INTRODUCTION

Ischia is an island lying off the Gulf of Napoli, some 30 km WSW from Napoli, 35 km west from Vesuvio and about 8 km from the mainland coast at Capo Miseno. Its highest peak, Epomeo, is not a volcano (as which it is frequently described), but is made up of an uplifted and tilted block (horst) of green ash-flow tuff (possibly erupted from Campi Flegrei). Numerous youthful and historic eruptive centers are scattered on its flanks, the most recent of which is Arso, formed in 1302 (or 1301). Continuing volcano-tectonic activity was destructively demonstrated on 28 July 1883, when the famous thermal resort of Casamicciola was levelled by the most violent in a series of local earthquakes, killing more than 2213 people.
Numerous eruptions from various sites have occurred on Ischia during the past 2500 years, indicating that the island is volcanically active and may well erupt in the future. Ground deformation (uplift, subsidence and tilting) are continuing to the present day. It is very likely that a shallow magma body is present below the island, causing these deformations. The reconstructed and/or documented style of eruption implies great hazards in the case of renewed activity.

General morphology and geology

Geological sketch map of the island of Ischia, from Orsi et al. 1996. I = scree and mud flows; II = volcanics younger than 10 ka (a = lava flows, b = craters, c = domes); III = volcanics dated between 28 and 18 ka; IV = volcanics dated between 55 and 33 ka(a = undifferentiated pyroclastic deposits, b = Monte Epomeo Green Tuff); V = volcanics older than 55 ka; VI = Faults. The numbered dots indicate locations of stratigraphic sections presented by Orsi et al. 1996.

Ischia, with a surface of about 46 km2, is the largest of the beautiful islands dotting the margins of the Gulf of Napoli, and it forms one of the most complex volcanoes of that region. Morphologically, it consists of the central-western highland (horst) of Epomeo which is surrounded by lower terrain punctuated by at least 40 volcanic centers. Most of these occur in the eastern half of the island, but a few are present in its SW and NW corners. Since the early 1980's, an old caldera of unknown age has been suspected below Ischia by Chiesa et al. (1985), whose relic moat is constituted by Castello di Ischia, Punta Imperatore, and possibly Monte di Vico, forming a half-ring around the southern circumference of the island. Rocks of the pre-existing volcanic complex are exposed mostly in the southern and SE parts of Ischia. The earliest established event in the geologic history of Ischia is the cataclysmic eruption of the Green Tuff, dated at 0.74 +/-0.09 Ma by Capaldi et al. (1976-77) and much less, that is, 53 ka by Gillot et al. (1982) and Chiesa et al. (1985). This large-volume ash-flow tuff was erupted from a site yet to be identified and covers an area conservatively estimated at 300 km^2, including all of Ischia and the Campi Flegrei. Its maximum thickness (at Ischia? no precise info) is ca. 1000 m, and still exceeds 600 m in the Campi Flegrei; it was probably erupted in a series of large-scale explosive eruptions (Capaldi et al. 1976-77). Compositionally, the potassic alkali trachyte Green Tuff magma is typical for volcanic rocks in the Campanian area. The much younger eruption date given by the more recent sources is of great significance for the understanding of Ischia's volcanism and reflects much faster processes than previously thought.
The Green Tuff was either deposited subaqueously or submerged below sea-level soon after, as evidenced by the presence of beach levels and glauconite in the strata. About 33 ka ago, probably even later (at 20 ka; Chiesa et al. 1985), vigorous uplift occurred below present-day Epomeo, forming the tilted horst of that mountain. Rittmann (1930) and Rittmann & Gottini (1981) speculated that this impressive uplift was due to the intrusion of a shallow magma body (laccolith), the same magma that then fed the younger eruptive centers. The total amount of uplift is at least 790 m (height of Epomeo summit) which implies, if the onset date of 20 ka of the uplift is accepted, an annual uplift rate of at least 3.9 cm (Newhall & Dzurisin, Calderas, p. 118). The presence of a shallow magma reservoir beneath Epomeo is confirmed by a positive Boguer gravity anomaly (Capaldi et al. 1976-77).

