Open University Geological Society (London Branch)
At each event we ask for a volunteer to record their experiences and impressions. These articles are published in our newsletter and eventually appear here as an archive of our activities. Note that these are only provided for reference, and are not endorsed by Event Leaders. Interpretations can change often and rapidly in Geology, any information stated should be verified from appropriate sources before being quoted.
Povington Hill and Durlston Bay • Vesuvius and Volcanoes in the Naples Area
Field trip: Povington Hill and Durlston Bay led by Paul Ensom
Sunday 16 September 2001
Well, Worbarrow Tout was the intended destination - Osama Bin Laden and the Military Mind were against us though. Some of us had driven down to Dorset the night before, staying in Lulworth, and hoping to visit the fossil forest on Saturday afternoon, only to find that the authorities had decided that morning to close the Lulworth range walks as a result of the crashes on 11 September. Quite how a few people walking over the Dorset countryside when the artillery ranges aren't in use can make any difference to the Defence Of The Realm, I'm not sure, but there it was - No Access.
So there were 12 members and a leader assembled by the locked gate across the road leading to Tyneham village and Worbarrow, wondering what to do. Strangely, the Military Mind hadn't seen fit to close the through roads across the ranges, so we could get to the viewpoint by Povington Hill (SY888811) where Paul gave us an overview of the geology. We were standing on the almost vertical part of the chalk in the Purbeck monocline, with the lowland of the Tertiaries stretched out to the north. Along the chalk ridge to the east we could see the ups and downs that indicated faults, and also the anomalous hill of Creechbarrow (SY921824), looking for all the world like a part of the chalk ridge, but which is actually formed of Tertiary strata capped by a limestone of Eocene age. The dips on Creechbarrow are about 10 degrees to the north, Paul told us, compared to the chalk less than 500m to the south which dips at about 85 degrees in the same direction. This makes it clear just how complicated the structures are in this area - the simple view of a monocline formed in mid- Tertiary times is a fudge. Paul explained how the Tertiary earth movements are superimposed on earlier tensional structures of mid-Cretaceous age, with discontinuities occurring in places throughout the succession. Looking south-west, we could see Tyneham Vale with Worbarrow Tout in the distance. These are of Purbeck Beds, also dipping to the north, but far less steeply than the chalk.
We had already decided to spend the rest of the day at Durlston Bay in Swanage which is essentially the same succession as we would have seen at Worbarrow (and that we had a log of in our handouts) but with significant differences. So off to Durlston we went. Our first stop was above the cliff at SZ035782 where we were shown the area where fossils of Purbeck mammals had been found for the first time in the 1850s by Samuel(?) Beckles. Paul explained that down on the beach we would see the bed thought to be the same horizon as Beckles' mammal bed, but that it must be a different facies since the matrix is different to that around Beckles' fossils.
We stopped at the coastguard station at Peveril Point to check tides and recent cliff-falls (our Worbarrow tide times weren't good enough here where there are double tides). It just wasn't our day - our intended way up, the Zig-zag Path, had been closed because of a landslip, and we would have to work along the beach and come back the same way, with a rising tide threatening to cut us off. Paul reckoned there'd be time to get as far as the Cinder Bed and the putative mammal bed before we had to turn back, so that's what we decided to do.
It was well worth it. The Purbeck Beds dip fairly shallowly at Durlston, but in the short time we had, we did indeed get down to the mammal bed, by way of strata showing a variety of shallow water environments. The beds are mostly thin limestones interbedded with shales and marls with some evaporites, indicating an unstable area with frequent marine incursions and some drying out. The highest beds we saw were the Unio beds characterised by a freshwater mussel. Below that was the Broken Shell Limestone which is fairly massive, and its base was nicely exposed in the cliff to show a convincing dinosaur footprint, probably Iguanodon. This is the earliest of the totally nonmarine strata, and there are shaley beds containing Corbula, a brackish water bivalve, below it. We were assured that they also contained evaporites (selenite and halite pseudomorphs), but we didn't see any. Insect fossils have apparently also been found in the Corbula Member.
In the end, despite the inauspicious start, it was a thoroughly good day. Good weather, good geology, good company, and an excellent leader. And of course we still have Worbarrow to look forward to Paul has promised to lead us there at a similar time next year. Don't miss it.
