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!

Chain of Dead Poets!

Amsterdam Island with visible craters.

The Amsterdam St Paul hotspot is one of the weaker hotspots around. It has created the St Paul and Amsterdam Islands, the now active Boomerang Seamount (last known eruption 1995), and an elongated chain of seamounts called the Chain of Dead Poets. These are remnants of the eruptive wake of the Amsterdam St Paul hotspot as the plates move on over it. The hotspot has had 2 episodes of increased activity after it became active. The first period lasted from 10 million years ago to six million years ago. The second period started 3 million years ago and lasts up until today. Amsterdam, St Paul and the Boomerang Seamount have all been produced during this second period of activity.

The hotspot is associated with the South East Indian Ridge and its rift system, and the chains volcanoes show evidence of changing in its chemical composition as the hotspot moved into the SEIR.

Amsterdam Island

The Island is the northernmost of the Antarctic sub-aerial volcanoes. It has had two eruptive centers down the line. Both with visible craters, the younger of the craters are far more visible on the image. Both of the craters are from periods of heightened activity, but later volcanism on the Island has primarily been of the flanking fissure type. Even though no eruption has been witnessed lava samples taken from the flanks of the younger crater shows that the volcano has indeed erupted during the last 100 years.

St Paul Island

The channel into St Paul natural harbour. One should keep slightly to the portside of the centerline of the channel when sailing in. The starboard side is much more shallow. By keeping slightly to portside of the middle you can get a 3 meter deep sailing ship into the natural harbour, well inside of it depth is not a problem, and you are quite safe regardless of weather. Stay away from the mammals on the beach, they are big and mean and are in no way to be compared to people in bikinis.

The island had a large eruption a few years before 1780 in which the predominant caldera formed. Even though the caldera is small for being a caldera it was probably formed by a Krakatoa style eruption starting with a for the volcanic system unusually large eruption with a subsequent magma chamber roof failure that let the ocean water down into the chamber. The ensuing steam explosion gutted the chamber.  In 1780 the vestigial remnants of the caldera wall facing the ocean crumbled and the ocean has during the following years carved out a fairly broad, but shallow canal that is open for smaller sailing ships due to its limited depth of around 3 to 5 meters.

Map of St Paul Island from Wikipedia. Note that the island is very small. The actual caldera is only slightly larger than 1 km across.

The Island is together with Isle du Kerguelen the best harbor in the southern ocean, and many trans-globe sailors make a port of call for repairs, or just general relaxation and landfall.

Boomerang Seamount

Not much is known about Boomerang that lies 18 kilometers north of Amsterdam Island. It rises 1 100 meters above the sea floor, but is still 650 meters below the ocean surface. During an expedition in 1996 they dredged up a lava sample and tested its Uranium/Thorium content. It showed that the lava had been erupted only 5 months prior to the visit.

The Seamount has a 2km caldera showing that the volcano has had at least one substantial eruption and probably have been a bit closer to the surface before.

CARL

El Hierro and the Physics of magma chambers

Image from Nature GeoScience. From Phillip A. Allens article Geodynamics: Surface impact of mantle processes.

Part 1

Not many people think about what is great with physics. People are normally more occupied with buying Prada hand-bags to carry their rat-sized yapp-dogs than physics. The great thing with physics is that the laws of nature are universal. And with that I mean that they can be transferred easily from the school books into real life, and from one part of science into another.

I am as most of you know not a volcanologist or a geologist, but I am a physicist. So every time I try to understand a volcano I do it from how it is behaving from the point of perspective of the laws of nature.

This time I would like to write about a few things regarding how magma chambers must be formed according to physics. I will mainly not talk about magma chambers because they are fairly hard to visualize since nobody has seen one in real life as it is forming. But most of us have for instance blown up a balloon.

In this case we will be talking about magma chambers that come from hotspot volcanism; the process will be slightly different in a subduction volcano. But first we need some background, this post will be about precisely that background.

Hotspots, weightlessness and Blobs

Let us start with what is required for a magma chamber to even start forming. And as a physicist I am always talking about basic forces. And there is only one basic force, and that is energy. There are of course many types of energy, and in this case we are talking about energy as mechanical pressure and heat.

