Mono Lake 13,000 years ago; now a flattened cone of basaltic debris. We had a good long look at Negit Island; built by several eruptive episodes between 1600 and 270 years ago. We goggled at Paoha Island created by a magmatic intrusion under the lake between 1720 and 1850; it has an exposed section of rhyolite and 7 (count em’) dacite cinder cones! There was a seismic swarm in 1980 including EQs of up to 6mag (estimated, richter scale) and another in the nearby White Mountain fault in 1986.

Mono Lake is not the whole story; to the south there are a series of domes, coulees, flows and craters stretching all the way to the Inyo Craters; many of these were formed in a series of violent eruptions ~600 years ago. When I say violent I mean phreatomagmatic explosions followed by the opening of a 6km multi- vent fissure, pyroclastic flows affecting the Mono Lake area and then (geologically shortly afterwards) a virtual repeat 40 kms south at the Inyo Craters, followed by coulee and dome building!!! The remaining features are thought to have arisen in the last 2000 years. Mammoth Mountain and the Long Valley Caldera are nearby… Quite a piece of volcanic real estate, I think you’ll agree:

The Big Picture… Approx 50kms top to bottom.
Wikimedia commons again.

This sums it up pretty well:

http://en.wikipedia.org/wiki/Mono%E2%80%93Inyo_Craters

The mishap? We were walking around in and admiring an awesome, starkly beautiful landscape, which:

“is considered one of the most likely sites for future volcanic activity in the United States”

according to Gates and Ritchie…

and I had absolutely no idea it was even a volcanic landscape until “yesterday” when I was glancing through their book!!!

Schtevie x

Disclaimer: The author is an amateur blogger and has absolutely no quailifications as a geologist or anything of the sort.

The article is not implying that “something is going on” and should give you no cause for concern at this time.

See the USGS website linked below for up to date information.

References

The United States Geological Survey:

http://www.usgs.gov/

Webcam:

http://www.monolake.org/today/mlcam

Inspiration for the article from:

Gates and Ritchie’s; Encyclopedia of Earthquakes and Volcanoes, 3rd edition. ISBN0816063028.

Not really a reference; (I nearly put my back out when I picked it up from under the tree!!!) but destined to be a new favourite:

Encyclopedia of Volcanoes; editor in chief Haruldar Sigurdsson. ISBN 012643140x.

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GeoLurking Link Recommendations for the nitty gritty. (Note, all links provided documents even though some are in paywall company sites)

“Monitoring Unrest in a Large Silicic Caldera, the Long Valley-Inyo Craters Volcanic Complex in East-Central California” Hill (1984)

http://link.springer.com/article/10.1007%2FBF01961568

“Comparison of risk from pyroclastic density current hazards to critical infrastructure in Mammoth Lakes, California, USA, from a new Inyo craters rhyolite dike eruption versus a dacitic dome eruption on Mammoth Mountain” Kaye et al (2009)

http://link.springer.com/article/10.1007%2Fs11069-009-9465-1?LI=true#page-1

“Elastic source model of the North Mono eruption (1325–1368 A.D.) based on shoreline deformation” Shaffer (2010)

http://lycaste.geology.buffalo.edu/monoinyo/downloads/publications/ShBuRe10.pdf

“THE GEOCHEMISTRY OF THE INYO VOLCANIC CHAIN: MULTIPLE MAGMA SYSTEMS IN THE LONG VALLEY REGION, EASTERN CALIFORNIA” Daniel E. Sampson and Kenneth L. Cameron (1987)

http://onlinelibrary.wiley.com/doi/10.1029/JB092iB10p10403/abstract

Copied from comments for completeness, Schteve.

…………..

 

Sheepy Dalek:

Alan C Evil Riddel:

Ladies beware! If you have badly fitting undies, you may find me!!

What am I?
What are my origins?
2 points
I hope you ladies aren’t offended by this one 

Riddle – Name Those Volcanoes 
7 Dings 7 points

Variety number 9280, No 1 is used to produce a splendid accompaniment to VC riddle solving!

French FIS WC silver medalist might have crashed on No 2 whilst he learnt to snowboard-cross

No 3 The ‘toy’ volcano (photo below)

16762 No 4 is potentially hazardous and unstable

No 5’s new island emerged, bravely flew the Portuguese flag & vanished just 20 days later

No 6 (photo below)

With a No 7 Bang Bang I wish us all a belated, but very happy & peaceful New Year!

Spica

Apocalypsathon; Post 21/12/12 Appeal…

I think Tyler Mannison found this one…

Send your urgent and much needed donations for those poor unfortunate endotheworlders who were not wiped out (they must be devastated) to schteve’sschwissbanking.ch

Please spare a thought and a dime for those not raptured up to heaven in the recent non- apocalypse; give generously, it’s nearly christmas after all…

Since this didn’t happen everywhere all at once…

I intend to set up a refuge high in the hills of La Gomera with a nice piece of (terraced) land and a look out tower; we’ll charge post 2012ers top- whack to come and contemplate… Me n’ Lizzie will be there most of the year looking after the goats and generally taking care of the place (and going for long walks and jaunts to El Hierro and stuff.) So once again Volcanocafers please dig deep for this very worthy cause…

