Photograph by Giovanni Mattera. Castle Aragonese seen from Ischia. The castle is sitting ontop of a resurgent dome plug from a flanking vent.
The World’s most ill begotten piece of real estate – Part III
The Chinese have a saying, “May you live in interesting times”. And it is in no way a friendly thing to say; on the contrary it is a rather magnificent curse. In Naples people live all their lives in interesting times. If it was not enough with being the poorest city in Italy, they also have to contend with the Camorra (local mafia), drug-wars, corrupt politicians, strikes and general civil unrest. To top it off even further they have built their city on top, or around, no less than 3 active super volcanoes. Could the times get more interesting than that? Well you could add large earthquakes and tsunamis to the list.
Ischia, or more correctly Monte Epomeo, started it’s activity about 350 000 years ago. Technically it is of the complex volcano type. During the first 300 000 years it grew and developed a large edifice paired with an over-sized volcanic sub-structure.
56 000 years ago the volcano had reached the critical level where the edifice was too large and heavy to be sustained on top of the very large magma chamber. The eruption probably started as a very large VEI-6 eruption that emptied out the magma chamber sufficiently for the roof to collapse. And since Ischia is an Island it then got messy as the ocean roared down into the open magma chamber. The ensuing VEI-7 explosion created the Green Tuff Ignimbrite. This Green Tuff Ignimbrite should not be confused with the even larger Pantelleria Green Tuff (Italy is rather interesting…) that covers most of the Mediterranean area.
Photograph showing Sant Angelo D’Ischia, another resurgent dome from a flanking vent.
After the eruption the Island was completely gone. As far as is known a 23 000 year long period of dormancy followed, but there might have been minor subsurface eruptions that helped to start healing the roof of the volcanic chamber system.
33 000 (Ar/K-dating) years ago a new phase started where the volcano had frequent effusive eruptions that helped to weld the tuff together healing the roof of the magma chamber along the entire 10 kilometer wide caldera.
28 000 years ago things started to get really interesting. By then the roof above the chamber was sufficiently structurally sound to hold for the increasing pressure inside the chamber. That caused the entire roof to be pushed upwards.
Most of the readers in here are familiar with the concept of resurgent lava domes. We have all seen them being pushed out of craters like odd plugs. For those interested in seeing the phenomenon I recommend Soufriere Hills at Montserrat. Thing is though that it is normally smaller craters that suffer from this rather dangerous condition.
The island of Ischia photographed from Castle Aragonese. The mountain area in the background on the island is Monte Epomeo, a resurgent dome formed as the caldera floor is lifted up above the caldera rim. Here be Dragons.
Problem here is that Monte Epomeo is a super volcano, and as such does things in super-size. And if you super-size a resurgent dome, then you have an entire caldera floor rising upwards. Just imagine the pressure needed to push up a ten kilometer wide plug 900 meters in 28 000 years.
I know, we are only talking about 3.2 millimeters per year on average, but it still requires rather stunning amounts of power. The uplift is though larger than that, the reason for that being failures in the resurgent dome with rock-slides and sector failures of the dome as it started to stick up above the caldera rim. 5 600 years ago the dome passed the rim. During the push up phase the dome had also dragged the caldera rim with it above surface, and around the island an elevated area has been created by the pressure. So, a lot of pressure has gone also into moving parts that technically are not a part of the resurgent dome.
Eruptive and other behaviors
The most common type of eruption at Ischia is smaller eruptions taking place between the resurgent dome and the caldera rim. There are quite literally hundreds of fissures, cones, and other volcanic vent types encircling the dome. These eruptions normally follow episodes of rapid surging (uplift) of the dome.
There are two more dangers on top of the island caused by the resurgent dome. The first one is quite simply sector collapses, landslides and rock-falls as the brittle welded tuff suffers structural failure. Some of these slides and rock-falls have reached as far as the coast line.
General volcanic map of Ischia showing major features of the volcano. Click for larger image.
The more dangerous version of failure is the lateral flank eruption. That happens as magma pushes upwards and builds up tremendous pressure and swelling of the side of the dome and the side of the caldera rim. Think Mount Saint Helens here and you get the picture. This causes a large pyroclastic flow going laterally over the island until it reaches the coast, then it will continue over the water. If it happens in the wrong direction it will hit inhabited land.
