Figure 4. Generalised stratigraphic log of the pyroclastic pile of Mt. Vulture Volcano (after Caggianelli et al., 1990).

The Basal Ignimbrite

The basal ignimbrite discontinuously crops out around the area and is mainly accumulated in palaeo valleys. Good exposures are at Albergo Farese or at the escarpment of the Valle dei Greggi fault which is also related to mineral springs rich in CO₂ (e.g. Gaudianello springs).

The basal ignimbrite consists of channelled pyroclastic flows (pumice-ash flow) and pyroclastic surges which may be related to different eruptions. Total thickness is about 30 m. Typical flow units are composed of highly vesiculated scoria (pumices) of white colour in a cream ash matrix. Most of the volume is formed by juvenile phonolitic material. Lithics are mainly melanite phonolite blocks from disrupted domes, associated with the flows and surges. Large amounts of clasts from the substratum are also present and are mainly of sedimentary origin.

In the Gaudianello area a sequence of seven aa lava flows occur at the top of the epiclatites covering the ignimbrite. At the top of the lava flows are present a variety of surge and wet flow deposits, which are very instructive for volcanosedimentary structures which indicate phreatomagmatic activity. The sequence is ended by surges produced by maar activity, large altered mantel nodules are found in this level.

 

The strato volcano  formation (II stage)

The cone is made up of a sequence of long and narrow lava flows and a much larger volume of tuffs and tephra layers. During this stage the volcano reached its morphological maturity. Rock composition varies from ol-melilitite, leucite-hauyne foidite to tephrite-phonolite. Soevite (intrusive carbonatite), hauyne-ijolite (hauyne mela-foiditolite) and a variety of coarse grained mica-amphibole ultramafic rocks are found in the tuff and possibly represent sampling by the magma of an alkaline ring complex beneath the volcano.

Exotic lava compositions include a century famous hauynophyre (local name Melfite) lava plateaux (dated 0.580 ma). The lava top plunges slightly towards E, and the city of Melfi was built directly on it using blocks of haunyophyre. Lava was emitted by an eccentric vent which poured it into a small lacustrine basin. Subsequent erosion isolated the lavic plateaux creating an inversion of the relief (phoenix effect). In the Cava Normanna area below Federico II’s castle an irregular columnar jointing is evident. Horizontal discontinuity surfaces are also an effect of contraction of the lava during cooling. The mineral assemblage comprises hauyne (lazurite), leucite, melilite, nepheline, clinopyroxenes, oxides, apatite, and zeolites.

Lava flows didn’t reach more distal areas where thousands of tephra layers related to hundreds of discrete eruption accumulated. A fantastic display of that succession is exposed at Cave San Antonio near to the town of Rionero. Spectacular Plinian pumice falls are characterised by bimodal composition changing throughout phonolitic (light colour) in the lower part, and foiditic (dark colour) in the upper part. This is believed to be produced by deflating of a zoned magma chamber. This kind of magmatic eruption alternates with phreatomagmatic ash-tuff with accretionary lapilli. Erosion surfaces, caliches, and paleosoils show gaps in the volcanic activity (Fig.6).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

      Figure 5. Stratigraphic log of cava San Antonio.

   

The strato volcano disruption (Stage III)

During that stage volcanic edifice was dissected by an E-W trending fault which produced a trench visible in the Landsat immage. In addition the S-W sector of the volcano collapsed. Probably the tectonic activity disturbed the equilibrium between the hydrothermal system and the magma subvolcanic reservoirs and produced phreatomagmatic eruptions. During those events ‘block and ash’ flow flowed away the fault trench towards Rionero area and a belt of thick pyroclastic flow accumulated around the volcano. The latter are related to the disruption of the volcano itself.

Notable is the exposure of Cava di Fosso della Gelosia where a thick, block and ash flow contains bombs of metric dimensions (bread crust bomb). In that quarry it is also possible to see epiclastic deposits accumulated in a paleo lake dammed by a lava flow.

A relatively small amount of magma finally rose along the fault as testified by several scoria cones along it.