Known activity

Ischia, as a volcano, already became notorious to the Greek, when they settled the area during the millenium before Christ. Eruptions were C14-dated at -1000, and -850. Further eruptions were recorded in -500, -470, -350+/-150, -150 +/-50, -91, in AD 0 (?), 69, 180, and 300 (?).
Several of the most recent eruptions of Ischia have been reviewed by Rittmann & Gottini (1981).

Porto d'Ischia eruption, about -150. The pictoresque harbor of Ischia Porto lies within a crater probably formed during an explosive eruption between 200 and 100 BC (according to Rittmann & Gottini, 1981, this eruption occurred as early as the 6th century BC). The eruption occurred in the NE prolungation of an eruptive fissure of a prehistoric eruption that produced a tuff ring (of which the Fondo Ferraro and Costa del Lenzuolo are remains) and a thick trachytic lava flow. Initially, the Porto d'Ischia eruption was phreatic-phreatomagmatic, piercing through the prehistoric trachyte flow. Following this initial stage, new trachytic lava fragments and scoria were ejected. No lava emission occurred during this eruption. Soon after the end of the activity, the maar-type explosion crater was filled with water. Rittmann & Gottini (1981) believe that the Porto d'Ischia eruption is identical with the one described by (the younger?) Plinius. In that event, "flames" were ejected and much damage was done to a citadel. The eruption is said to have "transformed a plain into a lake" - a scenario similar to that reconstructed for the Porto d'Ischia eruption.

Rotaro eruptions, -91 until ca. 300. The Monte Rotaro tephra cone and lava dome/flow complex formed by a series of eruptions, possibly coinciding with the historically documented eruptions of 91 BC and AD 69. The exact timing of the eruptive events is uncertain: Rittmann & Gottini (1981) describe Rotaro I as "prehistoric" while other sources attribute it to an eruption recorded for 91 BC (see, e.g., Krafft 1974). The Rotaro complex is a beautiful example of nested cones breached by lava effusion, located along an eruptive fracture that trends N20°ree;W. Also the dates of the later phases of the Rotaro eruptive sequence are uncertain. Rittmann & Gottini (1981) assume some date in the 3rd post-Christ century while Krafft (1974) gives AD 69 as the date (misprinted as 69 BC in the German translation of Krafft, 1974). Rittmann & Gottini (1981) point to the fact that Rotaro I is covered by tephra from the Montagnone-Maschiatta eruption (thought to have occurred around AD 140) while Rotaro II is not, neither are Rotaro III and IV. The entire sequence of Rotaro eruptions is here taken together even though it may to some extent overlap with the Montagnone-Maschiatta eruption.

In any case, Rotaro I was formed by an initial explosive phase with early lithic and later magmatic (pumice) tephra ejection, and successive large-volume lava extrusion. A major dome rose to about 340 m asl, with a short lava flow extending from its NW base. After the end of extrusion, a large (350 m diameter) pit crater formed on the dome summit, it still has a depth of about 120 m today.
Monte Rotaro II began with powerful phreatic-phreatomagmatic explosions from a new vent on the NNW flank of the Rotaro I dome (about 300 m from the summit), represented by a basal breccia in the deposit from the eruption. Immediately after, pumice and ash were ejected before a new lava dome with a scoriaceous surface largely filled the new depression. At a later date ("several decades later", according to Krafft, 1974), a new explosive vent exploded through the N base of the Rotaro II dome, some 300 m NNW of the Rotaro II vent. Thus the crater of Rotaro III was formed, being much smaller than its predecessor. Most of the magma, during this eruptive episode, was erupted quietly, forming a small dome that extended northwards outside the vent and extended into the sea in a trachytic flow some 600 m long. In its most recent eruptive gasp, the Rotaro volcanic complex extruded a small lava flow (Rotaro IV; not Monte Tabor - as written in Krafft, 1974 - which is a much older volcanic formation), about 350 m long. This flow is distinguished from the Rotaro III flow for its distinctly more alkaline composition and resulting brighter color.