Field Trip: Vesuvius and Volcanoes in the Naples Area led by Chris Kilburn
21st - 27th October 2001
Sunday 21st October 2001An Introduction to Vesuvius and the Naples Triangle
Having completed all the necessary car hire documentation, including the extra insurance option, our party of twenty was ready to leave for our first location by mid morning. Our leader Chris Kilburn had arranged a visit to the Observatorio Vesuviana (Vesuvius Observatory) where we would be given an introduction to the mountain and a guided tour of the observatory by one of his colleagues Dr. Mauro Di Vito. Minutes into the journey we realised why Chris had spent so long on giving instructions on which motorway exit to take and where to meet; because any thoughts of the three vehicles travelling in convey were quickly dispelled as we were introduced to the Italian Driving Experience. This was our first experience of the 15km road along the Sorrento Peninsular. I don't think that at any point during the week we were we able to negotiate this section of road in less than one hour. Not only did we have to compete with other cars, vans, busses and lorries but also thousands of scooters, double-parked vehicles, oncoming traffic, pedestrians and animals, all with their own individual interpretation of the Italian Highway Code (if such a document exists.)
We eventually arrived at the observatory to find that our visit coincided with one arranged for some local dignitaries which involved some shuffling of our itinerary and parked cars! Located at a height of 609m, the summit of Vesuvius is 1281m, the Osservatorio Vesuviana is the oldest observatory in the world dedicated to the study of volcanoes. Built as a research centre between 1841-1845 it is still in use today and hosts a major exhibition. From the observation terrace Mauro gave us a history of the volcano.
Vesuvius has evolved in a region forged by volcanism for at least 300,000 years. The present mountain is properly known as Somma -Vesuvius. Somma refers to the bulk of the volcanic edifice, while Vesuvius strictly describes the summit cone, constructed during the past 1800 years. Consisting largely of lava flows, growth of the Somma volcano was complete before 25,000 years ago. At about that time Somma's activity changed from modest effusions of lava to catastrophic plinian eruptions. Five further plinian events have occurred since then, at intervals of about 2000 to 8000 years. Of these, the most famous and most recent, is the one that destroyed Pompeii and Ercolona in AD 79. Since AD 79, volcanic activity has been restricted to sub-plinian and to the effusions of lava. By 1631, a summit cone (Vesuvius) had been constructed within the Somma caldera. The cone has since continued to grow, reaching its present height during the volcano's last outburst in 1944.
Mauro then guided us around rooms within the observatory, which hosts an exhibition. Here visitors are introduced to the world of modern vulcanology by posters, images, videos and computer simulations. Computer models of previous eruptions have been constructed to define a reliable scenario of the phenomena likely to occur in the case of a new eruption. From such models, the risk can be assessed and adequate measures can be adopted to protect or evacuate the exposed population. Another section of the exhib ition deals with volcano monitoring. It explains how from the recording and processing of seismic, geodetic and geochemical data precursory signs of an impending eruption can be recognised. A selection of historical instruments, rare scientific books and volcanic products were also on display.
After the observatory visit a quick snack of pizza was suggested for lunch. After some negotiation a room upstairs in a local pizzeria was made available to us and Chris ordered our "quick snack." Ninety minutes later we emerged having consumed a selection of dishes of anti pasta, pasta, fish, fried nibbles, breads and TWO METRES of PIZZA followed by fruit and coffee. All this for £8 a head including drinks and tip!
By now it was late into the afternoon so it was decided, that on the way back to Sorrento, we visit the remains of the Roman Villa of Poppaea at Oplontis destroyed by the AD 79 eruption.
Here several sections of deposits from the volcano are in evidence and they generated the first academic discussion of the week. Were the flows pyroclastic or mud? Hot or cold? Etc. etc. Unfortunately, at this stage we began to lose the light but many of the questions were resolved later in the week.
As with the Bermuda Triangle it is very difficult to navigate out of Naples once a vehicle is sucked into its traffic system. What appeared to be, on paper, a relatively straightforward journey back to Sorrento was not to be the case for the occupants of one of the cars. In an experience, that was to become quite familiar to us all, they extended their knowledge of the geography of Naples through a series of missed turnings, missing or disappearing signposts and driving down roads and motorways that took them in the opposite direction to Sorrento.
Monday 22nd October 2001Vesuvius
Monday started as a glorious clear day with views of Vesuvius across the bay from the hotel. Chris decided that we should seize the opportunity and make the journey to the top of the volcano. We were not disappointed, since the fine weather stayed with us all day with only a few clouds passing over.
We drove to the car park, which is at about 1,000m above sea level. At this height it was much cooler than lower down. From here it was about a 30-minute walk up to the crater rim. As we climbed the path there were panoramic views across Valle del Gigante, which contains the 1944 lava flow, towards Monte Somma. This forms the outer rim of the caldera (see panoramic photograph on website) within which Vesuvius has grown up.
