Ischia (pronounced: ‘Is-kyah) is a densely inhabited volcanic island in the Bay of Naples, on the mid-southwestern coast of Italy, some 30km from the Naples mainland. Most people know that the island of Ischia is of a volcanic nature. Many of them believe that it is just another extinct volcanic cone, layers over layers of lava, piled up throughout distant eruptive periods. Not so! The interesting thing is that it’s not your off-the-shelf volcano grown from a seamount before emerging above the sea surface. Ischia has a distinctly different geological history compared to the island volcanoes we know from subduction zones. And, of course, it is not extinct.
Everybody has heard of Vesuvius, many are concerned about the Phlegraean Fields – and rightly so. The island of Ischia is part of this same active volcanic area, but, if at all, is mostly mentioned as just an accessory. Millions of visitors flock onto the island every year for its Mediterranean flavour, its thermal springs and spas, the mild climate, or its Italian cuisine. But only recently travel companies have discovered an interest in the geological specialities of the island and offer geological tours. So I guess that quite a few of the regular guests learn only now that they are holidaying on a still active volcanic complex.
Ischia, as a volcanic field, is made up of several distinctive volcanoes. Scattered all over the island are more than 40 eruptive centers. Over 100 craters and vents have been traced. But, unexpectedly, 787 m high Mount Epomeo – roughly in the middle of it – neither is, nor has it ever been, a volcanic edifice.
The only historically documented eruption on Ischia took place in 1302 and produced the Arso lava flow in the north-east. Nevertheless, the writings of earlier times contain plenty of allusions to eruptions that have taken place between 500 BC and 300 AD.
Otherwise, the Island is seismically quite active and earthquakes are common – some of them have caused fatalities and serious damage in the past. However, at present there is not much going on below Ischia’s surface. A small swarm of tiny earthquakes occurred between November 2018 and 09.01.2019. These were caused by its very slow sinking process due to cooling magma underneath.
The latest bulletin for the island states no significant occurrences in seismicity and ground temperature monitoring. Deformation measurements show a slow sinking in the central-southern area, with the tiltmeter indicating the direction towards N-NNE. Gravimetric measurements between 9/2017 and 6/2019 show that the NE part was affected by a slight gravitational anomaly which, however, was at or below the limits of statistical significance.
PHLEGRAEAN VOLCANIC DISTRICT
Ischia is a part of the Phlegraean Volcanic District which includes Vesuvius, the island of Procida and the Phlegrean Fields. The PVD is the most widespread active volcanic system of the Mediterranean area. It developed inside the Campanian Plain half graben, following the opening of the Tyrrhenian sea basin. The Campanian Plain is bounded by a series of NW–SE (Apennine) and NE–SW (anti-Apennine) faults and tectonic movements generated a series of volcanic structures along such faults. The two large volcanic fields, Ischia and Campi Flegrei, are fed mainly by evolved alkaline magmas. A smaller volcanic ﬁeld on neighboring Procida island produced trachybasaltic–shoshonitic tuﬀ and scoria cones and minor lavas.
The question whether or not the two systems are somehow in communication seems not to be answered yet. “We know that there is a magma chamber below Campi Flegrei, around 7 to 8 kilometers deep”, said Francesca Bianco, director of the Vesuvius Observatory of the INGV in a 2018 news article, “but we are not aware of how far it is extended horizontally and whether it reaches Ischia”.
THE SEE-SAW VOLCANIC HISTORY OF ISCHIA
UP: First Volcanism
According to newer research the first volcanic activity in this place occurred about 150,000 years ago. The first volcanic craters were formed near Mount Campagnano. The rock on which the picturesque Castello Aragonese was built, off the east coast, is an ancient volcano itself and ~135,000 years old. This was followed by a long, and at times violent, history of eruptions and deformations.
DOWN: Caldera Forming
About 55,000 years ago, a large magma chamber had formed below today’s island of Ischia, and it followed a period of strong activity. The deposits of these mighty eruptions are called the Monte Epomeo Green Tuff. On the island of Stromboli(!), geologists found a prominent light-colored stratigraphic layer that represents a distal tephra deposit from an eruption of Ischia between 40 and 60 ka ago. When the magma reservoir was emptied so far as not to support its ceiling any longer, the latter caved in and sank into the void, forming a caldera.
