|
|
|
Outline Ischia
The island of Ischia is located on the north western border of the Gulf of Naples with an area of approximately 42 km2. The island is characterised by two prominent mountains, Mt. Epomeo in the central part and Mt. Torone in the south east; a lowland area is situated towards the north east extending from the foot of the mountainous areas to the coast. The island’s coastline is characterised by steep cliffs and promontories in the south and gentle slopes on the other three sides and the island itself by volcanic rocks, from the LK- series, landslide and debris flows with subordinate terrigenous sediments. The oldest rocks have been given dates of approximately 150ka, though they are not the from the lowest stratigraphic unit of the island. The widespread fumaroles, thermal springs and intense seismic activity indicate that the area is still active.
Volcano-Tectonic Outline
The island’s complex structural pattern can be related to the local and regional tectonics. Ischia is situated on a north west – south east trending fault system which extends from the mainland across the island of Procida to Ischia. The edifices of Vateliero, Molara, Cava Nocelle and a depression which lies between St.Angelo and Citara all lie along this fault system. The morphological caldera- like depression which lies in the central part of the island has been noted to be one of the most interesting tectonic-volcanic features. This structure has been related to the resurgence of the Mt.Epomeo block involving the geo-volcanic mechanism of "simple-shear block resurgence".(Orsi et al. 1991).
Volcanic Outline
The timing of the initial volcanism on the island is not known; the oldest dated exposures that belong to the island complex are related to small trachytic and phonolitic domes in the south eastern part of the island with ages of 150 and 74 ka. Since 55 ka, on the basis of stratiraphical, complsitional etc., techniques three periods of activity have been identified (Civetta et. al., 1991) each of which were characterised by the arrival of new, less differentiated magma.
Period I (from 55 to 33 ka): This period is marked by the eruptions of the, now uplifted, Monte trachytic green Tuff (Epomeo Green Tuff) which partially filled the central depression. Rittmann (1930) suggested that these were amongst the most powerful eruptions in the Mediterranean area. Sr and Zr data suggest that the magma chamber was zoned through a process of fractional crystallisation.
Period II (from 28 to 18 ka): This period is marked by the re-eruption of the Grotta di Terra trachybasaltic magma along the south-eastern coast. The significant variation in both chemical and isotopic composition of the erupted magmas leads to a model which implies that there was an arrival of new basic magma into the system, followed by a progressive differentiation and mixing with the resident trachytic magma.
Period III (from 10 ka to 1302 A.D.): This period is marked by effusive and hydromagmatic eruptions within the depression east of Mt.Epomeo. Most of the magma which erupted during this period was trachytic and subordinately latitic in nature with a negative correlation between chemical and isotopic compositions.
Each period experienced both explosive and hydromagmatic type events.
Sites to see
To the west of the port it is possible to see the hill in the Castiglione area which is a trachytic dome of undefined age.
To the south of the port one can see two hills corresponding to the volcanic complexes, younger than 2 ka, of Mt.Maschiata-Montagnone and Fondo d’Olgio-Punta la Scrofa. Within the Fondo d’Olgio exposure it is possible to observe (from the base working to the top) a strombolian deposit, a succession of pyroclastic beds with a contrasting grain sizes and a yellowish pyroclasic fallout deposit all of which overlie the Bosco dei Conti unit.
The Cretaio Tephra unit has a 14C age of 1860 bp; it commonly covers a palaeosol containing Roman pottery fragments. This unit consists of pyroclastic fallout deposits from a historical subplinian eruption; the deposits are principally composed of juvenile pumice, ash and lithic fragments. The vesicularity of the pumice clasts do not show any systematic changes throughout the eruption sequence indicating that there was constant flux, vesicle growth and volatiles during the eruption. This unit is overlain by the Bosco dei Conti unit.
The northernmost promontory of Monte Vico demonstrates the geometrical and morphological relationship between the Mt. Vico phonolitic dome (75 ka) and the Zaro lava flows (6 ka).
The beach cliffs of Citara offer one of the best exposures of the Pietre Rosse Tuff, a sequence of magmatic and hydromagmatic pyroclastic deposits. This exposure shows some well preserved bedding structures from both air fall and surge events.