A birthday treat on Sicily
To celebrate achieving my degree I persuaded my non-geologist wife that our holiday destination this year should be Sicily. In exchange for me agreeing to look round ancient ruins she agreed to explore a few of the island’s geological features, and in particular Mt Etna.
At 3300 m high Etna is the largest and tallest volcano in Europe and is situated at the zone of continental collision between the Euro-Asian plate to the north and the African plate to the south. There are now four main summit craters (100 years ago there was just one) along with around 300 smaller flank cones. We arrived on the evening of 11th May (my birthday) and as we drove to our hotel we noticed Etna starting to erupt. At 23.00 as we entered our hotel room we had a magnificent view of the volcano from our balcony only 25 km away. We could clearly hear the activity and were in and out of bed until 03.30 when fire-fountain and lava flows were clearly visible.
Increased seismic and volcanic activity had been recorded from 8th May and the eruption of 11th/12th started at 17.00 with lava overflowing the eastern rim of the Southeast Crater at 20.30. The fire-fountain reached its peak at 04.00 when it rose to over 200-300 m high, breaking up at its top and producing consistent emissions of ash and lapilli. This formed an eruption column rising to about 2 – 3 km before being blown south-south-eastwards by the prevailing wind. The leading edge of the lava flow came to a halt some 2 km for its source. All activity had ceased by 06.00 and all that remained visible was a gentle steam cloud. However, by then the ash cloud had deposited lapilli of varying size across a large area including Catania airport some 80 km away, causing it to be closed for the whole of 12th May while sweeping of the runway was undertaken.
We booked ourselves on a guided tour two days later and managed to get to a height of just under 3000 m and could see quite clearly the site of the new eruption. (Fig 1, right hand crater)
Fig 1
Where we were walking the eruption had covered the snow to a depth of up to 20 cm, with individual pieces of lava being 2 – 8 cm in diameter (Fig 2)
Fig 2
On our way down in the cable car we clearly saw cinder cones produced during previous eruptions. (Fig 3 - note the restaurant in the background for scale).
Fig 3
A few days later we visited other volcanoes, but this time not caused by any geothermal activities. These were the mud volcanoes at Macalube in the Nature Reserve of Aragona, 15 km from Agrigento. They are caused by the pressure of methane gas mixing with salt water and clay under the ground. On occasions larger eruptions can reach up to ten metres in height but when we visited they were typically only 30 cm high, but impressive none the less. (Fig 4)
Fig 4
It’s a fascinating place which covers something like three football pitches, and once the coach-load of school children departed we had the whole place to ourselves. There are also mini mud pools bubbling away with, presumably, methane gas escaping. I would like to have tried lighting the bubbles to check, but without a cigarette lighter or matches this wasn’t possible.
Our next geologically-themed trip was to the disused sulphur mines at Parco Minerario at Floristella-Grottacalda, south of the town of Enna, which opened in 1750 and closed relatively recently in 1984. We appeared to have arrived out of season as no-one was present and the buildings were either locked or boarded up. However there didn’t appear to be any restriction on wandering around the site which contains numerous white spoil-heaps formed into long banks (Fig 5), an abandoned quarry, the entrance to an underground mine, and several brick furnaces.
Fig 5
The sulphur is found in sedimentary deposits throughout the “gessoso solfifera formation” which is an evaporite formation of Messinian age (uppermost stage of the Miocene 7.3 – 5.3 Ma). The concentrations are very close to the surface, faults or tectonic events linked with circulation of phreatic waters, Different theories have been developed to explain its formation, although it is thought to be due to the transformation of gypsum in reducing environments.
Sicilian sulphur deposits typically contained between 12 % and 50% sulphur with the main contaminants being limestone and gypsum. Before being exported pure sulphur had to be extracted, and the traditional extraction method was to ignite and melt the ore, but as fuel was in short supply (and sulphur burns) it was simply used to melt itself. It was piled into conical mounds in an open pit and covered in soil. The sulphur at the bottom was set alight to melt what remained above, with the remaining limestone and gypsum producing the large spoil-heaps. This was an inefficient method of extraction and something like two-thirds of the sulphur was lost, while the escaping sulphur dioxide gas polluted the surrounding area for quite some distance.
Although I was tempted to enter the mine through the broken gate, as I didn’t have a torch and with my wife asleep in the car, I thought better of it. I later read that in 1870 the average depth of such mines was around 50 m and by 1931 it was nearly 200 m, with some being as deep as 300 m, so with hindsight I’m glad I didn’t venture down! Instead, I spent about 30 minutes wandering around and investigated the brick-built Gill furnace kilns (Fig 6) which in the 1890’s replaced the inefficient earlier process of simple burning. Coke was used as fuel in this new process and efficiency was vastly improved with nearly two-thirds of the available sulphur being recovered in the latter years of production. I picked up only one piece of pure sulphur but several fines specimens of gypsum.
Fig 6
Our last outing was to the impressive bright-white marl cliffs at Scala dei Turchi on the south coast a few kilometres west of Agrigento. They represent orbital-forced cyclic sedimentation in the Trubi Formation of Zanclean age laid down approximately 5 Ma at the start of the Pliocene.
They are not particularly well signposted but well worth the walk to find them as they are quite extensive with no restrictions on climbing all over them! (Fig 7)
/ Fig 7I hope I have managed to convey a flavour of some of what Sicily can offer the amateur geologist (and their initially reluctant partner!)
Peter Ellenger
References
For general information: Michelin Green Guide to Sicily
For information on Mt Etna
http://www.ct.ingv.it/index.php?option=com_content&view=article&id=230&Itemid=210
For information on the mud volcanoes of Macalube
http://travel.italyitalia.com/guides/sicily-sicily-western-side-agrigento/mud-volcanoes-aragon-agrigento
For information on Sulphur mining
http://books.google.co.uk/books?id=O4rzzkUQyzIC&pg=PA41&lpg=PA41&dq=sulfur+mines+sicily&source=bl&ots=9zcEnQnUno&sig=GIgwLJz8Lw5HtCIO20rDXKvbY4g&hl=en&ei=6xn3TceoKYPQhAfwxIXzCw&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEAQ6AEwBA#v=onepage&q=sulfur%20mines%20sicily&f=false
For information on the marl cliffs at Scala dei Turchi
http://www.stratigraphy.org/bak/Zanclean.pdf