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

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

Image by IGNS Ltd.

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

Tongariro

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

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

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

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

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

Image by Geonet.

Risks at Tongariro

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

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

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

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

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

Whaakari (White Island)

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

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

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

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

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

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

1 million viewers!

Image by Spica.

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

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

CARL

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

Monte Somma & Vesuvius

Painters rendition of the 79 AD Pompeian eruption of Vesuvius.

The World’s most ill-begotten real estate, Part II

Monte Somma is an old volcano, activity started 400 000 years ago. Over the next 375 000 years a massive strato-volcano was built up at around the same location as todays Vesuvius. The main geological component is guarinite, an epitaxy of hiortdahlite, wöhlerite and låvenite. There is no known record of any caldera forming events during this long period. At the end of the period Monte Somma had an edifice containing four times the rock volume of today’s Vesuvius (calculated conservatively).

The volcanicity in the area is driven by the back-arc subduction zone caused as the African plate slams into the Eurasian plate, and then being pushed under. On the European side melt from the friction of the plates is being released through the Campanian volcanic arc. Other close by members of the volcanic arc is Campi Flegrei and Mount Epomeo (Island of Ischia).

25 000 years ago Monte Somma suffered the Codolan eruption, an ultra-plinian eruption that eradicated almost the entire volcano in a cataclysmic failure of the magmatic chamber. The Codolan ash lies on top of the Campanian Ignimbrite caused by Campi Flegrei 34 000 years ago, making the Codolan eruption the youngest of the cataclysmic events caused by the Campanian arc. The highest remaining point after the eruption is today known as Punta del Nasone (Tip of the Nose), an 1 132 meter high edifice on the caldera rim. The eruption probably had a significant effect on the population size in southern Europe.

Google Earth Image of Vesuvius. On the upper left you can clearly see the caldera wall of Monte Somma with the Tip of the Nose (1132m).

Vesuvius is born

From the ashes of Monte Somma a new volcano started to grow almost immediately. During the first 8 000 years the new volcano had a fairly unevolved magmatic chamber system. As such it could not cause large eruptions, instead it slowly, but steadily built up.

That changed about 17 000 years ago when a cycle started consisting of frequent small to medium eruptions interspersed by Plinian eruption ranging between VEI-5 and VEI-6. To date there has been 8 of these larger events in the current cycle. Calling them large might seem ridiculous compared to the Codolan ultra-plinian event, but one should compare within the cycle. These eruptions are believed to range between 5 and 15 cubic kilometers of ash counted in Dense Rock Equivalent (DRE). Compared to the 0.25 cubic kilometers (DRE) of Eyjafjallajökull these eruptions are rather large.

These larger eruptions take place roughly every 2 000 to 3 000 years. This time interval makes sense if one takes into account that the magmas needs time to fractionalize enough to evolve to the highly explosive magmas involved in these eruptions.

The latest plinian eruption was of course the 79 AD eruptions that eradicated the cities of Herculaneum and Pompeii. I will cover this eruption in a separate article in the series about Neapolitanean volcanicity. This eruption is the reason we call these eruptions plinian. The reason for that being the historian Pliny the Younger (Plinius), writing down the quintessential record of the eruption.

The plinian eruption before that was the Avellino eruption (Pomici d´Avellino) that took place 3 800 years ago. Archaeologists have noticed that this eruption had a large effect on the regional Bronze Age population.

After the 79 AD eruption Vesuvius has had numerous small to medium sized eruptions ranging from VEI-1 up to VEI-5. Some of these have been notoriously ashy. The 472 and 1631 eruptions yielded ash that travelled as far as Constantinople.

Vesuvius today is rapidly getting known as the Garbage Dump of Italy. This is due to a large amount of both legal and illegal dumping of garbage and industrial waste in old flanking vents and cones. This has raised the toxicity around the volcano to a level where one should not eat anything growing on or around the volcano. Even the fabled wines of Vesuvius are now deemed not fit for human consumption. It is sad that Man’s folly is destroying one of the world’s most beautiful vistas.

Technically Vesuvius is a somma-volcano, a type of volcanoes named after its parent volcano. The term refers to a fully developed strato-volcano that has formed inside a caldera of an older destroyed strato-volcano.

Photograph by the US Air Force. Eruption of Vesuvius 1944 taken from a bomber plane.

Risks of Vesuvius

Vesuvius can theoretically have 3 types of eruptions if we look historically. These are in order of threat-level the regular eruptions, the plinian eruptions, and a possible recurring ultra-plinian Somman event. Let us look at them one at a time.

Before we go on I would like to say that the projected death tolls for the respective eruption sizes are from figures that have been calculated by INGV, The Italian Government, The regional government of Naples, independent catastrophe mitigation experts, EU and the UN Decade Volcano Program.