Somewhere like this, Pico del Teide is in the distance…

http://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/La_Gomera_1.jpg/1280px-La_Gomera_1.jpg

But seriously, and since we are still here; a genuine appeal (and some of my highlights):

This rather special place was started by Carl and Ursula after a group Volcanoholics decided they wanted their own place with their own rules… Those that wanted to go multidisciniplary, collaborative and friendly came here and (boy!) the discussion was, and still is, far ranging… The Welcome page and blog rules are here:

http://volcanocafe.wordpress.com/2011/11/15/volcano-cafe/

The average post rate is ~ one every 3 days, (that includes before and after Carl statistics…) some volcanoblogs manage more, but usually these are brief updates. What we get here are crafted pieces, made by amateurs in their spare time…

The hit rate is around ~150 visits per hour; this doesn’t include dragon visits…

I won’t lie to you; a blogpost can be quite a bit of work, depending on your skills… Carl once mentioned that he could write a 1200 word opera review in 20 minutes, and Geolurking seems to be able to get something revolutionary on tectonics done in only slightly more time…

Birgit deserves her own paragraph; she can research, compile, edit, post and get an intelligent layman up to speed on a particular subject in less time than it takes a crocodile to swallow an unwary victim!!!

Me? I’m at the other end of the scale; maybe 20 hours work on Teneguia Technicalities and Context, but that did include editing with wordpress which was a first for me… Don’t let me scare you, I can be quite ambitious…

I am asking everyone to keep the posts coming; think of it as an extended comment and you will do fine…

This one’s for our resident geologist… The little engine that could x

http://upload.wikimedia.org/wikipedia/commons/thumb/d/d2/Snowdon_Mountain_Railway_No_6.jpg/1280px-Snowdon_Mountain_Railway_No_6.jpg

Visits to volcanoes “a la Ukviggen” are always popular; (Mount Snowdon anyone? The narrow gauge, rack and pinion railway is the only one of it’s kind in the UK.) as are summaries of your favorites; (Karenz on Sakurajima is a very good example.) and memories of eruptions that were special to you; (Bobbi’s piece on Redoubt is a classic, and don’t forget Newby’s uncle on Erebus.)

Ascending eruption cloud from Redoubt Volcano as viewed to the west from the Kenai Peninsula April 21, 1990 (R. Clucas)

For the more ambitious how about an original piece of research? (Irpsit wrote a fascinating series about a big hole!!!) Controversial stuff is great, got an alternate theory? (Peter Cobbold on El Hierro is excellent.) What about something inter- disciplinary? (Diana Barnes on Scheeps helping to revive volcanic badlands is wonderful!) Technicalities more your bag? (Wagabond on marine seimic sounding; great insights.) Plotters, hows about “beefing up” a special plot? (Plotting for Beginners 2 may get done one day, but feel free to jump in!!!)

One of Birgit’s awesome SEM images of material from El Hierro…

If none of these inspire how about something outrageously off topic for the Scheeepy Dalek?

Nothing is like the smell of a Motorcade in Depresneyville in the morning. Remember that when people shoot at you, they just wish to greet you welcome to Ukraine.

So please, go and do yr research, track down the info on yr chosen subject and write something up… Include the standard Volcanocafe disclaimer and a reasonable list of references; and you’re done…

Posts are best submitted as plain text word documents; attached to an email. Pictures should be separately/ individually attached; most formats are fine but please no psd, crw or nef (they are too big and probably not supported by WP either; they need to be converted first). Jpg, gif, png, tiff are commonly supported formats and will do well.

However; when I asked Sissel about this, she said: “Just send it, I will edit what is neccessary!” (another inspirational blogger; remember The Little Prince?)

Have you ever made a comment that you (later) wished you’d saved for a guestpost? Then we want to hear from you; (give us as much detail as possible: approx dates, subject, etc. and we will go digging) dragons can search all 70,000 comments and extract that moment of inspiration…

My top tip (I know it’s environmentally unfriendly) is to print out the papers that you are really interested in; the references for yr article; that kinda thing…

Posts and comments are the lifeblood of the blog, there are (almost) no stupid questions or statements.

So there you have it, no more excuses for not handing in your homework!!!

With Love and Respect,

Schteve x

Links to inspirational articles:

• El Hierro : a tectonic compression/relaxation mechanism? Author Peter Cobbold ,September 19^th.
• A Volcanic ‘Grand Tour’ Part 1 Author UKViggen, October 26^th.
• Sakura-jima: Why is She Nearly Always Erupting? Author KarenZ, October 31^th.
• Redoubt Author Bobbi, October 16^th.
• The (ash) history of Iceland, in my backyard – Part I Author: Irpsit July 25th.
• Edinburgh – Volcanic heart of Scotland  Author: Alan C, February 28^th.
• Welcome to Ukraine – You’re arrested! (Sheepy Dalek Russian Style)  Author: Carl, December 9^th.
• Some points on El Hierro seismic  Author: WAGABOND, December 6^th.
• Biology, Sheep, and Human Survival  Author: Diana Barnes, November 21^st.
• Images of El Hierros magmatic system  Author: Geolurking, November 27^th.
• The Little Prince  Author: The Behemoth Dragon (aka Sissel), December 13^th.
• Mount Erebus, Ross Island and the Age of Exploration  Author: Newby, December 3^rd.

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