 

Diatremic activity (Stage IV) 

 

The Monticchio lakes are the morphologic expression of funnel-shaped conduits, whose craters are made directly on the prevolcanic sedimentary basement, therefore they are classifiable as maars. They formed  around 0.013 m.y. and produced a lapilli tuff which  crops out in a half moon shape surrounding two thirds of the volcanic-tectonic depression which contains the two lakes. The lapilli tuff covers most part of the lake’s depression and its walls. The deposit is layered and has a maximum thickness of 4 m. A lower part of the depression rim at Varco della Creta favoured the accumulation of the deposits with a dispersion axis toward W. On the contrary, the lack of deposits at E indicate that they weren’t able to reach the 500 m of difference in level between the emission area to the E depression rim.

Those pyroclastic deposits are mainly of lapilli grain size organised in dune structure of two m of wavelength and 0.5 m tall. Most spectacular are dune and other primary volcanic features at Case Agostinelli area (Fig. 6). Melilititic lapilli and bombs in carbonatitic ash tuff form the bulk of the deposits. Most notable is the presence of peridotitic nodules (spinel wherlites and lherzolites) reaching the size of a small melon (Fig. 7).

Grey spherical lapilli with evident concentric structures made of glassy lava shells around kernels of mafic megacrysts or peridotitic nodules must not be confused with more common accretionary lapilli which are formed by layers of wet dust. Concentric lapilli, known as tuffisitic lapilli, are believed to form in an eruptive column in abyssal conditions inside cylindrical conduits (diatremes) via agglutination and accretion of supercooled magma drops around nucleus of lithic fragments in rapid rotation. This mecahnism allows a semi-solid convoy to rise rapidly toward the surface directly from the mantle source, propelled by juvenile gasses (CO₂ dominated) and is typical of ultramafic magmas such as kimberlite, lamproite and melilitite.

Some thin carbonatite layers (carbonatite= a igneous rock with a volume of carbonate > 50%) occur and are composed by pelletoidal micrite matrix (very small carbonate-liquid droplets) mixed up with glassy ash and crystalline fragments.

 

Figure 6. Dune beds lapillituff near Case Agostinelli.

           

The mineral assemblage of the juvenile fragments (blocks or lapilli) consists of rounded xenocrysts of Al-Cr diopside, residual fosterite, Cr-spinel and very rare Mg-garnet. Those phases present deformation and compositions typical of mantle minerals and probably they derive from desegregation of peridotitic nodules. The ground-mass of the juvenile fragments contains melilite, microphenocrysts of spinel with magnetite rims, Ba-phlogopite, prismatic olivine, hauyina-lazurite, Si-OH apatite, calcite monocrysts, perovskite and pyrite veins.

             

 

 

Figure 7. A - large lherzolite xenolith collected at MLF by K. Bailey and F. Stoppa (scale bar in cm). B - Cored lapillus: the kernel is a lherzolite xenolith coated by melilitite lava (scale bar = 1 cm).

 

 

Geochemistry of the Mt. Vulture volcano rocks

 

            The Vulture rocks pose numerous classification problems when using conventional chemical diagrams, which are often inconsistent with the moda of the rocks. The Total Alkaly Silica diagram does not allow any distinction among Vulture rocks, which spread on it. R1-R2 de La Roche’s diagram better discriminates three main groups which are significant in term of Volcanic cycles but, again, it is not suitable for classification. Most of the Vulture rocks have no essential plagioclase and thus, term such as basalt or K-basalts or even, tephrite, should be avoided. This problem is typical of plagioclase-free rocks, which do not plot in the foidite field. Most of the latter rocks are foiditic phonolites with xenocrystics olivine.  Melilitites plot, according to the silica and CO2 content, in the foiditic to ultramafic field.

 

 

 

 

 

 

Figure 8

Trace elements distributions of Vulture rocks are consistent and indicate comagmaticity and substantial source homogeneity.  As a whole, a high LILE content and a depletion of Nb, Ba and Ti is shown. The latter feature may be explained with residual titanates in the source. LREE are highly fractionated (La_pm/Yb_pm =30) and there is no Eu anomaly. Carbonatitic rocks shows cross-over relationship with respect to carbonate-free rocks having much higher REE/HFSE+LILE. This figure is clearly not compatible with the postulated origin of carbonate and sulphur by country-rock gypsum assimilation which would have produced high Sr/LREE, In addition, S isotopes are in the range of mantle value being ….(Marini et al.,   )

 

 

 

Mt. Vulture: References

 

Caggianelli A., De Fino M., La Volpe L., Piccarreta G. (1990). Mineral chemistry of Monte Vulture Volcanics:petrological implications. Mineralogy and Petrology, 41: 215-227.