At that time the sea level was about 150-200 meters lower than today, so this pre-Ischia caldera was then on land. When the sea level rose again, the caldera “bowl” was flooded and consequently filled with sediments and tephra from later eruptions. The rocks, which should later form the island of Ischia, were now at the bottom of the sea.
UP: Volcano-tectonic Resurgence
This should change, though, beginning some 33,000 years ago. Due to the infiltration of fresh magma, the caldera floor (on the sea ground) began to rise gradually; it eventually emerged above sea level as an island – which is Ischia today. This deformation by refilling the old magma reservoirs is called resurgent uplift. The lid of the magma chamber was broken apart into several plates or blocks.
These blocks were then lifted differently high, creating a horst-and-graben feature, with present-day Mt. Epomeo as the highest point of the horst. Several authors link the resurgence of Mt Epomeo and the volcanic activity around the block to a laccolith-shaped very shallow magma body. The faulting would have been caused by emplacement of the magmatic intrusion which also drove resurgence. Other authors explain the cause of the resurgence by an increase in pressure in a shallow magma reservoir using a trapdoor model, or a simple shear model.
The processes of lifting blocks and relative lowering of the surrounding ground are still going on today. All the fractures that border the various blocks are active, this is where most of the hydrothermal phenomena throughout the island are now concentrated. More proof of the ongoing deformation are the historical eruptions (e.g. Arso lava, 1302 AD), large earthquakes (e.g. Casamicciola, 1883 AD) and numerous landslides.
Evidence of the submarine past of Ischia can still be found today, high up in the mountain, e.g. about 400-500 m above sea level near Buceto Spring – in the form of ancient beach debris and marine fossils. This proves that the ground here used to be a beach that was uplifted by the geologic processes. The total uplift on the northern flank is now 920–970 m, and 710 m on the southern flank, with an average resurgence rate of 3 cm/yr and 2.3 cm/yr respectively.
UP: Emplacement of Lava Domes
From ~10,000 years ago – and still ongoing at the present – several lava domes were emplaced along the eastern side of the Mt. Epomeo resurgent block. Also a series of pumiceous tephras, tuff rings and lava flows were erupted. The activity during the most recent period was on the eastern part of the island. Here most of the eruptive vents are aligned N-S along tensile faults at the eastern edge of the Mt. Epomeo block. A few of the youngest vents are located along an ENE-WSW-trending fault system that crosses the south-eastern corner of the island. The remains of the old caldera border are still visible today on the outer parts of the island: Monte Vico, Punta Imperatore, S. Angelo, Monte di Vezzi and the Castle of Ischia.
DOWN: Sector Collapse Events in the Resurgent Caldera
The active resurgence also created very steep and quite unstable slopes all over the island. Around or after 8.6 ka BP a it was affected by a series of major flank failure catastrophes. The three largest debris avalanche deposits cover the entire area from the Mt. Epomeo summit to the southern coast, at a distance of 3.5 km, and very probably extend into the sea. The combined thickness of these deposits is 200 m and the volume is estimated to be 1 km³. Debris avalanches entering the sea typically generate fast-moving tsunamis. So, the effects of these main collapses may well have affected other coastal regions, such as the Italian mainland and/or the southern Tyrrhenian Sea islands.
In the case of Ischia Caldera, it appears that the increase in volcano mass and shear stress are the most important factors that contribute to its instability. This is not a consequence of piling up of lava and tephra, but is due to the uplift during resurgence. In times of inflation by rising magma, part of the caldera is uplifted, with an increase of mass in the resurgent block. Another cause for flank failures could be deformation in the slopes, they get steeper and therefore more prone to failure. Also, due to a permanent system of fumaroles along the flanks of the resurgent horst, hydrothermal alteration would weaken the rocks and further contribute to their instability.
Still DOWN: Cooling of Magma Chamber
Presently, the GPS time series shows a general subsidence of the island with higher values in the central-southern sector. This is due to the continuing cooling of the magma body at about 2 km below the island. The magma cools, contracts, and the island lowers. This causes fractures along the faults, with the main fracture passing below Casamicciola and Lacco Ameno in the N/NW. Casamicciola Terme was destroyed in 1883 by a violent earthquake of magnitude 5.8. The damage was considerable and more than 2000 fatalities were recorded. The M 4.0 earthquake of Casamicciola Terme in August 2017 led to a toll of 2 victims and 42 wounded. It also caused the collapse of numerous houses that were not built to earthquake-safe standards.