Montagnone-Maschiatta eruption, about AD 140. One of the most recent eruptions of Ischia occurred only about 1800 years ago, on the same eruptive fracture that had been the site of the prehistoric Fondo Ferraro and the ca. 150 BC Porto d'Ischia eruptions, right between those earlier eruption centers. Following the typical sequence of Ischian volcanic eruptions, the vent-clearing phase ejected abundant clasts of pre-existing rock, immediately followed by abundant pumice and ash that left a thick deposit in the NE part of the island.
After the explosive stage of the eruption, degassed viscous magma rose within the crater and formed a voluminous dome that overflowed the crater rim in the SW and NE, thus assuming an elliptical shape. Later, more fluid lava broke through the dome's carapace at its SE base and flowed in the direction of the later Arso crater. Due to this partial "emptying" of the dome's interior, the top of the protrusion collapsed, forming a pit crater.
Pottery found below the basal pumice of the Montagnone-Maschiatta eruption indicates an age of no earlier than the 2nd post-Christ century. Rittmann & Gottini (1981) cite as further evidence for a 2nd century eruption date the submersion of the ancient town of Aenaria, situated between the NE tip of Ischia and the Castello di Ischia. Pottery found among the ruins of that town, now 6.5 m below sea-level, indicates a date of about 130-140 AD for the event that led to its catastrophic submersion. This, according to Rittmann & Gottini (1981), occurred in response to the eruption at Montagnone-Maschiatta.

1302 Arso eruption. The most recent eruption on Ischia occurred during January or Feb 1302 (Krafft, 1984, gives 18 January 1301 as the beginning of this eruption), after about thousand years of volcanic inactivity on Ischia. It opened a new crater at a site named Cremate and emitted a lava flow (named Arso) to the NE coast. Most info for the following summary derives from Rittmann & Gottini (1981).
Prior to the eruption, the site was known for its solfataric activity ("solfonaria"). Nothing is known about any precursory phenomena (but Newhall & Dzurisin, in Calderas, p. 119, cite reports of an earthquake preceding the eruption). The beginning of the eruption was sudden and violently explosive, phreatic or phreatomagmatic, as the hydrothermal system flashed into steam by some unknown depressurization process. The basal deposits of the 1302 eruption has abundant fragments of fumarolically altered rock, torn from the surface of the "solfonaria" geothermal site, and mixed with fragments of Rotaro and Montagnone-Maschiatta pumice. Old written sources describe the fall of "ash mixed with sulfur". Inhabitants of the area must have been completely surprised by the disaster, leaving money and artefacts when trying to escape from settlements near the new vent. The eruption area was populated and Ischia Porto was only 1.5 km distant. The only escape way was across the sea. It was probably during this phase that "many" persons and animals were killed and great damage was done to inhabited and agricultural areas. Some deaths were apparently due to asphyxiation due to strong SO2 emission.
The phreatic or phreatomagmatic phase of the eruption was followed by the emission of large volumes of fresh pumice and ash, darkening the sky and causing ash falls to 300 km distant (location of distal fallout is not indicated by Rittmann & Gottini, 1981). The eastern part of Ischia including the town of Ischia Porto was buried under a deep cover of pyroclastics (no thickness data in Rittmann & Gottini, 1981). Then, a brief period of relative quiet ensued before more degassed and more mafic magma reached the surface. Very porous and large scoria were then ejected, building a tephra ring around the 500 m-wide crater.
During the final, least violent, but probably longest, phase of the eruption, viscous lava rose within the crater, first forming a dome that then overflowed the crater rim at its lowest point, on its SE side. Just east of the crater, the flow turned NE, pushing before it a part of the crater rim. The higher part of the descent was across steep terrain but nearing the shore, flatter terrain was encountered, causing the lateral extension of the flow. When the lava finally arrived at the coast, it entered the sea along a front about 1 km wide, displacing the coastline 200 m into the sea and creating a new headland (Capo Molino). This phase of the eruption lasted about 2 months. The final length of the flow is 2.7 km, and its average thickness 9 m, total volume is about 1.3 x 10^7 m^3.