 

Lavecchia G., Boncio P. (2000). Tectonic setting of the carbonatite- melilite association of Italy. Mineralogical Magazine  64, 583-592.

 

Stoppa F. and Principe C. (1998). Eruption style and petrology of a new carbonatitic suite from the Mt Vulture (Southern Italy): The Monticchio Lakes Formation. Journal of Volcanology and Geothermal Research. 80, 137-153.

 

Stoppa F. and Woolley A.R. (1997). The Italian carbonatites: field occurrence, petrology and regional significance. Mineralogy and Petrology 59, 43-67.

 

Campi Flegrei and Vesuvio

 

 

 

 

Figure 9. Landsat image of the Somma-Vesuvio and Campi Flegrei area

 

Phlegrean Fields: the caldera

The volcanic activity in the Phlegrean Fields (Fig. 9, 10A and 11) produced, in the last two million years, pyroclastic rocks and lava flows inter bedded with larger marine sedimentary deposits. These sedimentary and volcanoclastic deposits filled up the graben of the Campanian Plain. Most of the recent sediments are related to pyroclastics deposits produced by the activity of the Flegrei, Ischia Island and Vesuvio volcanic areas. The first two are quite different in volcanic stile and the geochemistry of the Vesuvio rocks indicates a distinct magmatic source.

 

 

The Phlegrean Fields are composed of 50 monogenic inland volcanic centres and an unknown number of off shore vents, distributed in a caldera of about 150 km². Most of them are tuff rings and maars but lesser tuff cones and domes are present. There are no discrete lava flows. The caldera (12 Km in diameter) is possibly related to the emission of a large volume of ignimbrites, namely the Campanian Ignimbrite, dated 35 ka, and the Napolitan Yellow Tuff dating from 12,000 years ago. Both ignimbrites have an estimated volume greater than 100 km³, have a thickness of tens of metres and are widespread in the Campanian Plain. Campanian Ignimbrite is composed of plinian pomice at the base, surges and a thick layer (up to 20-30 m) of a welded facies, with fiamme structures in a ash matrix, called

 

 

 

       

 

Figure 10. A) Shaded relief of the Phegrean Fields volcanic area. B) Sample of Piperno (11 x 8 cm). “Piperno” (Fig. 10B).

 

Napolitan Yellow Tuff is composed of dune beds and layers with chaotic structures made of pumices and lithics set in a fine-grained matrix often cemented by late zeolites. This deposit has been coerupted from several  submarine and subaerial vents and/or fissures.

After this event, volcanic activity of decreasing energy occurred inside the caldera. Different stages of post caldera activity have been recognised: submarine, older than 10,500 y.b.p; sub-aerial 10,500-3,700 y.b.p.; Historical 3,700 y.b.p. to 1538 a.C.

            Resurgence of the caldera is testified by a marine terrace (ledge) (La Starza) that rims the Pozzuoli coast and whose formation predates a phase of intense volcanism between 4.600-3.700 years ago. This activity produced the Cigliano, Agnano, Olibano-Accademia, Solfatara, Averno, Astroni, and Segna vents (Fig. 11). These vents are tuff rings, maars, scoria cones, and dome structures. The pyroclastic surges and pomice airfall levels are the most common deposits, organised in different facies. The presence of accretionary lapilli and vesiculated tuff matrix, suggests the occurrence of phreato-magmatic phenomena during this cycle of the phlegrean volcanic activity. However, most of the juvenile physical material is composed of  highly vesiculated lapilli (pumice) that testifies an original high contents of juvenile volatiles. In the last 2000 years the Phlegrean Fields had been subject to repeated periods of subsidence and uplift (bradyseims). Seismic swarms accompanied historical eruptions of 1233 at Arso - Ischia and that of 1538 at Monte Nuovo next to Pozzuoli (Fig. 11). The most recent uplift during 1970-1980 has not been followed, so far, by an eruption (Fig. 14).

 

 

Figure 11. Geological Map of the Phlegrean Fields. Redrawn from Di Vito M., Lirer L., Mastrolorenzo G., Rolandi G. e Scandone R. (1985). Vulcanological Map of Phlegrean Fields. U. degli Studi di Napoli. Ministero della Protezione Civile.

