L. La Volpe

Thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from Molten Fuel Coolant Interaction experiments

[1]xa0Thermohydraulic explosions were produced by Molten Fuel Coolant Interaction (MFCI) experiments using remelted shoshonitic rocks from Vulcano (Italy). The fragmentation history and energy release were recorded. The resulting products were recovered and analyzed with the scanning electron microscope. Fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode. These clasts relate to the thermohydraulic phase of the MFCI, where most of the mechanical energy is released; they are here called “active” particles. The total surface area of such particles is proportional to the energy of the respective explosions. Other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime. These fragments, called “passive” particles, form after the thermohydraulic phase, during the expansion phase of the MFCI. In order to verify thermohydraulic explosions in volcanic eruptions, we compared experimental products with samples from phreatomagmatic base-surge deposits of Vulcano. Ash particles from the experiments show features similar to those from the deposits, suggesting that the experiments reproduced the same fragmentation dynamics. To achieve discrimination between active and passive particles, we calculated shape parameters from image analysis. The mass of active particles in base-surge deposits was calculated. As the material properties for the natural samples are identical to the experimental ones, the energy measurements and calculations of the experiments can be applied. For a single phreatomagmatic eruption at Vulcano, a maximum mechanical energy release of 2.75 × 1013 J was calculated, representing a TNT analogue of 6.5 kt.

Fragmentation versus transportation mechanisms in the pyroclastic sequence of Monte Pilato-Rocche Rosse (Lipari, Italy)

Abstract Monte Pilato is a rhyolitic pumice cone formed about 1400 years ago during the last volcanic activity which occurred at Lipari. Its activity consisted of numerous explosive episodes, ending with the extrusion of the Rocche Rosse obsidian lava flow. We have identified nine different pyroclastic lithofacies, of which four main lithofacies represent 90% of the erupted products; they include: (1) deposits consisting of decimeters- to meters-thick well-sorted, graded beds of pumice lapilli, which are of fall origin; (2) massive deposits consisting of decimeters-thick beds of pumice lapilli in an ash matrix, interpreted as being of pyroclastic flow origin; (3) deposits of centimeters-thick ash beds of surge origin; (4) decimeter-thick fine ash layers with scattered subcentimetric accretionary lapilli of turbulent “flow” origin. The complex grain size distributions of the main lithofacies were distinguished using a multivariate statistic analysis (R-mode factor analysis). Variations of the grain size characteristics in relation to the distance from the vent area have been investigated in order to discriminate the roles played by fragmentation and transportation processes. In the lithofacies 1 fall deposits, the grain size distribution is always unimodal and varies from a Rosin-type distribution near the vent to a Gaussian one in the distal outcrops. This is caused by the transport process and the different particle trajectories. In lithofacies 2, 3 and 4, the grain size distributions are always polymodal. The polymodality is not caused by preferential processes occurring during transportation, but seems, rather, to be connected with the original fragmentation of the pyroclastic material. The pumice lapilli present both in lithofacies 1 and 2 have mean vesicularity index values always in excess of 65%, and the value is homogeneous throughout the sample. These data suggest that for these two lithofacies, the pumice lapilli originated from exsolution of magmatic gases. In contrast, the fine ash particles in lithofacies 2, and also the main mode of lithofacies 3 and 4 deposits, show typical features of hydromagmatic fragmentation processes. The grain size population is always centered at 4–5 ϕ (0.032–0.064 mm) and, interestingly, the same grain size has been obtained recently (Zimanowski et al., 1991) for fragments produced by experimental magma/water interaction. Field and laboratory data suggest that during Monte Pilato-Rocche Rosse eruptions magmatic and hydromagmatic mechanisms were frequently operative at the same time.

Petrogenesis of Monte Vulture volcano (Italy): inferences from mineral chemistry, major and trace element data

The paper presents major and trace element data and mineral compositions for a series of foiditic-tephritic to phonolitic rocks coming from Monte Vulture, Southern Italy, and investigates their origin, evolution and relationship with the other centres of the Roman province.Major and trace element variation in the foiditic to tephritic suite agrees with a hypothesis of evolution by simple crystal/liquid fractionation, whereas the early erupted phonolitic trachytes and phonolites have geochemical characteristics which do not support their derivation from tephritic magma by crystal fractionation. Foiditic and phonolitic rocks have mineral compositions which are interpreted as indicating magma mixing. However geochemical evidence shows that this process did not play an important role during the magma evolution.The Vulture rocks have compositional peculiarities such as high abundance of Na2O, CaO, Cl and S, when compared with other Roman volcanics. Instead, the distribution of incompatible elements is similar to those of Roman rocks, except for a lower content of Rb and K, higher P and lower Th/Ta and Th/Nb ratios which are still close to the values of arc volcanics.The high contents of Na, Ca and of volatile components are tentatively attributed to the interaction of magma with aqueous solutions, rich in calcium sulphate and sodium chloride, related to the Miocene or Triassic evaporites occurring within the sedimentary sequence underlying the volcano. The distribution pattern of the incompatible elements is interpreted as indicative of magma-forming in a subduction modified upper mantle and of the peculiar location of M. Vulture.