THE VOLCANOES OF ISCHIA ISLAND
The volcanic craters in the eastern part are all located in the subsidence area of the Ischia graben. When entering the port of Ischia Porto (north-east corner) you are first greeted by the flank of the Montagnone-Maschiata crater. This volcano was active in the 1st century BC and was created in an older crater of the Fondo-Ferraros volcano. That produced alkalitrachytic lavas some 2700 years ago, as well as a huge maar-like tuff ring. Today, only the southern flank of the Fondo Ferroros is preserved, as the opposite flank was blown out by the younger Montagnone-Maschiata volcano.
In addition, the picturesque port of Ischia Porto itself is also a volcanic crater, dating to about 180 years BC. Its alkalitrachytic lava flows and scoria deposits can be seen today east and west of the port. After the eruption, the volcanic structure subsided and filled with water. It is believed to be the same event that the (younger?) Plinius described, where “flames” were ejected and much damage was done to a citadel, and the eruption “transformed a plain into a lake” – a scenario similar to that reconstructed for the Porto d’Ischia eruption.
In Ischia Ponte, the Castello Aragonese was built on an approximately 135,000-year-old alkalitrachytic lava dome. Today only the central area of the volcano is preserved. By various weathering processes, the flanks were completely removed and the central area exposed. A special feature can be seen in the volcanic structure: the western part has sunk. This volcano was created on a fault bounding the graben, and its western part was located on the subsidence area, so this part is now below sea level.
The south-eastern corner is the oldest part of the island. It is separated from the youngest part by a NE-SW fault. At this fault, several eruption centers line up like a string of pearls. Today, these craters are barely visible. The first three volcanic craters were created around the 3rd century BC and produced latitic scoria and lava flows. West of these eruption centers and the important fault line is the Ischia graben. In 1301/2, the last volcanic eruption on Ischia took place in this area, at Fiaiano, where a tens-of-meters thick trachytic lava flow has made its way in a north-easterly direction to the sea – the Arso lava flow.
In the central area of the island of Ischia around the highest point Monte Epomeo, the green Epomeo Tuff is exposed. These are Ignimbrite – welded ash that was still hot and sticky when deposited. The characteristic greenish color of the tuff is caused by chlorite, an iron-containing phyllosilicate. The tuff layer has a thickness of about 200-300 m.
At the southern coast is the small town of Sant’Angelo with a rocky peninsula jutting south into the sea. The latter is a former cinder cone which had erupted alkalitrachytic lava about 100,000 years ago. Today, only the solidified core, or the lava neck, can be seen. The flanks of the cinder cone are almost completely eroded. The alkalitrachytic volcanics are overlain by tuff and pyroclastics of later eruptions.
At the Maronti Beach, east of Sant’Angelo there are several fumaroles, some discharge into the sea, others through the sand on the beach. This area of the beach is closed off, as the hot gases escape here with about 99 °C.
In the highlands of the Forio municipality, the bed rock is Green Epomeo Tuff, which has deposited in an explosive volcanic eruption over the entire island of Ischia.
Near the S. Maria al Monte church there are several hot fumaroles. These are primary water vapor of about 40-70°C. Another strongly degassing fumarole field is on the western flank of the Bocca di Serra ridge. The gases here are largely composed of water vapor and CO2, but also contain an amount of H2S, so that a smell of rotten eggs can be perceived. All these indicate that a slow-cooling magma reservoir is sitting below the island of Ischia, at a depth of about 2 km.
In the northwestern coastal area of Forio is the mighty Zaro lava flow, today about 60-80 m thick, which has flowed in a northwesterly direction to the sea. This was a highly viscous A’a’ lava flow, rich in SiO2. On cooling the lava flow broke up along polygonal fissures and disintegrated into irregularly shaped columns in some places. As such viscous lava flows have a low flow velocity they can sometimes form very rough lava crusts or ridges. In S. Francesco, above the church, such bizarre lava ridges are well-developed. The eruption center of the Zaro lava flow is located to the SE of it.
In the southwestern part of Forio is the peak of Punta Imperatore. This is an old volcanic eruptive center which, in addition to pumice-rich pyroclastic deposits, produced powerful lava flows about 117,000 years ago (now mostly overlain by the volcanic deposits of the Scarrupo di Panza crater). The Campotese crater has an extension of about 900 m and is located east of the Punta Imperatore. About 12,000 years ago, this volcano had another explosive eruption, which formed pumice fall deposits or flowed down the flanks in hot cinder avalanches.