Critical lateral collapse of the resurgent dome towards the Bay of Naples.
During the last 12 000 years there has also been 3 sub-surface collapses of the island causing massive debris flows running out into the Tyrrhenian Sea. And there are several spots along the coast line where parts of the Island have calved off into the ocean. When this happens large tsunamis will race into the Bay of Naples destroying any part not high up. The latest known widespread tsunami in the area is known to have happened 800BC according to written records.
Debris flow from a sub surface failure of the shelf around the island. The surge direction caused a large tsunami to go into the Bay of Naples.
In the end though it is probably the super part of Monte Epomeo that interests people more than anything else. Because however you look at it, there is between 70 and 210 cubic kilometers (conservative estimate) of magma in various grades of fractionalization down under that ever uplifting plug. The volcano also has an ample supply of fresh water to drive up the pressure for a larger eruption, and when that happens the same thing that happened to Krakatoa and Santorini will happen to Ischia. And as with the two more famous volcanoes, it has happened before.
Current status of Ischia
Even though Ischia is currently not showing any sign of erupting other than the steady uplift she is deemed by INGVs Director Guido Bertolaso to be the most likely volcano to erupt due to the rapid buildup of magma that they have recorded. Bertolaso even went so far as stating “if I had to say which is the volcano with the most loaded gun barrel, I’d say it’s not Vesuvius but the island of Ischia”. He though went on to state that no eruption is imminent. This becomes evident if one looks at the lack of heightened volcanic tremor, and minimal amount of magmatic earthquakes.
Risks of Ischia
The risks are roughly discussed below in the order of likelihood. Ischia is the volcano most likely to have a large eruption in the Naples area. One should though remember that it is most likely to have a normal VEI-1 to VEI-4 eruption when it erupts next. This would mainly affect the 60 000 residents on the island, and the same amount of tourists.
Rock falls, dome failures and landslides from Monte Epomeo is also fairly likely to happen in the foreseeable future due to the resurgent dome uplifting. This will also only affect the local residents and tourists.
Large landslides either at the coast, or out on the elevated shelf that surround the island is fairly likely to happen within the next few thousand years as the pressure building up raises the land up and weakens the structure of the flanks. When this happen large tsunami waves will hit the Bay of Naples causing widespread destruction. This is also the risk that is hardest to predict and mitigate.
In the same timeframe there could be another partial dome collapse causing a Mount Saint Helens style eruption. This would destroy all buildings on the island, cook the inhabitants, and depending on the direction of the pyroclastic surge hit areas far into the Bay of Naples. I do not think we need to contemplate the effects of a hydro-magmatic eruption at the VEI-7 scale. I would only like to point out that Ischia is the most likely candidate of having such an eruption in the neighbourhood of Naples. Right now there is nothing pointing towards it happening within the next millennia, but in the end it is likely to happen within the next ten millennia due to catastrophic failure in the resurgent dome.
Ischia early in the morning. The sleeping Dragon rests calmly.
Ischia is more likely to kill people than any other volcano. This is due to the absolute lack of places to run to quickly since it is a heavily populated island, and that half of the inhabitants at any given time are tourists not knowing where to go. So even the smallest event will get messy, best case scenario is probably a VEI-1 eruption with clear precursors for INGV to order a complete evacuation. Anyhow, anything interesting happening at Ischia is more likely to kill thousands up to millions than any of it’s siblings due to it having more modes of operation.
Not only do we live in interesting times, now we have an inflamed Ischia.
Short addendum on the Turkish quake
There has been an earthquake just south Antalya. It ranged between 5.8 and 6.2, figures are going to be revised. The distance from Antalya, and depth is very likely to cause damages to houses and fatalities.
The associated beach ball has a rather odd look to it. But this is also likely to change. The EMSC-CSEM site has gone down due to pressure from people trying to get info. USGS is open for business. Here is a link to their beach ball and other technical data.
http://earthquake.usgs.gov/earthquakes/eqarchives/fm/neic_b000ac4h_fmt.php
Oddball beach ball of Turkey.
CARL
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