Somma volcano grew to a height of about 2,000 metres and was complete some 25-30,000 years ago, when it changed to catastrophic Plinian eruptions. The collapse of Somma (the rim now reaches a maximum height of 1,130 metres) means that about 40 km3 has gone in the last 25,000 years. The Somma wall is about 300m of pyroclastic material with sub-vertical lava dykes that jut out from the wall. The dykes are oriented radially around Vesuvius, fed from a central conduit that moves outwards. We also saw evidence of the pyroclastic deposits and lava flows that cover the caldera floor.
The Vesuvius cone is now 1,281 metres high, reaching that height in the 1944 eruption. Each Plinian event released about 1-3 km3 of pumice in a few days. The most famous (and most recent) Plinian eruption took place in 79 AD (1,922 years ago), destroying Pompeii and Herculaneum. There had been six Plinian events between 25,000 years and AD 79.
Between 79 and 1631 there were about 10 eruptions separated by decades or centuries. Between 1631 and 1944 there have been 18 cycles of eruptions, i.e. virtually continuous. Most of them erupted material of about 10-30 million m3 over a few days. During this period there were a few major events of 100 million m3 of material.
Just past the ticket office we reached the rim of Vesuvius' cone. The view is dramatic, looking into the crater that is 500m across by between 200-300m deep. The walls are near vertical with scree nearer the base and around the crater walls there are many active fumaroles. The lava layering of eruptions between 1913 and 1944 were clearly observed, as well as the 1944 pyroclastic material that lies on top and at one point a discontinuity marks the junction between the 1906 and 1944 eruptions could be clearly seen.
We saw examples of where leucite and pyroxene phenocrysts were formed where magma was in the crust some time before eruption.
The eruption of March 1944 had been bubbling away for a few days while the magma entered and pushed out magma that was near the surface. Blocks of magma were seen that were deposited by the lava fountain. The flow was trapped between the base of Somma caldera and cone where it flowed down and then turned north to San Sebastiano.
Vesuvius has been quiet now for almost 60 years and since development of eruptions during Plinian phases tend towards more evolved magma, concentrating on more volatiles there is likely to be greater explosivity. Therefore, the volcano is monitored continuously and plans have been developed for eme rgency evacuation that assumes 0.06 km3 of magma will be erupted. Seismic measurements are made but to a poor resolution - the style of eruption has oscillated therefore it is not known quite what will happen.
After descending back to the car park it was time to have lunch of rolls, cheese and salami. Driving down the slopes of Vesuvius, we made a short stop by the road to look at fine examples of pillow lava and the view back to the peak of the volcano.
Our next stop was in the town of Massa di Somma on the northern flank of Vesuvius. This and the neighbouring town of San Sebastiano were partially destroyed by the 1944 lava. The lava was clearly vis ible in foundations of buildings (there was even a helpful sign on the flow) that had been erected since. On looking towards Vesuvius, the trail of the lava illustrated how it had flowed down towards the towns.
The last stop of the day was at a disused quarry near Pollena, which contained deeply eroded pyroclastic material. Here we could also see a very good stratigraphic sequence representing the period from the Sarno Plinian event (17,000 years ago) through to the Pompeii eruption (AD 79). Here we saw evidence of a Strombolian cone with dykes and lava flows while magma is rising quickly. The eruption probably oscillated between fountaining where fragments come out as a jet and Strombolian - the cone being formed in a few days.
After leaving the quarry, the three cars separated to make their own way back to Sorrento. We all thought it would be an easy journey. However, the vagaries of the Italian autostrada and road direction signing caused all three to go the wrong way. One car ended up taking a tour of the autostrada in the suburbs of Naples before getting onto the right road. Another car took the right road south but ended up going towards Salerno before realising they had gone too far and turning round. The third car managed to get all the way to the right turnoff for Sorrento to then take a wrong turn to end up in Casellammare di Stabia. Rather than go through the congested town it turned round to find out about limited access to get onto the Sorrento road. In the end, we all arrived back within half-an-hour of each other, just in time to get an evening meal.
Tuesday 23rd October 2001Herculaneum
Today, we visited the ancient town of Herculaneum, buried nearly 2,000 years ago by the AD 79 eruption of Somma-Vesuvius. Beyond the sound of the evernoisy traffic, we stood overlooking the uncovered remains of the Roman town, before descending to the base of the excavations to examine the details of what had overwhelmed the settlement.
Peppe Rolandi, professor at the University of Naples, explained to us that the town had been built on a much earlier Plinian eruption of Somma, and evidence of this was found under the foundations. There are beach deposits showing that the town was originally on the edge of the sea.