Thankfully for the poor fledgling magma chamber there is one thing that causes both pressure and heat, and that is your basic magma. So, let us drop up a ball of nice hot juicy magma from the hotspot under El Hierro.

It is not entirely clear how magma travels upwards via a hotspot, but we know there are two types of hotspots. First we have the deep Icelandic type that brings up material from the depth, this magma is hot and arrives at high (relatively) speed and with great force. It brings with it an assortment of rare and heavy metals from deep down at the boundary between the core and the mantle. The other type is a colder and less deep hotspot. The magma here is either brought up from within the mantle, or created as the hotspot heats up material close to the MOHO boundary either through heat or pressure, perhaps even a mixture between them. This type creates magma that is low in precious metals, and gives a low Uranium-Thorium (UrTh) count which in turn is a dead giveaway that it comes from a shallow source. The Canarian hotspot seems to end up somewhere in the middle of these two types, it is definitely not melting crust as a part of the magma creation, the almost pure basic basalt tells us that, on the other hand it is not from the core/mantle boundary since the UrTh count is wrong for that option. Let it suffice to say that the Canarian hotspot is a bastard mongrel of a hotspot.

So, where does now the pressure to drive any hotspot come from? Well, once again the answer is not simple. We have at least two sources. The first is heat; the Earth is producing loads of juicy heat due to at least 3 different processes. The first one is UrTh and other atomic nuclear processes. Yepp, we live on an atomic reactor. The second one a form of pressure called overburden pressure. That is the combined weight of the planet pushing downwards, this creates compression heat. The third is through the dear old gravity slowly massaging the planet, this is by far the smallest of powers creating the heat. Here I have simplified a bit, there are more forces at play than this.

Image of nested magma.

So, how come then that magma travels upwards? The answer might surprise you a lot. If you are getting a headache from this it is normal. Let us imagine that you where hanging at the exact mid-spot of the planet. The pressure would be phenomenal from the overburden pressure; still you would notice something odd. For the first time in your life you would be completely weightless. This would be due to the entire planets gravitational pull would be affecting your entire body in every direction at the same time, effectively cancelling out any gravitational effect.

What does this now have to do with magma? Well, you have magma under tremendous pressure that does not weigh a lot. A cubic decimeter of magma at the mantle/core boundary is considerably more lightweight than the same volume of water. And at the same time it is squeezed by tremendous pressure.  Here we enter a nice little simple physics, when you squeeze a fluid it will try to run away, in this case it can’t go down, it is fairly buoyant and will try to float. Now we just need one small thing, a conduit. Enter the heat.

Energy will always go from a high state to a lower state; this is the nutty little physics law that also gives that order will always go towards disorder, in other words, entropy and enthalpy. So, the core will try to lose heat, and the heat will always be able to escape, and once a convective current of heat has started to run upwards it will jolly well keep on going. When magma finds a stream of heat going upwards it will follow that stream because the fluid will follow the point of least resistance. And that is why a mantle plume and a hotspot is the same thing (simple physics). The mantle plume cannot exist without a hotspot, and the hotspot will sooner or later create the mantle plume.

Now our blob of magma is finally moving upwards towards El Hierro, the trip started a long time ago, it takes a while to go through all that semi-permeable heated pipe that runs up through the mantle. One day, let us say on the 24^th of June 2012 our blob of magma arrives at the bottom of the crust under El Hierro.

The speed with which it arrives is very slow even compared to a human walking, but the weight is enormous, the same goes for the amount of heat energy and the buoyancy pressure. Let us just say that it is like a comet sized blow-torch hitting the almost melted MOHO boundary. It will cut through the first layers in a rather short time. As it goes on up through the bottom of the crust it decelerates fairly quickly, and that is the point where all the fun starts, the formation of the magma chamber.

Until the next time!

CARL

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

Edge Driven Convection: BoB’s Back-Story And Malcolm in The Middle.

