The eruptive and tectonic history of the Island of Ischia is very complex and the surveyed age of its volcanic materials dates back to 150,000 years ago.
Volcanic activity on the island of Ischia includes two distinct cycles: the first developed between 150,000 and 75,000 years ago and the second cycle started with the great eruption that generated the green tuff of Mount Epomeo and is characterised by consistent vertical movements that concern the central part of the island. Added to this volcanic activity are the volcanic-tectonic phenomena and those connected with the action of exogenous agents, the effects of which translate into the erosion, transportation and sedimentation of melted pyroclastic materials.
The most commonly accepted hydrogeological model provides the existence of two basic superimposed tables (3): the lower one is fed by the ingression of seawater, therefore having a high saline content, and is delimited above by a concave surface that faces upwards; the other is nourished by rain water, which contains less minerals and “floats” on the previous one due to its lower density and is delimited above by a convex surface, since it thickens as it departs from the coast and heads towards the internal part of the island.
The area of contact between the two is characterised by a diffusion belt where the waters assume an intermediate chemism. The thickness of this belt varies from area to area and can even vary in the same area during the year. These variations are due to the different intensity of rainfall. Maximum rainfall takes place in the twomonth period of November and December, while minimum rainfall occurs during the quarter that goes from June to August.
Water heating takes place through conduction (transfer of heat on contact between the waters and the hot acquifer rocks), but especially by convention, following the circulatory motions through the reticule of fissures (divergent faults and fractures) triggered by thermal imbalances.