The AD 79 Plinian eruption consisted of a series of surge units followed by massive pyroclastic flows, and it was the hot (600 degree C) surge units that killed the people. The surge units are caused when the column of erupting material from the volcano starts to collapse. The head of the advancing mass becomes dilute and settles as the surge, and the main body over-rides it forming the flow. We could identify 6 thin surges separated by 6 flows in the 20 metres of pyroclastic material. This all happened within a few hours and completely buried the town, killing at least 200 people as they tried to escape by sea. They were unable to launch their boats because of the tsunamis.
We spent an hour or so exploring the remains of the ancient town before eating bread, cheese, salami and fruit "on the hoof," standing outside the entrance.
We then made our way to the Rainieri Quarry, which excavates the 1834 lava flow for building stone. Here we saw the remains of a Roman villa and more of the eruptive material of AD 79. We were unable to find the basal pumice but examined horizons of surge and flow deposits. We found a layer of accretionary lapilli, which are pisolites made by concentric layers of fine ash round a tiny fragment. These are formed when the eruptive column contains large amounts of moisture. Lack of time terminated the discussion and we eventually found our way back to the hotel.
Wednesday 24th October 2001A Visit To The Campi Flegrei
After once more braving the Neapolitan traffic and the road signs aimed at sending you the wrong way down the autostrada, we arrived at Pozzuoli and and our first stop at the Temple of Serapis. The columns of which "are among the most famous symbols in the history of modern geology" (Kilburn and McGuire, 2001) having formed the frontispiece in Charles Lyell's Principles of Geology.
The marble columns are marked from a height of 3.6 m. above ground for a further 2.7m. with a darkened area caused by the attack of the marine bivalve (lithodomus lithophagus) showing that the columns have been below sea level and have emerged from the sea at least once since they were erected 2,000 years ago.
35,000 years ago, to the terror of the Paleolithic settlers in the area, the whole 50 square km. of the Campi Flegrei erupted throwing 100 cubic kilometers of lava and ash into the sky (about 300 times as much as Mt. St. Helens in 1980). This ash or Campagnian ignimbrite can be found over an area of 30,000 square km.
12,000 years ago an inner caldera covering about 30 square km. erupted forming the Neapolitan Yellow Tuff, NYT, which has been used as the main building material around Naples for at least 3,000 years.
What is so astounding about the whole area of the Campi Flegrei is its sheer size. Before visiting the area the writer had imagined an Icelandic type of deserted landscape of fumaroles and cones. The actuality is two ring structures the outer one with a diameter of approx. 15 km through which the 'route tangentiale' of Naples tunnels, and an inner caldera of approximately 7 km. diameter. Around this inner caldera there are a number of pyroclastic cones about 300 km in diameter andabout 80 km high. Within the 100 square kilometers of the Campi Flegrei crater and caldera, which are still active, two million people live.
From the Temple of Serapis we walked to the ferry port and noted the line of mooring bollards stranded when the land uplifted 1.8 m from 1982-4. Further round the bay you reach the old fishing port and Rione Terra, the old centre of Pozzuoli, which was evacuated during the first of the present earth movements between 1969-72. For someone brought up near the tides of the North Sea it is easy to miss the new lower line of mooring bollards and the steps to the water stranded in mid-air. There are of course no tides in the Mediterranean and the land in Pozzuoli has now risen 2.3m in the last 32 years.
Across the bay we could see Monte Nuovo about 3 km. distant and the Starza marine terrace which has risen 40m in the last 4,500 yrs. Monte Nuovo is 133m high and 700m across. This hill formed in the two days leading up to the 27 September 1538. The ground shook, the sea receeded, & cold water poured from cracks in the ground. The cold water was followed by steam and mud then a violent hydromagmatic eruption sending wet surges which flattened buildings within 5 km. Within 48 hours the town of Triperzole was buried by Monte Nuovo.
From 1501 - 1538 the ground rose 12m. Today so far the ground has only risen 2.3m in the last 30 years. Undoubtedly magma has again been moving 9km down and the possibility of a new eruption perhaps within the decade cannot be entirely discounted.
From the old port we again braved the traffic of Pozzuoli, this time on foot, to climb past the very impressive Roman amphitheatre, the third largest in Italy. We were aiming for the cone of Solfatara which features in all the tourist guide books of the area. The cone is 300m across and 80 m high and inside gives the illusion of space and lack of population. There are fumaroles, boiling mud pools and even a campsite surrounded by Tamarisk trees. Solfatara is about 4,000 years old and is of a shape typical of hydromagmatic eruptions.
