Beschreibung: The entry of groundwater into volcanic conduits has been proposed as a major modifying agent of eruptive dynamics, influencing magma fragmentation and pyroclast dispersion. Although several external water sources and interaction mechanisms have been proposed, the nature and effects of magma-water interaction are still largely unclear, as well as its controlling factors. A common postulate for phreatomagmatic activity to occur is that pressure in a conduit crosscutting a subsurface aquifer should drop below the aquifer pressure, which depends on the properties of the aquifer and the ascending magma. In agreement with most phreatomagmatic eruptions, we show that the injection of large mass fractions of groundwater during silicic explosive eruptions (e.g., 〉5 wt%) is only physically feasible for low-eruption-rate events; while high-intensity eruptions with evidence of magma-water interaction are probably related to other interaction mechanisms (e.g., the involvement of surface water or the destabilization of aquifer-hosting rocks during collapse phases). Because conditions for access of groundwater to the conduit are preferably reached above the fragmentation level, magma-water interaction seems not to induce dramatic changes to the features of a primary ‘dry’ vesiculation, as commonly claimed. Hence, the low vesicularity indexes often attributed to phreatomagmatic eruptions are difficult to explain by the quenching effect of groundwater on not-fully developed vesicularity. Instead, these indexes may be related to the low eruption rates needed for effective magma-water interaction, generally characterized by significant lateral gradients of vesicularity in narrow conduits.

Beschreibung: The Plinian columns formed during the magmatic phase of the A.D. 79 eruption of Vesuvius alternated several times between fully stable, buoyantly rising regimes and unstable regimes of partial or total collapse. Six pyroclastic density currents (PDCs) were produced during unstable regimes, and ultimately caused the destruction of Roman towns around the volcano. Through new measurements of juvenile clast density and estimations of ascent parameters, we show that four partial collapses were likely triggered by increases in the abundance of dense juvenile clasts within the eruptive column. In contrast, the total collapse probably occurred in response to an increase in the wall-rock content injected into the plume during a progressive widening of the conduit. A sixth low-energy, small collapse resulted from high abundances in both dense juvenile clasts and wall-rock material. Simulations of eruption column behavior already account for the effects of variations in conduit radius, mass discharge rate, and particle size, but have yet to include variable clast density and wall-rock abundance that cause temporal variations in plume density. Our results suggest that both parameters can exert a significant control on the potential for generation of PDCs.

Beschreibung: Cotopaxi volcano, situated in the Eastern Cordillera of the Ecuadorian Andes, is one of the most active volcanoes on Earth. The volcano is well known for the magnificence of its almost perfectly symmetrical cone topped by ice and snow and for the destructive power of its large-scale, syneruptive lahars. This paper presents a stratigraphic study of the post-twelfth-century eruptive products that reveals the existence of 21 continuous tephra beds. Most of them were characterized from both a physical (dispersal areas, deposit volumes, peak Mass Discharge Rate [MDR] of the eruptions) and compositional point of view. New 14C dates, linked with a new examination of historical chronicles, allow us to create a new chronostratigraphic scheme for this period of activity, which is bracketed by the emplacement of a regional tephra marker (A.D. 1140 ash bed from Quilotoa volcano) and the present day. The first period (A.D. 1150-1742) included only two moderate-intensity explosive eruptions, the oldest being possibly related to a dome disruption. In contrast, the period A.D. 1742-1880 started with two high-intensity, Plinian eruptions (maximum column heights of 25 and 29 km), followed by several short-lived but sustained, convective episodes. Deposits of pyroclastic surges and scoria flows were emplaced during some of these short-lived events and may have been related to column collapse and boiling over activity, respectively. Post-1880 activity, reported in 1904, 1906, and 1912, likely consisted of minor explosions that affected only the crater area. Our study of recent activity at Cotopaxi shows that high dispersive power (peak mass discharge rates from 1.1 to 9.3 x 107 kg/s) is associated with the eruption of only moderate amounts of magma (1.1 x 1010-6.0 x 1011 kg, or [~]0.005-0.2 km3, Dense Rock Equivalent [DRE]). Additionally, al-though the past 2000 yr of activity at Cotopaxi have been interpreted to reflect a fairly uniform magma supply rate, detailed analysis of the past centuries, and a reanalysis of data from the past 2000 yr show that Cotopaxi's eruptive activity is characterized by clusters of eruptive events that are separated by periods of long quiescence punctuated by isolated eruptions, often of slightly more evolved magma. No systematic variations in composition emerge in the time sequence. Although new magmatic phases commonly start with the eruption of mafic magma, this is not always observed. Additionally, eruption clusters may show either compositional trends of increasing SiO2 content or abrupt compositional changes within a cluster. We interpret the temporal and compositional variations in eruptive activity to reflect the complex interplay of deep versus shallow magmatic processes. An important result from the perspective of volcanic hazards is our conclusion that, over the studied period, no clear relation exists among repose time, eruption magnitude, and magma composition. This conclusion contrasts with the periodic eruptive behavior that has been postulated at many central volcanoes worldwide, thus inviting a reexamination of other intermediate-composition volcanic systems and a reassessment of the assumption of periodic activity.

Beschreibung: This work has been partially supported by the project V1 “Stima della pericolosità vulcanica in termini probabilistici” funded by Dipartimento della Protezione Civile (Italy).

Beschreibung: Published

Beschreibung: 5V. Processi eruttivi e post-eruttivi

Beschreibung: The Somma-Vesuvius (SV) volcanic complex has shown in his history a moderate variability of eruptive styles associated with a significant spatial variability of the associated vent locations. This is proved by the presence of numerous eccentric vents which fed effusive eruptions and also by the variability of the vent area of the large explosive eruptions that showed a shift within the present multistage SV caldera. Numerical simulations of explosive eruptions with variable vent locations inside the caldera have shown that this variability, although restricted to an area a few square kilometers, can have a major effect on the associated hazard, particularly for the threat associated to the occurrence of pyroclastic density currents produced by column collapse. This work summarizes some of the activities that have been put forward with the aim of producing a first long-term vent opening probability map for the area of the Somma-Vesuvius caldera. These activities mainly consist in the recognition and collection of key volcano features that can be linked to the spatial distribution of volcanic activity as well as used for their probabilistic treatment. Key variables that have been considered so far include: a) location of Plinian and sub-Plinian volcanic vents; b) location of moderately explosive eruptions (Violent Strombolian to ash emission); c) location of parasitic vents and eruptive fissures; d) distribution of subsurface faults from DTM analysis; e) distribution of the main deep lineaments, as deduced from published geophysical inversions. Locations of Plinian and sub-Plinian volcanic vents have been represented considering their uncertainties based on the available reconstruction of deposits and expert judgment. Parasitic cone and eruptive fissure locations have been also compiled after a comparative analysis of different bibliographic sources, including geological, geomorphological and topographic maps. Distribution of faults and fractures have been finally derived by integrating data from literature studies and new analysis of different digital terrain models (DTM). All the data have been imported into a GIS-based workspace that allowed to organize, analyze and elaborate different datasets. By assuming that each dataset can contribute to the probability distribution of vent opening through the assignment of appropriate weights (e.g. based on expert elicitation), preliminary vent opening (susceptibility) maps will be produced. Results will be used in the production of more accurate hazard maps of the range of expected explosive phenomena in case of a future reactivation of Somma-Vesuvius.

Beschreibung: Published

Beschreibung: Milano

Beschreibung: 6V. Pericolosità vulcanica e contributi alla stima del rischio