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studies, drilling and gravity anomaly data, they recognized four volcanic activities
in the Vulsinian volcanic district. They are as follows (oldest to youngest):
(1) Paleo-Bolsena,
(2) Bolsena,
(3) Monte
ascone and
(4) Latera.
The third and fourth one were evolved contemporaneously. The volcanic
activities range between 580,000 and 127,000 years B.P. (Fornaseri 1985; Santi
1990). According to Nappi et al., the volcanic complex of Vulsini district is
characterized by one or more eruptive cycles, which may be summarized as
follows:
(a)
Initial phase involving eruption of mainly effusive leucite-bearing lava
ows
associated with Strombolian type activity,
(b) Explosive activity of Plinian type near the intersection of major regional faults,
and
(c) Final phase, depicted by volcanic collapses associated with hydro-magmatic
and/or magmatic activities.
Nappi et al. thought that not all calderas were generated by rare catastrophic and
voluminous ash flow eruption, which is associated with caldera depression and
involves following mechanism (Williams and McBirney 1979):
Subsidence of coherent crustal blocks and rift fractures, and
Collapse due to Krakatoa type of eruption resulting in the formation of caldera.
Bolsena caldera (Fig.
4.22
) is older of the two calderas located south of the
Siena-Radicofani graben. Nappi et al. (1991) suggested that down-sagging might
have been the reason for the downwarped topography of the basin resulting in the
formation of a downward sagging of the caldera. The topology of Bolsena caldera
most likely have been controlled by the geometry of the Mesozoic-Cenozoic car-
bonate basement, which re
ects a structure hinged to the southwest with large scale
lowering to the northeast as shown by Bouger gravity anomaly in the eastern and
northern part of the caldera. They think that the caldera may not be solely due to the
collapse of magma chamber occurring during ignimbrite eruptions of Paleo Bolsena
and Monte
ascone volcanic complexes. This resulted due to the eruption of basal
ignimbrites and Orvieto ignimbrites. These were largest explosive events during the
activity of the three complexes. According to them the Bolsena caldera was pro-
duced by several phases of tectonic subsidence since the beginning of volcanic
activity in the Vulsini district till Recent, resulting in the drowning of neolithic
villages in the lake.
Nappi et al. consider that the Monte
ascone caldera occurring within the main
Bolsena caldera represents the youngest volcanic collapse due to an explosive
activity of short duration and its topography is nicely preserved.
Ignimbrite eruption from northeast to southwest of caldera resulted in an
incremental growth of Latera caldera, which is associated with piecemeal and
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