 

Petrology

Composition of Phlegrean Fields rocks range from potassic peralkaline phono-trachyte to alkali trachyte with very sub-ordinate trachy-basalt (Fig. 12). Large volume eruption of peralkaline phonolitic magma raises the problem of a suitable reservoir. It seems unlikely that thinned crust, which characterises the area, would allow very large magma chambers. In our opinion, not present in current literature, an alternative hypothesis would base on mantle melt underplating at the mantle-crust boundary. Large “evolved” mantle melts may be generated by a fast lithosphere thinning, which affected the area in the past 2 million years.

Figure 12. Normative diagram  of the Phlegrean Fields rocks, after Di Vito M., Lirer L., Mastrolorenzo G., Rolandi G. e Scandone R. (1985). Vulcanological Map of Campi Flegrei. U. degli Studi di Napoli. Ministero della Protezione Civile.

 

La Solfatara

The pyroclastic products of the La Solfatara centre have a limited spatial distribution and reach a maximum thickness of 10 m at the crater edge (Fig. 3). The deposit comprises of an explosion breccia made up of previous tuff blocks, lava fragments, pumiceous bombs and lapilli within a fine-grained matrix. At the top of the opening breccia lie coarse-ash dune and pisolitic ash deposits produced by repeated wet pyroclastic surges. Within this deposit there are layered, vesiculated-lapilli beds interpreted as product of airfall.

Owing to the monogenic nature of Phlegrean centres, a possible minor explosive event described in XII century should be interpreted as phreatic eruption produced by disturbance of the hydrothermal system. The exhalative activity at the Solfatara is related to fractures striking NW-SE and N60. These fractures crop-out along the eastern and southern crater edge crossing the Monte Olibano trachytic dome.  The crater bottom has been a location of human activity since Roman Times. Monoclin sulphur and kaolin extraction was the main activity followed by thermal steam baths. Other sublimates are mostly realgar (AsS) and K-allume (KAl(SO₄)₂ 12H₂O), alunite [(K,Na) Al₃ (SO₄)₂ (OH)₆].

On the vent floor the main fumarolic vents are:

- Soffione or Forum Vulcani, sited in the southern part of the vent with emissions at 145°C of temperature.

-Friedländer Observatory area (the last century, small observatory collapsed during the seismic activity of the 70’-80’s). It is a 500 m² area in which opens the Bocca Grande that is the hottest spot of the Solfatara (158°C).

- La Fangaia. In this area of ca. 500 m² there are pools of boiling mud, a couple of metres deep. At the end of 1983, during the apex of the bradyseism crisis, new pools originated within a fracture of 12-13 m in length and 0.5 m wide.

Figure 13. Fumarole vent from the La Solfatara floor.

 

In the La Solfatara the emissions are 99% of water vapour with the exception of La Fangaia whose emissions consist mainly of CO₂.

The Macellum of Pozzuoli and the Bradyseism

 

The Macellum (the Market) of Pozzuoli traditionally called Serapeo, during its two thousand years of history experienced and records repeated subsidence and uplift phenomena (bradyseism) Fig 14. In Roman age, the Pozzuoli-Averno area was very different from today. The Lucrino Lake did not exist and the Averno Lake was linked

 

 

 

 

 

Figure 14. A) The macellum flooded by seawater before the bradyeseism. B) A recent immage

 

to the sea through a channel. In the I century A.D. it was necessary to elevate the roads in Pozzuoli and rebuild the Erculea coastal road that was flooded by the sea. The Romans built a coastal dam in front of the Averno channel creating a harbour. The dam in the fifth century was partially flooded thus Teodorico rebuilt it. In the following five hundred years the coast and the imperial roman buildings were submerged down to 8 metres below the sea level. During this period the columns of the Serapeo hosted Lithodomus (tidal mollusc) colony and their wells are still visible. After the year 1000 the subsidence stopped and a slow uplift phase begun. A dramatic uprise occurred in 1537-38 that produced the emersion of the Roman dam and formed new beaches along the Lucrino lake area. The uprise peak preeced slightly the Mt. Nuovo eruption. After the eruption the soil slowly subsided until 1969. In the 1969 a sudden uprise occurred in Pozzuoli area. In the period 1969-1972 it rose in places to 170 cm, and after a standstill phase, the soil rised another 160 cm in the 1982-83. An area of 80 square kilometers was uplifted, damaging homes, the harbor, and the tourist industry. An intense seismic activity accompanied the uplift, with hundreds of quakes a day, with events reaching the magnitude 4 (e.g. 4 October 1983). The earthquake epicentres have a U-shaped distribution with two maximums, one between Pozzuli and the Solfatara and the other in front of the coast between Baia and Miseno (Fig. 6). In the Solfatara a new fracture opened and the vapour/gas ratio and H₂S emission increased. Ultimately 36,000 people were relocated