The lower end figure is the best possible figures. Basically it would require functioning scientific volcano predictions, and a high-powered highly ordered Government ruthlessly enforcing evacuations and other protective measures. Basically we are talking about northern European style Government with heavy military aid here. The high figure is based on INGV being disregarded for political reasons, week or no mitigative measures taken, lack of functioning roads being accounted for, and the general nonchalant attitude in the region. I would here say that INGV will do their work; they are highly capable and very diligent in performing their duties. I just hope that they will be allowed to do it by the highly corrupt Neapolitan local politicians.

The risk is of course heightened by the high population numbers, and that people live close to, or even on the flanks of Vesuvius.

Central crater of Vesuvius.

Normal Vesuvian eruption

Vesuvius is a highly prolific volcano, and it is known to have had several instances of magmatic intrusion since the 1944 eruption. The last major intrusive episode was taking place between 1996 and 1999. So far this is the largest of the intrusive events post 1944.

It is highly likely that Vesuvius will have an eruption during this century. When it happens it will almost certainly be in the range of VEI-2 to VEI-4. One should though note that there have been two out-layer small VEI-5 eruptions since the 79 AD eruption and also that there has been a few VEI-1 eruptions. Median eruption (most likely) would be a VEI-3 size. Ash, volcanic bombs and pyroclastic flows would be the largest risk for the population.

Death rate would be between 0 and 100 000 depending on size of the eruption, and the amount of protective measures taken.

Vesuvius in the background photographed from Herculaneum.

Plinian Vesuvian eruption

Vesuvius is from a short geological time-frame ranging in on a plinian eruption. Nothing points towards that the eruptive cycle that started 17 000 years ago has changed to the better. Judging by previous behavior the next plinian eruption will occur during the coming millennium.

The risk of a plinian eruption is driven by the rate of fractionalization of the magmas. Normally this type of explosive eruptive behavior requires the volcano to not erupt for a few centuries before the plinian eruption, thusly giving the magma time to evolve as intrusions bring in new material that mixes with older colder magmas to revigorate the explosivity until the volcano quite literally explodes. This seems to not be the case with Vesuvius. One suggestion might be that there are different magma chambers that are responsible for the larger eruptions and small shallow chamber responsible for the smaller eruptions. Be that as it may, do not expect a long period of repose between a normal eruption and a plinian. Risks for a plinian eruption would be large amounts of ash, large pyroclastic floods, and lava bombs ejected up towards 40 kilometers. There is also risk of tsunamis causing additional deaths in the low laying parts of the Bay of Naples. Larger pyroclastic flows can rush over the water’s surface and hit areas that are not close.

Death rate between 10 000 and 1 000 000 depending on prevailing wind and the amount of people evacuated.

Photograph from Whiteynet. Vesuvius encircled by Monte Somma caldera.

Ultra-plinian eruption

This option is highly unlikely in the foreseeable future. Why? Compared to the size of the Monte Somma edifice we know what the maximum size the volcano can grow to before it suffers a catastrophic fail. Even if we count in the secondary caldera formation normally are smaller than the first one due to damages to the crust we still known that it will take quite some time to build the volcano up sufficiently both above ground and below ground.

If we calculate the growth rate of Vesuvius and compare it with the size of Monte Somma before the caldera event we see that it will take a minimum of another 75 000 to 100 000 years to grow to comparable size. Statistically we know that secondary caldera formations are 50 to 75 percent of the original event size. So, we are most likely looking at something in between 25 000 to 75 000 years of continuous growth before we need to worry about it.

The major risk of an ultra-plinian eruption would be ash covering a very large area, the explosions involved would instantly crush anyone within 25 kilometer. Think a hydrogen nuclear bomb shock-wave here. Between 25 and 50 kilometers there would be an initial 50 percent mortality rate due to high aerial ash content, lava bombs, and enormous pyroclastic flows covering large parts of the Bay of Naples. After the event pretty much no buildings within the 50km radius would be left standing up. Nationally deaths would occur due to ash and gas contamination. The coming year southern Europe would suffer crop failures. There will be an increased likelihood of hemispheric rapid cooling causing additional deaths and famine.

Death rate, 100 000 to 4 000 000. Supervolcano as a term is nuisance, but if one would erupt in a population the size of Naples it would have major impact. Regardless of the term, the effect on the population of southern Europe would be truly “super”. Remember, it is highly unlikely to happen.

This was the second installment in a series that will be five posts long. Remaining are the two other supervolcanoes encircling Naples, and of course the mentioned Pompeian eruption.

CARL

Adriatic Bop – Italian Quakes

Picture from IB Times. End of Time.

Recently there was a rather significant Earthquake in Northern Italy along the Po valley. Rescue and recovery efforts are still underway. With luck there will be no additional injuries due to aftershocks and building/infrastructure failure. Though unfortunate, this quake does afford us the opportunity to look around to see what is going on… geologically.