Volcanic hazards. No volcanic eruptions have occurred on Ischia since about 700 years, but seismic activity (very intense in the 18th and 19th centuries) as well as continuing ground deformation point to magma movements at shallow depth. While subsidence occurs in the coastal areas of the island (which may correspond to the overall subsidence observed in many other places around the Gulf of Napoli, as noted by Newhall & Dzurisin, Calderas, p. 121), the central part with Epomeo continues to be uplifted. It would require more continuous and precise repeated measurements - especially in the case of earthquakes - to judge if the Epomeo uplift may correspond to the uprise of magma within the suggested shallow reservoir. Eruptions have occurred repeatedly during the past about 53 ka (after the eruption of the Green Tuff) and were particularly frequent during the period from about 500 BC until AD 300. During this period, the NE part of the island has been affected by a total of 5 significant eruptive events (Porto d'Ischia, Rotaro I, Montagnone-Maschiatta, Rotaro II, Rotaro III). There is no ground on which to place speculations whether these eruptions represent one major cycle, but it is evident that that period was much more volcanically active than the following 1700 years (with only one eruption).
Regarding future eruptions, the following points are of particular relevance: - the eruption will likely occur from a new vent somewhere in the lower regions of the island, most probably in its NE sector,
- like all recent major eruptions, the onset of activity is expected to be violently explosive and will result in the fall of very large, dense lithics followed by large volumes of pumice,
- initial activity is likely to be influenced by the presence of ground or sea water. Surge clouds are a common feature of such eruptions and would extend to distances of several kilometers, a range within total destruction is to be expected.
A hazard which is even greater than that of renewed volcanic activity is that of earthquakes (because they usually have no predecessors). The history of Ischia knows of at least 5 destructive earthquakes that have also caused fatalities (1228: 700 deaths, probably in a landslide caused by the earthquake; 1796: 7 deaths; 1828: 28 deaths; 1881: 129 deaths; 1883: 2213 deaths). The shallow focus of the latest of these quakes and the very limited areas affected (the devastating 1883 temblor was not even felt on the nearby mainland) point to a connection between these events and movements of the Epomeo horst, maybe in response to magma movements in the postulated shallow reservoir. More geophysical studies are required in order to reveal the internal structure and processes of Ischia volcano.

References

Capaldi, G.; Civetta, L. & Gasparini, P. (1976-77) Volcanic history of the island of Ischia (South Italy). BV 40: 11-22

Fusi N, Tibaldi A & Vezzoli L (1990) Vulcanismo, risorgenza calderica e relazioni con la tettonica regionale nell'isola d'Ischia. Memorie della Società Geologica Italiana 45: 971-980 (published in 1992)

Gillot PY, Chiesa S, Pasquarè G & Vezzoli L (1982) 33,000 yr K/Ar dating of the volcanic-tectonic horst of the isle of Ischia, gulf of Naples. Nature 229: 242-245

Krafft, M. (1974) Guide des volcans d'Europe. Delachaux & Niestle Neuchatel: 1-412

Orsi G, Gallo G & Zanchi A (1991) Simple-shearing block resurgence in caldera depressions. A model from Pantelleria and Ischia. Journal of Volcanology and Geothermal Research 47: 1-11

Orsi G, Gallo G, Heiken G, Wohletz G, Yu E & Bonani G (1992) A comprehensive study of the Cretaio Tephra oof Ischia (Italy). Journal of Volcanology and Geothermal Research 53: 329-354

Orsi G, Piochi M, Campajola L, D'Onofrio A, Gialanella L & Terrasi F (1996) 14C geochronological constraints for the volcanic history of the island of Ischia (Italy) over the last 5000 years. Journal of Volcanology and Geothermal Research 71: 249-257

Rittmann, A. & Gottini, V. (1981) L'isola d'Ischia - Geologia. Bollettino del Servizio Geologico d'Italia 101: 131-274

Vezzoli L (ed) (1988) The island of Ischia. CNR Quaderni de "La Ricerca Scientifica" 114 (10): 1-122