Image processing analysis in reconstructing fragmentation and transportation mechanisms of pyroclastic deposits. The case of Monte Pilato-Rocche Rosse eruptions, Lipari (Aeolian islands, Italy)

Abstract A quantitative study of particle morphology of samples from Monte Pilato-Rocche Rosse deposits, Lipari (Aeolian islands, Italy) shows a significant variability in particle structure and texture. The study consisted of image processing techniques and multivariate statistical methods in analysis of the quantitative particle morphology data, with the goal of identifying the genetic processes responsible for the formation of the deposits. From these investigations we numerically define the form of the clasts, thus avoiding subjectivity in the classification of clast morphology. Starting from this clast definition, we then construct diagrams in which clasts due to different genetic processes (i.e., magmatic vs. hydromagmatic fragmentation), are discriminated based on simple nondimensional parameters. The application of the methods used seems to be of general interest in discriminating the fragmentation and transportation dynamics responsible for the formation of pyroclastic deposits.

Stromboli and its 1975 eruption

On November 4, 1975 in the evening, an eruption took place at Mt. Stromboli. On the following day lava flowed on the Sciara del Fuoco downward to the sea, accompanied by an intense explosive activity at the crater plane. Direct observations on the volcanic activity were carried out since November 6 while a seismic survey was made from Nov. 7 to 12. The total volume of the lava outpoured during this period of activity that lasted 21 days, was estimated to be about 104 m3. This paper reports the results of direct observations, and of the petrological, radioactive disequilibria and seismic activity studies performed for this eruption.The eruption was preceded by an insignificant change of seismic activity, which was monitored by a seismic station located about 2 km East of the crater. A shallow seismicity was strietly related to crater explosions accompanying the eruptive phenomenon. Radioactive disequilibria showed a lack of disequilibrium between228Ra and232Th explainable in terms of a fast rising of magma in the conduit.Chemical analyses of lava samples and deep seismic sounding data indicate a correspondence between the depth (10–15 km) at which crystallization pressure of phenocrysts occurs and a low velocity laver.

Volcanic history of Lipari (Aeolian Islands, Italy) during the last 10,000 years

Abstract Examination of the volcanic stratigraphy of deposits younger than 10,000 years on Lipari indicates four principal periods of volcanic activity related to specific centers. The products from each different volcanic center are defined as volcano-stratigraphic unit (VSU). From the oldest these are: the Canneto Dentro, Gabellotto-Fiume Bianco, Forgia Vecchia and Monte Pilato-Rocche Rosse VSUs. The study of textures and dispersal of the deposits permitted the vents to be localized and the recent volcanic history of Lipari to be reconstructed. The oldest event formed a small explosion breccia cone with a final obsidian lava in the Canneto Dentro area. Immediately afterward, a complex series of explosions produced the widespread dry-surge deposits of the Gabellotto-Fiume Bianco sequence. This activity ended with the extrusion of a domical lava flow. The renewal of activity occurred in the Pirrera area with an explosive eruption that produced explosion breccia deposits. The last eruptions from this vent were coeval with the first eruption of M. Pilato. The rim of the explosion breccia cone was partially destroyed by the Forgia Vecchia lava flow. M. Pilato cone grew in a very short period of time due to a continuous swarm of explosive events. After a short repose time, a series of more energetic and superficial explosions occurred through a vent slightly to the south. The extrusion of the Rocche Rosse lava flow (about 729 A.D.) ended this cycle of activity. All the volcanic centers follow a quite similar stochastic pattern starting with a fall or surge eruption and ending with effusion of viscous rhyolitic lavas. The four centers are aligned along either NW-SE or NE-SW fault systems according to the structural pattern of the island. They can be placed into two groups: the Canneto Dentro-Gabellotto centers and the Forgia Vecchia-M. Pilato centers. A long quiescence ( ⋍ 3,500 years ) separates the activity of these two groups while inside of each the activities were nearly contemporaneous or in rapid succession.

Volcanic hazards at Fossa of Vulcano: data from the last 6,000 years

Stratigraphic reconstruction of the complete sequence of deposits that formed the Fossa cone of Vulcano has distinguished four principal eruptive cycles: Punte Nere, Palizzi, Commenda, and Pietre Cotte. At least three additional eruptive cycles, one of which ends with the Campo Sportivo lava, occur between deposits of the Punte Nere and Palizzi cycles. However, exposure is inadequate for their characterization. The assignment of the modern deposits that follow the Pietre Cotte lava is uncertain.Deposits of each cycle follow a similar stochastic pattern that is controlled by a decrease in the effect of water/melt interaction. The normal sequence of pyroclastic products for each cycle starts with wet-surge beds, followed by dry-surge horizons, fall deposits, and finally lava flows. Absolute age determinations have been made on each cycle-ending lava flow.Wet-surge deposits normally occur near the crater rim, whereas dry-surge deposits are more widespread, reaching the surrounding caldera wall in many places. Thick fall deposits are confined to a zone extending about 800 m from the crater rim. Lava flows normally reach the base of the cone. The greatest hazard at Fossa is related to surge eruptions. The thickness of dry-surge deposits on the flanks of the cone increases away from the crater, but they pinch out toward the source near the crater rim. SEM analysis of the surface textures of juvenile glass clasts from dry-surge deposits confirms that the dominant control on the eruptive mechanism is water/melt interaction. Only slight modifications are induced on grain surfaces during transport. Particles from the Palizzi dry-surge beds lack surface textures characteristic of fall pyroclasts which suggests that ballistic fragments were not incorporated into the dense portion of the turbulent surge cloud. A quantitative analysis of the dispersal of products from the Palizzi cycle allowed creation of a computer-generated map for this eruption.