In the village of Panza there is a dry maar of 300m diameter. A maar occurs when rising magma strikes an aquifer, causing phreatomagmatic eruptions. A tuff ring forms around the vent. Usually the depression in the center fills with groundwater, but here it is filled with sediment.
The Rotaro complex is a beautiful example of nested cones breached by lava effusion, located along an eruptive fracture. It has a complicated history of several eruption phases and craters, dated between ~100 BC and 300 AD. There are four eruption centers: The Rotaro I (Fondo d’Oglio) is a typical cinder cone from about 700 BC., which produced pumice-rich ash deposits and alkali-trachytic lava flows.
Rotaro II began with powerful phreatic-phreatomagmatic explosions from a new vent to the NNW. Another batch of magma erupted still further northwest and so Rotaro III was born. In its most recent eruptive gasp, the Rotaro volcanic complex extruded a small lava flow – Rotaro IV, about 350 m long. This flow is distinguished from the Rotaro III flow for its distinctly more alkaline composition and resulting brighter color. In addition to pyroclastic material, the volcano also produced scoria and agglutinates (formed from partly fused hot volcanic bombs). One of its lava flows went north to the sea and formed what is today called the Punta la Scrofa.
In the northwestern coastal area we have Monte Vico, an alkalitrachytic lava dome that originated 75,000 years ago. On its SE flank crop out the unwelded ash and pumice deposits of the Green Epomeo tuff. The alkalitrachytic rocks of the Monte Vico lava dome are overlayed by pumice-tuff deposits of the Citara formation (pyroclastics and tuffs, 43,000 years) and alkalitrachytic pumice rich pyroclastics of the Zaro eruption (6,000 years ago).
Further interesting geological sights of the island are for example the features created by “Tafoni” weathering. These rock structures are formed by the action of sun, wind, salt and water on the Epomeo rock. By weathering from inside to outside (“core weathering”) spherical to kidney-shaped cavities are created, with a diameter of a few centimeters to a meter or more.
Saline water penetrates into the rocks, through the capillary forces from bottom to top. In intense sunlight, salt crystallizes in the capillaries, and the crystals break up the rock. Sometimes weathering rinds are preserved and form overhanging upper layers. The “Tafoni weatherings” have created strange structures, such as the “Mushroom” in Lacco Ameno and the “Bird’s head” in Forio. Honeycomb or sieve-like structures – small-scale Tafoni weathering – can be found scattered all over Ischia.
Also, by chemical and mechanical weathering, large blocks have come off at slopes of Mt. Epomeo that were exposed to erosion. The boulders accumulate in a “sea-of-rocks” landscape at the foot of the mountain. Some blocks are so large that the locals have created habitable rock houses by hollowing them out. These curious houses are very worth seeing.
South of the village Noia another special form of erosion can be seen. These “Pizzi Bianchi” are bright, white tuff stones, eroded a conical shape. Such earthy pyramids are generally found in steep slopes of soft rocks, such as tuff or sandstones. Often a block or layer of harder material on top protects the underlying material from total erosion.
As this is a densely populated island with additional millions of visitors throughout the year there truly is a need to keep an eye on possible volcanically induced hazards. This island’s rocky structure is very fragile due to its morphology and its volcanic-tectonic activity. There have been destructive earthquakes (the last occurred in 2017 causing two casualties and severe damage), as well as formation of mud ﬂows and landslides, sector collapses and related debris avalanches linked to volcano-tectonic uplift and/or subsidence phenomena and general volcanic deformation processes. So… it is nice to holiday on a volcanic island, and it is still more relaxing when you know its activity is monitored by experts.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP, Ischia
– INGV, Ischia
– June Monthly Bulletin Ischia, INGV
– Structural, seismic and hydrothermal features at the border of an active intermittent resurgent block: Ischia island (Italy) (2003)
– Volcanology of Ischia (2018)
– Bathymetry of Ischia Island 
– …pumice formation and dispersal: the Cretaio Tephra of Ischia (1992)
– Buried volcanic structures in the Gulf of Naples… (2005, PDF)
– A new type of volcano flank failure: The resurgent caldera sector collapse, Ischia (2004)
– Ischia volcanic complex, Italy (1996)
– Bartolomeo Garofalo’s Blog
– Eurogeopark – Ischia