VOLCANOLOGICAL HISTORY OF ISCHIA ISLAND
(Partially based on "Valutazione del Rischio vulcanico nell'isola d'Ischia"; Laurea Thesis of Massimo Mattera, 1995, Università degli Studi di Napoli, Italy)
1 - Introduction
Ischia island is located 33 km SW of Napoli (Naples), Italy. It covers 46.3 km2 and is the largest island in the gulf of Napoli.
The highest peak of the island is Monte Epomeo surrounded by several volcanic vents. Mount Epomeo, covers a surface of 16 km2 or about the 34.5% of the entire surface of the island. It is almost entirely made up of volcanic rocks locally called Tufo Verde di Monte Epomeo (Green Tuff of Mount Epomeo). The top part is covered by sedimentary and volcanic deposits of younger age (Tufite of Monte Epomeo and Colle Jetto formation). The central- eastern part of the island is occupied by a triangular-shaped depression called the Ischia Graben comprised among the villages of Casamicciola, Barano and Ischia Ponte. The southern border of the graben is limited by a fault system extending from Carta Romana to Maronti and to the west by Mount Epomeo. The depression is divided into two halves by a fault along the valley of Rio Corbore.
The volcanic activity of the island of Ischia is related to the same causes that produced the volcanic activity along the western margin of peninsular Italy. About 10 millions year ago the areas now represented by the italian peninsula and the islands of Corsica, Sardegna (Sardinia) and Sicilia (Sicily), were bound toghether. A slow anticlockwise rotation of the italian peninsula separated it from Corsica and Sardegna and caused the opening of the Thyrrenian sea. The consequent stretching and thinning of the crust caused deep faults which favoured the formation and eruption of magmas.
The first modern geological studies of the Island of Ischia were made by the Swiss volcanologist Alfred Rittmann, who compiled one of its first geological map .
2 - Cronology of volcanic activity
The hills bordering Mt Epomeo are old volcanic vents formed during the last 150000 years. Absolute datings by means of radioactive methods and volcanological studies permitted to summaryze the volcanic activity into 5 different cycles. The first two occurred before the eruption of the Green Tuff which forms most of Mt Epomeo. The last three phases of activity occurred after the eruption of Mt Epomeo.
The activity before the Green Tuff (Phases 1 and 2)
The products of these phases outcrop mostly in the southern part of the Island (between Punta della Signora and Carta Romana, Monte di Vezzi, peninsula of Sant'Angelo, Capo Negro, Punta Chiarito and Punta Imperatore), in the north western sector (Monte Vico), and eastern part (Castello d'Ischia). Also the lava flow outcropping at Rione Bocca on the western side of Epomeo is thought to belong to these phases. The products of the largest eruption of this period are found at the base of the sequence of Scarrupata di Barano. Most of these products have been eroded by the action of the sea and of subsequent explosive eruptions. The result of this activity was the building of a volcanic edifice probably larger than the present one. A caldera (a depressed circular structure) was formed after this phase with the collapse of the central sector of the island. Annular fractures border the caldera and represent the preferred pathways for the upcoming magmas.
Phase 2 is characterized by the formation of several scoria cones and domes (Punta Imperatore, Capo Negro, Punta Chiarito, Sant'Angelo, Punta della Signora, Capo Grosso, La Guardiola, Monte di Vezzi and Castello d'Ischia). The dome of Monte Vezzi emitted two viscous lava flows. Other lava flows form the peninsula of San Pancrazio, Punta della Cannuccia and Parata. Also Monte Vico belongs to this phase. After this activity there was a long period of quiscence until 55000 years.
The volcanic activity starts again at 55000 years with numerous explosive eruptions whose produts are called Pignatiello Formation, Green Tuff of Monte Epomeo and Citara formation.The eruptive vents are no more identified. The possible vent of the Green Tuff eruption is possibly located in the southern part of the island, between Sant'Angelo and Maronti. Part of the products of this eruption filled the pre-existing caldera and covered the eastern part of the island. The green colour, typical of marine alteration of volcanic products, suggest that the the caldera was already submerged and lately subsided more as a result of the eruption. The products of this eruption outside the caldera (Monte Vico, Punta Imperatore and Scarrupata di Barano) were deposited in a typical sub-aerial environment. The Green Tuff of Monte Epomeo remained below sea level until 28000 years ago. At this time most of the Epomeo was raised by the intrusion of magma at shallow depth. The eruption of the sequence called Citara Tuffs occurs between 44000 and 33000 years. These products were erupted during at least three separate eruptions with a typical phreato-magmatic activity (explosive interaction between magma and sea water). The Citara Tuffs overlie the Green Tuff at Monte Vico, and between Capo Negro and Punta Chiarito and at Punta Imperatore.
The first products of this phase are the lava domes of Grotta del Mavone. The subsequent activity is characterized by thick lava flows and scorias emitted by the vents of Pilaro. Then there were eruptions of scoria and pumices found at Scarrupo di Panza. After this activity there is a phase of quiescence
Numerous eruptions occured around 10.000 years ago and the volcanic activity continued until the eruption of Arso in 1302 AD. Most of the eruptive centers are located along the eastern border of Epomeo in the area called the Ischia Graben. It is possible that these eruptions were fed by the magma rising along the fractures that borders the Epomeo. During this phase were formed Monte Rotaro, Montagnone, Monte Moschiata, Vateliero, Molara and Cava Nocelle.