The Yemen trap series: Genesis and evolution of a continental flood basalt province

Abstract The Yemen basaltic plateau (Trap Series) was built up by fissure eruptions related to tensional tectonics in the Afro-Arabian plate. Two main phases of activity have been identified and dated between 30 and 26, and 23 and 19 Ma, respectively. The first phase was dominated by eruptions of alkaline to transitional basalts with a few rhyolitic ignimbrites. During the second phase, rhyolites with minor transitional basalts were erupted in the southern and southwestern part of the area. Geochemical and isotopic data indicate that the basalts are the result of combined processes of crystal fractionation and crustal contamination on mantle derived parental liquids. The degree of crustal contamination generally decreases from south to north.

Wet surge deposits at La Fossa di Vulcano: Depositional and eruptive mechanisms

Abstract Wet surge deposits of different volcanic cycles of the recent Fossa activity at Vulcano have been measured on a bed-by-bed basis, with data recorded to millimeter detail. The wet surge layers are varicoloured with variable thickness, with the most recurrent thickness being about 1 cm. The beds consist of fine ash without internal structures. Textural features include: (a) accretionary lapilli, of maximum size of 0.5 cm, dispersed thoroughout the layer or forming continuous layers of submillimeter size; (b) vesiculated layers which represent 10% to 65% of the total deposit; vesicles have different shapes and smooth walls, varying in volume from 1% to 15–20%; (c) soft-sediment types of bedding deformation, such as gravity flowage ripples, load cast and slumps. The slope angle has not influenced either the concentration and size of the accretionary lapilli or the shape, size, and distribution of vesicles. Only the thickness of the layers decreases with distance from the vent. SEM investigations show features indicating the hydromagmatic origin of the deposits and stressing the role of the fluid phase. Noteworthy is the presence of vesiculated grains, produced by magmatic exsolution, which show chilling effects on the internal walls of the broken bubbles. Grain size analyses reveal that the layers are not graded and most of the samples have a median size finer than 50 μm. The grain size distributions are frequently polymodal, suggesting several closely timed explosions. As all the beds exhibit the same textures, grain size, and particle morphology a single mechanism can explain their deposition. The depositional unit formed at the base of the cloud through the lateral expansion on the ground of a sticky muddy medium consisting of ash and fluid. In general each layer lost its plasticity before the deposition of the next layer. The deposition occurred in a nearly continuous sequence without periods of rest long enough to permit erosional discontinuity. The eruptions are hydromagmatic and occur where magma, at least partially fragmented, comes into effective contact with subsurface water. The process follows a model suggested by Wohletz (1983b, 1986), developing in more than one fragmentation event. In the turbulent surge cloud both severe hydration and alteration of glassy grains and the formation of accretionary lapilli occur.

Geology, volcanic history and petrology of Vulcano (central Aeolian archipelago)

Abstract Vulcano is an active NW–SE-elongated composite volcano located in the central Aeolian archipelago. Based on available radiometric ages and tephrochronology, the exposed volcanism started at c. 127 ka and spread through eight Eruptive Epochs separated by volcano-tectonic events and major quiescent stages. Various eruptive centres and two intersecting multi-stage calderas resulted from such evolution. Vulcano geological history displays several changes of eruption magnitude, eruption styles and composition of magmas through time. Vulcano rocks range from basalt to rhyolite and show variable alkali contents, roughly increasing during time. Magmas with low to intermediate SiO2 contents and high-K to shoshonite affinity prevail in the early Epochs 1–5 (c. 127–28 ka), whereas intermediate to high-SiO2 shoshonite and potassic alkaline products dominate the last three Epochs (<30 ka). This sharp increase in silicic products is related to the shallowing of the plumbing system and resulting major role of the differentiation processes in shallow-level reservoirs. Radiogenic isotope compositions are variable (87Sr/86Sr=0.70424–0.70587, 143Nd/144Nd=0.51254–0.51276, 206Pb/204Pb=19.305–19.759, 207Pb/204Pb=15.659–15.752, 208Pb/204Pb=39.208–39.559) as a result of both source heterogeneities and shallow-level interaction of magmas with continental crust. The compositional variations of mafic magmatism with time suggest that the source zone changed from a metasomatized, fertile, ocean island basalt- (OIB-) like mantle to a metasomatized depleted lithospheric mantle. DVD: The 10 000 scale geological map of Vulcano is included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic. Also included is a full geochemical data set for Vulcano.