These phenomena may indicate the fracturing of a small, superficial magma reservoir roof. Because the eruption did not occur it is not possible to understand whether or not the magma rose. However on the basis of the phenomena that occurred in the 16 century that leaded to the Mt. Nuovo eruption, and considering volcanic activity precursor indicators, the 1983 crisis, indicates that a relatively small scale phreatomagmatic eruption in likely to occur in the future, with a serious risk for the 400,000 inhabitants of the area surrounding Pozzuoli.

 

The Monte  Nuovo eruption

In 1537-1538 an intense seismic activity began and the fumarole of Tripergole increased their emission volume. In the last decade of September 1538 an abrupt uplift occurred in the same area and the beach shifted by 50 metres.

 

 

 

 

 

 

 

 

 

 

Figure 15. Mt. Nuovo on Febraury 1540, painted by Francesco De Hollanda

 

This phenomenon was accompanied by an intense seismic activity and in the thermal area of Tripergole new fumaroles and water steams opened. In the 29 September at 1 o’clock a fracture opened in the thermal area and suddenly phreato-magmatic eruption started accompanied by an intense explosive activity. In few hours the Tripergole area was covered by wet ash deposits, and ash fell also in Naples transported by the wind. Strombolian activity built a cone with a height of 130 metres in three days. In the following days the activity was discontinuous, but on the 3th October, from a lateral vent (south mouth) a violent explosion produced pyroclastic surge that knocked down  the trees up to 5 km in the W direction down to Lucullo caves. The rest period following the initial volcanic activity encouraged many people to visit the new volcano. In the evening of the following Sunday a sudden unrest killed 24 people that had ventured on the volcano flanks. That was the last eruptive event of Monte Nuovo. Today, hot springs and small vapour emissions are still present at Monte Nuovo.

The Monte Nuovo volcano has a volume of 25 million m³ and it is made of chaotic beds of scoriae and surrounded for a distance of 3-4 Km by surge/flow ash lapilli tuff that erupted during the phreatomagmatic events. The pyroclastic surges had a relatively small flux potential since a bastion height few meters was able to shelter a roman temple called “Tempio di Apollo” sited on the estern shorelane of the Averno lake. The Mt. Nuovo eruptions devastated an area of 3-4 Km radius from the vent, but the shock waves and the pyroclastic fall-out heavily affected an area of 7 Km radius.

 

 

 

The Somma-Vesuvio complex

The Gran Cono of Vesuvio (1277 m) is a young volcano, which is nested inside the remains of a 20,000 year old strato-volcano, the Monte Somma. The latter collapsed about 17,000 years ago and its highest  peak is now the Punta Nasone (1132 m). Between the Vesuvio cone and the Punta del Nasone opens the Valle del Gigante valley the western part of which is called Atrio del Cavallo (horse hall) and the eastern side is called Valle dell’Inferno.

 

Figure 16. Volcanological Map of the Somma-Vesuvius volcanic complex. Redrawn from Di Vito M., Lirer L., Mastrolorenzo G., Rolandi G. e Scandone R. (1985). Vulcanological Map of Phlegrean Fields. U. degli Studi di Napoli. Ministero della Protezione Civile.

 

The Somma - Vesuvius complex alternates between violent explosive eruptions followed by long effusive activity with centenary restings. In the Somma-Vesuvius history at least 8 main plinian eruptions of some Km3 of volume are recorded. A larger number of sub-plinian events also occurred. In 5960 B.C., 3580 B.C. and 79 A.C., Vesuvius had eruptions that rate among the largest known in Europe. The Avellino pumice eruption (3580 years ago) and the 79 d. C. eruption are particularly famous for producing a big impact on the human civilisation and sealed important archaeological layers. In general each eruption produced thick pumice airfall followed by pyroclatic flow and surge along with lahars or mudflows. The latter can occur much later. On 5 May 1998 secondary lahars, mobilising 1944 ashes, killed about 300 people in Sarno. The local term lava was originally referred to mudflows and following Hamilton it changed meaning in the XVIII century when Vesuvius was permanently emitting large volumes of lava flows. Eccentric vents commonly produce effusive eruptions of fluid and degassed lava mainly flowing towards W-SE.