Cumbre Vieja, La Palma, looking south, Teneguia is hidden beyond San Antonio (huge crater, top centre), 70 kms beyond that is the North coast of El Hierro:

http://www.miguelbravo.com/VARIOSTEMAS/volcanes%20canarias/canari203.jpg

(Sorry, I don’t have credit for this picture… found it via Google though…looks like an old postcard)

Whether you are new to volcanology (like me) or an old hand; if you have been following the BoB happening, you will hopefully be familiar with the work of Juan Carlos Carracedo. A brief biography may be found here: http://www.cienciasmc.es/web/biografias/juancarlos_carracedo.html

He is, pretty much, the foremost authority on the volcanology/geology of the Canaries, and he has (apparently) had a proper row with Nemisio Perez about BoB!!!

What follows is essentially a digest of some of Carracedo’s work; anything in bold is wild amateur schpeculation on my part.

Before we go into the Canaries themselves, a few points about the Hawaiian Islands, which may be considered the archetypal intra-plate, hotschpot driven, volcanoscheanic island chain (iphsdvoic)…They are located approx 2000 miles from the coast of North America and are resting on thin, fast moving Pacific Ocean crust. The Big Island is ongoingly active; the last confirmed eruption on another island was Halekala on Maui (approx 50kms from Big Island) which occurred sometime between 1480-1600. Kauai (580kms from the Big Island) is long since extinct and eroded/ subsided to a nub. The Hawaiian archipelago shows an orderly progression from youngest and most active volcano to oldest and most extinct.

Bathymetric map of Canaries and Madeira. Thick dashed lines indicate possible hotspot tracks:

(Hoernle and Carracedo, 2008)

The Canaries are approx 100kms (at their closest) from the African continental crust and are resting on thick but slow moving Atlantic Ocean crust. (Hotspot volcanism does not occur if the crust is too thick and/or moves too fast.) Forming a near perfect curve, they emerged in sequence starting with Fuerteventura which went sub-aerial approx 25 million years ago, then Lanzarote, Gran Canaria, La Gomera and Tenerife (approx 7.5 million years ago). La Palma and El Hierro are the youngest of the islands; they surfaced between 1.2 and 2 million years ago. The hotspot is somewhere between La Palma and El Hierro and the African plate is moving northeast and rotating “anticlockwise”. Incidentally the Canaries show little or no subsidence; the lower (older) islands are that way due to erosion alone. So far, so normal for an iphsdvoic…

Delving deeper and comparing with the Hawaiian iphsdvoic shows that the Canaries actually have an entirely different pattern of active volcanism:

Look again at the sequence in which the Canaries were formed; Fuerteventura before Lanzarote, La Gomera before Tenerife…

Also note in the map above; Salvagens seamount is way off the postulated hotspot track as is Lars, as indeed are Gran Canaria, Fuerteventura and Lanzarote…

El Hierro and La Palma formed (and are forming) “simultaneously”; despite being as far apart as Tenerife and Gran Canaria…

La Palma has had lots of historic activity (as may be expected), but so has Lanzarote (400 kms from the hotspot); there was a devastating eruption (though with few, if any fatalities) from 1730 to 1736 and a minor one as recently as 1824. …

El Hierro, apart from BoB has had no confirmed historic eruptions; there was a seismic crisis in 1793 but it is not known if there was an associated (presumably undersea) eruption. The most recent (proven) eruption was in approx 500BC.

Tenerife has experienced several eruptions since the beginning of the conquest in 1402 and a seismic crisis in 2004…

There is an undersea volcano 500m high at a depth > 2500m between Tenerife and Gran Canaria. Volcan Enmedio (thanks Judith! x) is in the vicinity of the strongest EQ (5.2mag, lat27.9 long-16.2, depth 36km, intensity V, 9/5/89) detected in the Canaries. This area also has regular EQs; the last one was on 10/5/12, depth 21km, 2.5 mag… The research I have seen (Carracedo) suggests that there wasn’t an eruption as such…

However, that 5.2mag as well as approx 50 EQs > 2.5mag in and around that location between 3/89 – 6/89 is perhaps quite suggestive… Imagine how many “tiddlers” were not detected!!!