3 - Mount Epomeo
Mount Epomeo is interpreted as an entire raised block (volcanic horst). The uplift is dated between 33000 and 28000 years ago, and was possibly due to the intrusion of magma at shallow depth. The pressure increase caused the formation of fractures representing the sliding planes along which the block was raised. The uplifting block was tilted to the south as a result of a differential movement, and, as a consequence, the northern side was raised more than the southern side.On the northern side there are several fractures feeding the thermal springs of Casamicciola and La Rita and a lineament of fumaroles along a fracture cutting Monte Cito. The southern part of Epomeo was raised less and was more affected by a sequence of landslide and mudflows occurring along the inclined surface of the block.
4 - Fumaroles and thermal springs
The volcanic nature of the island is well evidenced by the presence of fumaroles and thermal springs, that have been since long exploited as a terapeutical and touristic attraction. The fractures bordering Mount Epomeo are the preferred pathway for the uprisings volcanic gases that provide the heat to raise the temperature of the water table. Fumaroles with temperature near 100 °C are found on the flank of Monte Nuovo and Monte Cito and along the Maronti coast. In other areas (San Michele, Monte Rotaro, Fundera e Scarrupo di Panza), the temperature of the fumaroles does not exceed 46 °C.
The hot springs have temperatures between 20° and 80°C. They are located at Forio in località Monterone (Castaldi and Castiglioni springs), at Casamicciola ( La Rita and Bagni springs), at Barano in località Maronti (springs of Cava Scura, Olmitello and Cava Fredda), and the springs of Cartaromana, Punta Chiarito, Bagnitiello, San Montano and Porto d'Ischia. The areas where the thermal springs are localized ,can be considered as possible sites for future euptions. A fumarolic field called Solonaria, exploited for the extraction of sulphur, was located in the area where opened the vent of the Arso eruption of 1302. Deep wells drilled in all the island provide the evidence of temperature at depth as high as 200 °C and the occurrence of different water tables heated by geothermal fluids probably coming from a depth of about 3000 m.
5 - Eruptions and earthquakes of pre-historical and historical times
The natural history of Ischia island has been characterized, since long time, by natural catastrophes caused by eruptions and earthquakes. A pre-historical site of iron-age is buried under the ashes of an eruption occurred in the area between Ischia Porto and Casamicciola. The first greek colony of southern Italy (called Pithecusa) was settled in Ischia in the VIII century BC. The greek colonists arrived around 770 BC from Eubea and settled on the Promontory of Mount Vico at the north-western side of the island. Earthquakes and eruptions compelled the colonists to leave the island and settle on the coast of Campania where they founded the colony of Cuma which caused the economic decline of the island. Around 600 BC, the eruption of Monte Rotaro caused the final decline of the colony in Ischia. In 476 BC the island is occupied by the Syracusans, which were driven off the island by a new eruption occurred between 474 and 470 BC. This eruption possibly occured where now is sited the Porto d'Ischia. After some time the island was occupied again by colonists coming from Naples (possibly the same that left the island before).
Volcanic activity occurred also during roman time. There are evidences of an eruption occurred in 91 BC and another one during the emperorship of Augustus who gave back the island to the neapolitans and got in change Capri! There are other evidences of eruptions under the emperors Titus, Antoninus and Diocletian.
The last eruption occurred in 1302 when a vent opened in the area of Flaiano and emitted a lava flow which lasted for two months. The lava flow is between 500 and 100 m wide and is presently called Arso (once it was called "Le Cremate"). It reached the beach between Porto d'Ischia and Ischia Ponte and destroyed the old village of Geronda. The eruption caused much panic and compelled the escape of many people to the nearby islands and continental coast. Since then, the only volcanic activity are the fumarolic emissions all over the island.
Numerous detructive earthquakes occurred on the Island. In the last period we recall those of 1881 and 1883. The quake of 1881 was felt for 7 seconds and had an highr intensity in the territory of Casamicciola. There were 129 casualties (124 in Casamicciola and 5 in Lacco Ameno). The wounded were 100 in Casamicciola. 290 buildings were destroyed in Casamicciola and 159 were damaged. In Lacco Ameno there was the destruction of 194 buildings (among these the church of S.Giuseppe al Fango). Also most of the buildings of Forio were damaged.
The earthquake of 28 July 1883 was felt for 16 seconds and was localyzed in Casamicciola where there were 1784 casualties over 4300 inhabitants. 448 people were wounded. Of the 672 buildings of the village, 572 collapsed and 134 were damaged. In Lacco Ameno there were 146 dead and 93 wounded over 1800 inhabitants. 269 buildings collapsed and 102 were damaged out of 389. The church of Santa maria del Rosario was destroyed. In Forio there were 345 casualties and 190 wounded , out of 6800 inhabitants.1344 buildings were destroyed and 977 damaged out of a total 2713. The church of S. Sebastiano also was destroyed. At Serrara-Fontana there were 28 dead and 21 wounded . At Barano there were 10 dead and 10 wounded. At Ischia the quake was barely felt and a few buildings were damaged. The quake caused a total of 2313 casualties.
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Modified 28-October 97