            Since 79 A.D. Vesuvius has erupted at least 19 times and the most destructive occurred in 1631. Pyroclatic flows, surges and lahars killed 4000 people in the coastal area between Pompeii and Portici. From 1694 to 1944 Vesuvio was in effusive activity almost continuously and produced very high lava fountains and long lava flows. Large strombolian or phreatostrombolian eruption occurred, among others, on 1822, 1872, 1906 and 1944 (Fig. 17).

 

 

 

 

Petrology

Vesuvio rocks are mainly tephritic phonolitic leucitites and leucite phonolites. Monte Somma rocks are potassic trachybasalts and trachites. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 18. Seminormative composition of the Somma-Vesuvius rocks, after Di Vito M., Lirer L., Mastrolorenzo G., Rolandi G. e Scandone R. (1985). Vulcanological Map of Campi Flegrei. U. degli Studi di Napoli. Ministero della Protezione Civile.

 

Mantle nodules at the Vesuvius

Recent discovery of peridotites nodules at the Vesuvio (Fig. 19) opens new hypotheses on source composition and magma propagation through the mantle and crust. A mechanism is required to erupt both magma and mantle xenoliths from mantle depths. Ascent needs to be rapid, 1-5 m/sec for basanites and higher for less viscous magmas, to avoid xenolith settling. Only a fracture-conduit flow mechanism is sufficient, diapiric ascent and melt percolation are too slow. The only realistic agent of acceleration is gas (Anderson 1979; Bailey 1984). Phreatomagmatism is a shallow process and cannot provide volatiles at the requisite level. Thus, current Vesuvio eruptive mechanisms based on very superficial magma chamber and phreatomagmatism should be seriously questioned.

 

 

 

 

 

 

 

 

 

Figure 19. Sp-lherzolite nodule of 11.5x9.5cm dimension, and 1.3 kg weight.

 

The 79 A.D. eruption of the Vesuvio

 

            Tacito and Seneca describe the effect on building and human beings of the 62 A.D. earthquake which struck the Vesuvio area causing large damage in Pompeii, Hercolaneum, Stabiae and Neapolis. Effects are depicted on a larario in the house of L. Cecilio Giocondo in Pompeii (Photo on the left).

After 17 years, on the 24^th august at 1 p.m. Vesuvius started one of the most tragic and important eruptions that history records. Eruption impacted on a populated area and buried the main cities of Hercolaneum, Oplontis, Pompeii and Stabiae and an unknown number of villages and villas. The eruption, where Plinius il Vecchio (Gaio Secondo), was described by Plinius il Giovane (Gaio Cecilio Secondo) some years later in two letters to the roman historian Publio Cornelio Tacito. Plinius il Giovane gave the first detailed account of a Volcanic eruption. An unparalleled case both in the western and eastern civilisations. The eruption is composed of three main phases:

1)      vent opening phase,

2)      plinian eruption phase with formation of a steady pyroclastic column,

3)      phreato-magmatic phase with intermittent collapses of the pyroclastic column.

The first eruptive phase was caused by phreato-magmatic explosion and its roar terrified the inhabitants of the Vesuvius area and a grey, pisolitic, thin ash layer covered the eastern flanks of the volcano. The opening of the vent allowed the release and expansion of the volatiles stored in the magma chamber causing the magma fragmentation and its throwing at an estimated velocity of 400 m/s. Plinius described a big eruption forming a pine shape column. The region of the column sustained by the gas thrust may have reached 3 Km in height, while the convection may have transported gasses and pumices up to 30 Km in height.

In the first 8 hours pumice fell on the city of Pompeii (phonolitic in composition) at a rate of 10-15 cm per hour. In the proximal area (distance < 15 Km from the vent) white pumices fall was followed by another fall-out phase characterised by grey pumices with grey colour and tephritic-phonolite composition. During this plinian phase the vent was probably 100 m in diameter and the eruptive flow was of ca 4 x 10⁴ m³s^-1.