Picture/ bathymetric map of Enmedio:

http://imageshack.us/photo/my-images/28/cangeovolcndeenmedio.jpg/

(Credit as in picture)

So why don’t the Canaries show an orderly progression like the Hawaiian Islands? Carracedo asserts that edge driven convection is the reason.

Edge driven convection diagram:

http://www.rainer-olzem.de/typo3temp/pics/dc102ce961.jpg

(Juan Carlos Carracedo, 1998)

The hotspot warms and “circulates” the nearby athenosphere. This warming and circulating doesn’t amount to much (it gradually cools and sinks) except where the circulation goes east; and runs into the African continental crust (approx 500kms from the hotspot). When it hits the cold crust it cools and sinks; this in turn pushes more of the athenosphere up and past the plume “driving” the convection.

Hopefully we can see how edge driven convection could explain some of the “anomalies” in the mantle plume model of the formation of the Canary Islands. Specifically; the historic eruptions at sites far removed from the hotspot.

So what could account for the geologically simultaneous formation of El Hierro and La Palma and the seemingly “off track” formation of Salvagens, Lars, Gran Canaria, Fuerteventura and Lanzarote? Carracedo says that this may simply be a function of the thick, slow moving plate. He has a further theory as to why La Palma and El Hierro are evolving simultaneously, albeit with apparently alternating eruptive periods. He suggests that changes in the tectonic stress regimes caused by large gravitational landslides are the explanation. I haven’t found a paper which goes into this, but a short discussion can be found at the end of the Giant Quaternary paper and in the book referenced below.

Carracedo also discusses the idea that edge driven convection is more “chaotic” than the diagram implies i.e. it’s not just a simple “loop”; slow moving as it is, there may be some turbulence in the system.

This got me thinking about possible causes of turbulence:

Eruptions have diverted energy from the system, as have the conduits and chambers under the islands.

There are an (increasing?) number of islands resting on the oceanic crust; which distorts it.

The African continental crust is not a perfect rectangular slab.

The hotspot is not a perfect “tube”.

Some (but not all) of the currents will interact with the continental crust: Those that do will interact with an uneven crust at varying distances from the hotspot, having passed a variety of “obstacles”.

The convective currents themselves (given the above) presumably interact.

The chaos in the system could therefore be “perturbing” the mantle plume itself… Eruptive action has moved from the southern tip of La Palma (possibly via Malcolm Enmedio) to just south of El Hierro (which is about 100kms) in only 40 years. This wild theory of mine may also go some way towards explaining the “anomalies” in the hotspot model of the Canary Islands discussed above.

 

Conclusion and discussion:

 

The Canary Islands and their volcanoes are complex and enigmatic, they’re not in Iceland’s league, but for an iphsdvoic they have a certain, as yet unresolved mystery about them…

Punchline:

I’m hoping that this post will raise as many questions as it answers and provoke as many arguments as it settles and that those questions and arguments will get answered, debated and settled (and so on and so forth!)…Thanks for reading.

Schteve.

 

P.S. Big thanks to Lizzie for all her help x.

References and further reading:

 

Hot spot volcanism close to a passive continental margin: the Canary Islands:
http://www.atan.org/geologia/articulos/Carracedo1998.pdf

Los Volcanes de las Islas Canarias IV; La Palma, La Gomera, El Hierro. Juan Carlos Carracedo, Editorial Rueda S.A, ISBN 978-84-7207-190-2

Growth, structure, instability and collapse of Canarian volcanoes and comparisons with Hawaiian volcanoes:

http://www.atan.org/geologia/articulos/carracedodeslizamiento.pdf

Giant Quaternary landslides in the evolution of La Palma and El Hierro, Canary Islands:

http://www.atan.org/geologia/articulos/PalmaHierroCarra.pdf

Youngest lava flows on East Maui probably older than A.D. 1790:

http://hvo.wr.usgs.gov/volcanowatch/archive/1999/99_09_09.html

http://www.ign.es/ign/layoutIn/volcaFormularioCatalogo.do is the source of EQ data.