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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
two-month 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.
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