The maximum thickness of the plinian depost made of white and grey pomices reaches 280 cm in Pompeii, 200 in Oplontis, 150 cm in the Sorrento area, and 30 cm at Agropoli, 70 Km SE of Vesuvius (Fig. 9). The volume of the white pomices is 1 Km³ and the grey pomices is 2.6 Km³. During the emplacement of the grey pomice tuff, the eruption dynamics changed probably in response of hydrothermal system – magma contact possibly due to subsidence of blocks of the magma chamber roof. The water content of the eruptive column increased causing increasing density and agglutination of particles. During the night the eruptive column partially collapsed forming pyroclastic surges and flows that hit the southern and western flanks of the volcano travelling for a distance of 15 km from the vent (Fig. 10).

The first pyroclastic surges occurred 12 hours after the beginning of the eruption and travelling at 30 m/s reached Hercolaneum in less than 4 minutes. Only a few ashes fell on Hercolaneum, being windward, from the beginning of the eruption. Most of the inhabitants had already escaped but hundreds, waiting along the beach, did not survive the surge. The eruptive column recovered its convective stability and only fall of pomices  characterised the following hour. Around 2 o’clock on the 25^th august another pyroclastic surge (S2) came down along the volcano flanks. It was three times bigger than the first and caused huge damages in Hercolaneum but stopped before Pompeii.

After the second pyroclastic surge the pomices fall-out started again but with more abundant lithic fragment fraction suggesting a widening of the volcanic conduit. At 6:30 am a wet pyroclastic flow ran over Hercolaneum but did not reach Pompeii.

At Hercolaneum the volcanoclastic deposits related to the 79 A.D. eruptions have a thickness of ca 20 m and lacks in the plinian pomices level, but at the base there is the dune deposit produced by the first pyroclastic surge. On top of it there is a deposit of pyroclastic flow containing building material and statues, burned wood and degassing pipes near the top. This sequence is repeated another time and then at the top there is a lahar (mud flow) deposit. At Oplontis the volcanoclastic sequence is similar to Hercolaneum having two pyroclastic flows but lacks in the lahar deposits. The two pyroclastic flow episodes are missing at Pompeii. Oplonti and Hercolaneum are both near the valleys that channelled the pyroclastic flows towards the cities.

Meantime at Pompeii the fall-out of the lapilli and ashes attenuated, even if the pyroclastic material accumulated so far reached 2,5 m of thickness, obstructing doors and windows of the first floors. Many people were still in the town, hoping to leave at the first morning lights, while others were coming back to recover their goods from their houses. At 7:30 the eruptive column collapsed and new and bigger pyroclastic waves (S4, S5 and S6) rolled down over Pompeii and covered wide areas at SE of the volcano. At 8 am of the 25^th august the widest pyroclastic surge (S6) flooded the Pompeii area reaching Stabia where Plinius il Vecchio died.

This last eruptive phase produced the most abundant piroclastic waves and fluxes of an overall extimated eruptive flux capacity of 10⁵ m/s. The estimated volume of the tephra erupted in 19 hours was ca 9 Km³ corresponding to 4 Km³ of magma.

A description of the Pompeii pyroclastic deposit and the its relationships with the eruptive phases along with the effects on the people and buildings is in Fig. 20. The human casts have been recovered from the first level of ash tuff surge deposit above the pumice layer and below of other 2 metres another surge deposits. Most of the people carried small goods and tried to cover themselves with cloths and pillows. Most of the people (200,000) however left the town in the first 18 hour from the beginning of the eruption.

 

 

Figure 20. Pyroclastic deposit of the 79 A.D. and the its relationships with the eruptive phases that destroied Pompeii.

 

 

 

Oplonti

The Roman Villa of Oplontis is located about 5 km from the ruins of Pompeii. It is one of the best preserved villas in the world.

 

 

 

 

 

 

 

 

 

Figure 21. The Oplonti Villa.

 

This urban villa was begun in the mid first century B.C. and progressively enlarged and modified for about 100 years, until the mid first century A.D. Fashions in Roman wall paintings changed over time and can be dated accurately. Four styles have been identified. The second (80 - 15 B.C.) and third styles (15 B.C. - 63 A.D.) are well represented in the Villa. It is worth noting that at the time of the eruption the villa wasn't being lived in. Heaps of building and decorating materials were found which suggests that in 79 the villa was being renovated. This may well be associated with the severe earthquake of 62 A.D.

At Oplonti the stratigraphic record of the 79 A.D. eruption can by subdivided in two main eruptive units with a total thickness of about 8-10 metres.