I would like to thank KarenZ whose plots put me on to this line of inquiry.

There is a lot going on in this region, and the structures there are somewhat complicated (to me) but in essence, the Adriatic or Apulian Plate broke off of the African plate and is wedged between the two. Where it is pushed North , the Alps were formed, to the Southwest, the Apennine Mountains formed and make up the familiar “spine” that runs down the Italian peninsula. The northern section of this range between the Po Valley and the Ligurian Sea is the region of interest. It seems that there is a pretty ancient subduction structure here that has a plate section hanging almost vertically underneath the mountains. (see Fig 1 of Margheriti et al). It is suggested that this is not a classic “subduction zone” but could be some exotic structure made up of continental crust fragments frozen in place in said paper.

Why do I bring that up? Well, the focal mechanisms for the two largest quakes show faulting similar to that of a subduction zone… specifically reverse faulting. The dangling slab in the last paragraph is not it.

USGS Moment focal tensor solutions (beach balls) of a fore-shock, the main-shock, and an after-shock of the large Italian earthquake.

In reverse faulting, the headwall is pushed up over the other side of the fault (relative to the other side) or the other side is being pushed under the headwall. (same motion, just different ways of looking at it) For this quake, it is actually oblique reverse faulting since it is pushing off to one side a bit. (the ball isn’t perfectly lined up).

The question about the Bulgarian quakes came up , but those have a completely different solution. They show normal faulting where one side slides down and away from the other. (or up and away). The only things those two quake sets have in common is.. um.. nothing. They were along the northern boundary region of the Agean Sea plate and the Eurasian plate. There may be some regional stress that caused them both, but as for fault lines, totally unrelated.

So.. what is with the Apulian Plate and how did it get there? Well, that’s the really wild thing. It seems (according to diagrams in reference 4) that the toe and heel of Italy, and part of Greece, originated in the gap in the North African coast down around Tripoli. During this drive north the Alps were formed. Massive folding and crumpling occurred as the land was tortured into position. Anticlines and Synclines formed and eroded, and the leading edge of the collision warped and formed a basin…much like the Persian Gulf between the Arabian and Eurasian plate collision or the Ganges valley on the Indian Plate to Eurasian Plate collision. As some of you know, the top of the Matterhorn is African crust. Did you also know that it is upside down? That’s how extreme the collision is. (pg 14 of Ref 5) In fact, one anticline was an island in a shallow northern Adriatic sea during the Pleistocene, the Ferrara Anticline, buried about 20 km northeast of Modena in the Po river plain. (ref 2 and 3).

Okay, enough rambling.

From reference 3, a modified Figure 1.

In this document I noticed that the study area covered a rectangle directly covering the quake area. Taking a position on the Northeast end of that box, I was able to calculate the distance to each quake and plot them in relation to the cross sectional strata of the study area. As you can see, the fore shock and mainshock occurred in the Mesozoic era limestone that was laid down when this area was part of the sea. Most of the aftershocks are along the interface of that layer and a lower ancient Tethyan crust. Only one quake in the USGS set shows as being in that part of the crust.

The dangling slab is not shown in this plot, and I did yank the mountains off the top. (They were represented in a different scale).

Thank You for your time.

GeoLurking

References:

1) “The subduction structure of the Northern Apennines: results from the RETREAT seismic deployment” Margheriti et al, ANNALS OF GEOPHYSICS, VOL. 49, N. 4/5, August/October 2006

http://earth.geology.yale.edu/~jjpark/Margheriti_etal_Annali_2006.pdf

2) “HYDROGEOLOGICAL FEATURES OF THE PO VALLEY (NORTHERN ITALY)” Bortolami et al

http://iahs.info/redbooks/a120/iahs_120_0304.pdf

3) “A new active tectonic model for the construction of the Northern Apennines mountain front near Bologna (Italy)”, Picotti et al JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B08412, doi:10.1029/ 2007JB005307, 2008

http://www.ees.lehigh.edu/ftp/retreat/outgoing/preprints_and_reprints/picotti_pazzaglia_2008_Apennines_final.pdf

4) “FROM THE TETHYS OCEAN TO THE MEDITERRANEAN SEAS: A PLATE TECTONIC MODEL OF THE EVOLUTION OF THE WESTERN ALPINE SYSTEM” Biju-Duval et al lNTERATlONAL SYMPOSIUM ON THE STIUCTUIAL HISTORY OF THE MEDITERIANEAN BASINS. SPLIT (YUGOSLAVlA) 15.29 OCTOBER 1976.

http://archimer.ifremer.fr/doc/1977/publication-5197.pdf

5) “Tectonic evolution of the Alpine orogen” Jacques Charvet

http://www.sklable.ac.cn/uploads/file/Jacques%20Charvet.pdf