based on "Proximal facies of a caldera forming eruption. The Parata Grande Tuff at Ventotene Island (Italy)." by Annamaria Perrotta, Claudio Scarpati, Lisetta Giacomelli and Anna Rita Capozzi on Journ. Volcanol. and Geoth. Res., vol. 71, pagg. 207-228. Article.pdf

1 - Geological setting of Ventotene and Santo Stefano Islands

Ventotene Island, and the nearby islet of S. Stefano, are located on the western margin of the continental shelf of the coast between Rome and Naples. They belong to the Pontine Archipelago constituted also by the islands of Ponza, Palmarola and Zannone. Ventotene island elongates for less than 3 km along a NE-SW direction and its maximum width is less than 800 m. The islet of S. Stefano is at a distance of 1,5 km east of Ventotene. It has a circular shape with a diameter of about 500 m. Bathymetric data indicates that Ventotene is the south-eastern flank of a composite volcano, with a basal area of about 300 km2, rising more than 700 m from the sea bottom.

All volcanic rocks outcropping on Ventotene and S. Stefano may be referred both for their ages and composition (< 1 Ma; Metrich et al, 1988) to the potassic association of the Roman Comagmatic Province. Ponza, Palmarola and Zannone are characterised by a different age and composition (the first erupted products were ryolithic lavas at 4.4 Ma, Barberi et al., 1967).

The deposits of the most recent explosive eruption occurred on Ventotene island (<0.15-0.3 Ma; Metrich et al., 1988) cover entirely the island and have been termed 'Parata Grande Tuff' (PGT), as the best outcrop of the series occurs at that locality. The reconstruction of this eruption is important to understand the volcanological and structural evolution of the island because this explosive event may be associated with a caldera collapse.

2 - Stratigraphy of S. Stefano and Ventotene islands

The stratovolcano of which Ventotene and S. Stefano represent part of the south-eastern flank, has experienced at least 27 eruptions over the last 0.8My. The products of ten of these eruptions are exclusively formed by fallout tephra of subplinian or plinian and subordinately strombolian activity and should be addressed in the tephrostratigrapy of the Tyrrhenian basin.

Figure 1

Ventotene island. The upper part of the cliff is formed by the last eruption products. Undulated beds with red paleosols separate this formation from the older deposits.

Nine explosive eruptions generated only pyroclastic flows or surges that probably had limited or different distribution, as the associated deposits have been recognised only on one island. Each of the remnant five explosive eruptions began with a pumice fall phase and culminated with the emplacement of pyroclastic flows. The effusive activity produced lava flows and a dome that alternated with explosive eruptions with the time. The oldest products crop out only at S. Stefano, meanwhile most of the youngest fall deposits are recognisable on both islands.

The last activity was a climatic explosive event that produced a >30m thick sequence of flow deposits overlying basal pumice and ash fall beds. The products of this eruption have been called 'Parata Grande Tuff'.

Figure 2

The Parata Grande Tuff.

Table 1 shows stratigraphy and description for the deposits outcropping at Ventotene and S. Stefano islands. The stratigraphic succession is devided in 27 members which are interpreted as being composed of a deposit or a succession of deposits demarcated by paleosoils. Each member is denoted by upper-case letter and is considered to represent the product of a single eruptive event. The members are further subdivided into units. Each unit is denoted by the upper-case letter of the member which it belong, followed by a number. The presence of subaereal erosional surfaces between units belonging to the same member suggests a pause in the deposition.

3 - Stratigraphy of the products of the Parata Grande Tuff.

The Parata Grande Tuff (PGT) consists mostly of pyroclastic flow deposits overlying basal pumice and ash fall beds. The basal fall deposits usually mantle underlying topographic irregularities while retaining a uniform thickness.

Figure 3

Parata Grande Tuff. Note the abundance of coarse lithic clasts in the upper part of the picture.

The lowest bed occurs as a massive, grain supported deposit up to 30 cm in thickness and is composed mainly of angular lithic clasts with minor pumice clasts both up to 1cm in size. The lithic clasts consist of dark lava fragments. Somewhere this bed is interstratified with thin ash layers which are poorly sorted, variable in thickness and bounded at the base by erosional surface.

Figure 4

Basal fall deposit of Parata Grande Tuff. The lowest bed (30 cm) is composed of lithic clasts; the uppermost bed is composed of pumice clasts (1 m)

These ash layers contain rare rounded pumices (<1cm in size). This bed is overlain by a well sorted, approximately 1 m thick, pumice fall deposit. It shows reverse grading of angular pumice and lithic clasts which maximum diameters vary respectively between 4 and 6 cm and between 2,7 and 4 cm. The lithic/juvenile ratio increases with stratigraphic height. A few larger angular pumice blocks, up to 20 cm in size, are present in the upper part of the deposit. Numerous ballistic blocks of about one meter size are found in the top part of this bed and form impact sags up to 6m deep.