The first is composed of pomices deposited by fall-out accumulated with a rate of about 10-15 cm per hour. This deposit is related to the sustained column magmatic phase. Heavy lithics are present but not very abundant. Small surge level are present.

The second is made of ash and lapilli tuffs deposited by pyroclastic surges and at least two pyroclastic flow units corresponding to the phreato-magmatic phase occurred in the morning of the 25^th of August. Impact on the building, goods and humans is impressive. About 70 bodies were found in the excavation area, mostly from the houses surroundings the villa.

Figure 21. Images of collapsed colonnade (now restored) on the left. Wooden doors and window partially buried in the pumice which was pulled and banded (right).

 

 

Phregrean fields and Somma – Vesuvius: References

 

F. Barberi, G. Corrado, F. Innocenti & G. Luongo (1984). Phlegrean Fields 1982-1984: brief chronicle of a Volcano Emergency in a Densely populated area. Bull. Volcanol. 47,2, 175-185.  Inoltre Autori Vari nello stesso fascicolo dedicato ai Campi Flegrei.

Peter J. Baxter (1990). Medical effects of volcanic eruptions. Bull. Volcanol., 52, 532-544.

S.Carey & H. Sigurdsson (1987). Temporal variations in column hight and magma discharge rate during the 79 A.D. eruption of Vesuvius. Geol. Soc. Amer. Bull.,99, 303-314.

R.Cioni, P. Marianelli & A. Sbrana (1992). Dynamics of the A.D. 79 eruption: stratigraphic, sedimentological and geochemical data on the succession from Somma-Vesuvius southern and eastern sectors. Acta Vulcanologica, 2, 109-123.

R. Cioni, Civetta L., Marianelli P., N. Metrich, R. Santacroce & A. Sbrana (1995). Compositional layering and syn-eruptive mixing of a periodically refilled shallow magma chamber: the A.D. 79 plinian eruption of Vesuvius. Jour. Petrol., 36, 3, 739-776.

M. Di Vito, L. Lirer. G. Mastrolorenzo & G. Rolandi (1987). The 1538 Monte Nuovo eruption (Campi Flegrei, Italy). Bull. Vulcanol. 49, 608-615.

D. Faraone (1994). Attività vulcanica. Dipartimento di Scienze della Terra, Centro Stampa Università degli Studi di Perugia, 273 pp.

M. Rosi & A. Sbrana (1987). Phlegrean Fields. Qaderni del "La Ricerca Scientifica, C.N.R., 114,9,  175 pp.

 

 

 

Questions and considerations

• What is the dominant volatile in carbonatitic and ultralkaline Italian magmatism?

• What makes Italy first in the world for production of natural fizzy water and commercial travertine marbles

• What was the vesiculating agent of Campi Flegrei and Vesuvio eruption?

• Is there any other factor but phreatomagmatism, which may entirely explain Italian volcanism character and tuffisite formation?

•  What was the suffocating agent, which produced victims inside the buildings during 79 A.D. eruption?

Presumably, the answer is the CO₂.

 

The origin of Italian CO₂ was so far interpreted according to the old Rittman’s theory and its variations, that is trachytic magma desilicisation via limestone digestion. 

The first Italian carbonatite was fully described in 1992 at Polino, other five were added with time. They are high temperature extrusive rocks with high Ni+Cr content (1000 ppm), very high Mg# value (92), along with high REE, Zr, Sr, Ba (summing >5000 ppm). In addition, mantle xenoliths composed of Cr-phlogopite, Cr-diopside, forsterite and chromite forms up to 20% vol. of the rock. They are not in dilution-concentration relationship with other Italian primitive rocks, such as melilitites and leucitites to which displays substantial geochemical and isotopic equilibrium. It is thus clear that these rocks cannot be generated from one another by assimilation of crustal material and that is not possible to produce carbonatite through limestone assimilation by silicate magma, as contaminant should be in excess compared to contaminated.

These rocks may change current views on the Italian magmatogenetic theories owing to it clear primary nature, which can pose a firm point also on Italian C02 origin. There is no doubt that Vulture and Vesuvio magmas, among others, are from the mantle and associated CO₂ should have the same origin.

The nature of the Italian magmatism implies that the high concentration of deep-seated juvenile propellant resulted in extremely violent volcanic activities and that the potential volcanic risk implied by CO₂ occurrences must be re-evaluated in terms of this factor.