We had a fascinating day of "hands - on" geology in this small section of the Campi Flegrei but it is still difficult to imagine why so many choose to live in so dangerous a place.
(Reference - Italian Volcanoes by Chris Kilburn and Bill McGuire, Terra Publishing 2001)
Thursday 25th October 2001Pompeii
Thursday was a free day, but most of the party elected to catch the Circumvesuviana train from Sorrento to Pompeii.
Friday 26th October 2001Monte Nouvo
Plan A for the day was to drive to Pozzuoli and take the ferry to the island of Procida for a day of geology and swimming. We started well, and all three vans arrived in Pozzuoli without getting lost. Unfortunately the ferry had broken down! Chris quickly drew up plan B, we would take the train to Arco Felice and spend the day on Monte Nuovo, the youngest volcano in the Phlegraean Fields. Unfortunately the train wasn't running due to track repairs!
With great presence of mind Chris produced plan C, take the bus to the next station and take the train from there. This one worked, and after a short walk from Arco Felice we arrived at the Monte Nuovo nature park.
Monte Nouvo consists of a cinder cone produced during a series of explosive events in 1538, destroying in the process the village of Tripergole and much of the port Agrippa. The park, which combines geological and ecological conservation, was established in 1996 by the combined efforts of the Municipal Corporation, the Liceo Scientifico E. Majorana, Pozzuoli (NA), and various local organisations.
Our first stop was an extensive outcrop close to the entrance of the park. A lower pale yellowish grey unit was seen, tapering out to the right and overlain by a dark grey unit. The pale unit consisted mainly of poorly sorted angular pumice fragments centimetres or less in size with some dense glassy fragments. A crude bedding was apparent in places, and a few large clasts were seen to have impact sags. The overlying dark grey unit was similar in comp osition but contained a greater number of larger clasts. Closer inspection revealed that some of the pale fragments had dark grey interiors and some of the dark fragments had pale exteriors, suggesting that the colour difference was due to alteration. It is known that the overlying unit is a scoria flow which occurred on the 5th day of the eruption, killing 24 people who had walked into the valley. Analyses of the two units has shown that they are chemically indistinguishable, both consisting of K-phonolite. The colour difference is thought to be due to the earlier magma being more vesicular and therefore more interactive with groundwater.
After a picnic lunch we walked up to the crater rim with magnificent views over the Gulf of Pozzuoli and Lake Averno. The crater walls drop steeply to the floor some 120m below. An exploratory geothermal borehole in the floor of the crater recorded a temperature of 400°C at a depth of 2km. The Monte Nuovo cone has a low width/height ratio typical of eruptions with restricted hydromagmatic activity. This is in contrast to the adjoining craters of the Averno complex which are very wide in respect to their height, a characteristic of strong hydromagmatic activity. The last eruption in the Averno complex took place about 3700 years ago. The name is taken from the Greek for 'without birds', and it is known that the crater was actively degassing about 2100 years ago. In 37 BC a canal was constructed to the Gulf of Pozzuoli so that Lake Averno could be used by the Roman fleet.
Other points of interest seen from the crater rim were Capo Miseno from where Pliny the younger observed the eruption of Vesuvius, the ruins of the Temple of Apollo, and on the north west rim of Lake Averno the entrance to the tunnel which forms part of the Roman canal leading to the west coast. The twin peaks of Monte Gauro and Monte Covara could be seen to the northeast.
It was now time to return to the vicissitudes of the Italian transport system at the end of another excellent day, thanks to Chris's instant improvisation.
Saturday 27th October 2001
On our final day in Italy, we had a special treat planned for lunch. We caught the local bus outside the hotel. This flew up the winding roads, hurtled around hairpin bends and finally deposited us in the centre of St Agata, a small town in the hills above Sorrento.
Lunch had been arranged for us in the Restaurant Margharita by Professor Antoino Pouzzilli, Chair of Mineralogy at the University of Naples, a friend of Chris Kilburn. The restaurant is family owned and run. Professor Pouzzilli joined us for lunch, despite being caught up in the Neapolitan traffic.
As soon as we had taken our places at the table, the food and wine started to arrive and did not stop coming. What a wonderful meal! All too soon, we were forced to stagger back to the bus stop and were transported at break neck speed back to the hotel in time for the coach to take us to the airport.
And so we arrived back in the UK with many happy memories of a great field trip. A vote of thanks is due to Chris Kilburn for his leadership, to Barbara Cu mbers for organising the trip and not the least to the drivers who took their sanity in their hands in the Italian traffic!