Figure 5

Impact sag of a large ballistic clast that perforated all the underlying paleosols and deposits

These sags dip consistently eastwards. The basal fallout sequence end with a massive ash bed with a maximum thickness of 10 cm . The base of the pyroclastic flow succession is welded and thickens in topographical depressions (thicknesses ranging from 1.5 to 12m). The welding increases within channels and towards the centre of small valleys.

Figure 6

The welded base (dark lense) of the pyroclastic flow unit of the Parata Grande Formation occurs within depressions of the original topographic surface. See also fig. 1

Most of the bed that form this part are massive or with inverse graded large pumice clasts and occasionally are very rich of lithic swarms.

The flow deposits that overlie the welded succession are about 18m thick and show a drastic increase of lithics content. Lithic blocks as large as 1m frequently occur as well defined lithic concentration zones that are laterally discontinuous (see fig. 3). Single beds are generally more than one meter thick and are massive or show the grading of the coarser lithic and pumice clasts.

The uppermost part of the sequence has a thickness of about 13 m and is made by a succession of layers forming dunes and cross stratification, alternated with massive beds.

Figure 7

Uppermost part of the flow sequence of Parata Grande Tuff with dunes and cross stratifications alternated with massive beds

These deposits are well exposed at Punta Eolo, which possibly was a depression of the ancient volcano strongly controlling the deposition of the flows. The characteristics of the PGT, such as the large average size of the clasts, the abundance of lithic clasts and the presence of ballistic blocks suggest a very close source area to Ventotene Island. Bomb sags indicate a vent location near the western coast of the island.

4. Eruptive Dynamics

The first products of the PGT eruption overlay a paleosoil. They are lithic-rich fall layers and thin ash flow layers indicating an initial phase of the eruption characterised by small phreatic explosions, alternated with brief phreatomagmatic phases.

The paroxysmal phase, which develops with increasing violence, is characterized by the formation of a sustained plinian column and the emission of pumice clasts. The pumice layer overlaying the phreatomagmatic deposits, testifies this shift of activity after the opening of the conduit and the precursory phase. The increase of clasts size toward the top of the deposit is attributed to increasing discharge rate and column height with time. The fast drainage of magma and the consequent pressure decrease in the magma chamber possibly favoured the exsolution of gas from the saturated magma.

The phase evolves with intraplinian pyroclastic flows, derived mostly from non-convecting ash emission because peak discharge rate erupts too much material to be effectively transported by convection. The products are poorly sorted ash beds few centimeters thick, interlayered with pumice layers originated from the still convective and low concentrated region of the eruptive column. The top of this layer is encroached by many ballistic impacts of lithics up to 2 m in diameter. This peculiarity, along with the general change in the depositional mechanisms of the products, after the early plinian phase, provides a further evidence of the new style of the eruption. The increase in the discharge rate did not allow any more conditions for a sustained eruption column and a voluminous emission of pyroclastic flows began at this stage.

The subsequent collapse of a relatively low column determines the emplacement of a poorly expanded, hot pyroclastic flow with the formation of the welded beds which fill topographic depressions. The rapid drainage of magma in the foregoing eruptive phase produces a depressurization of the magma chamber. The collapse of the roof of the reservoir begins as soon as magma pressure is lower than the lithostatic load minus the tensile strength of the rocks. The collapse of the roof produces pyroclastic flows rich of coarse, meter sized, lithic clasts. The presence of many lithic breccia beds testifies the occurrence of different and successive steps of collapse. The presence of lithic concentration zones with coarse (up to 1m) lithic clasts within the stratigraphic sequence indicate the onset of a caldera collapse that probably occurred in incremental fashion. This produced also the collapse of the hydrothermal system into the magma reservoir that caused a change in activity from magmatic to phreatomagmatic. The abundance of accretionary lapilli and plastic deformation structures in the deposits associated with this phase suggest the beginning of a magma/water interaction possibly produced by the incipient collapse of the hydrothermal system into the magma reservoir.

As most eruptions, after the climactic phase, represented by the emission of a lithic rich flows, there is a waning phase with a relatively minor activity. The topmost beds are made up by a repetition of numerous and thin massive and sandwave deposits. They probably records a waning explosive power with deposition of diluted pyroclastic flows.


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L. Giacomelli

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