Activity, Unrest, and Hazard Evaluation at Stratovolcanoes and Calderas III
Presiding: G De Natale, INGV-Osservatorio Vesuviano; C Troise, INGV-Osservatorio Vesuviano
V33A-01 INVITED 13:30h
Volume-Time Relations in Large Silicic Volcanic Fields - Clues to a Thermomechanical Control on Eruption Frequency and Mechanism
In assessing the hazard associated with large calderas it is crucial that the broader context of the formation, evolution, and eruption of these systems be factored into any assessment. The existing paradigm is that as large volumes of intermediate to silicic magma accumulate and evolve at high-level, a pressure build-up during second boiling results in overpressures of 20 - 25 MPa that result in the fracture of the magma chamber roof and consequent eruption. This may not be the case for the largest ignimbrite eruptions. These are typically associated with a regional ignimbrite flare-up and we should consider a more holistic approach that takes into account the spatiotemporal and volume-time evolution of the entire flare-up in understanding the operation of these systems. I illustrate this with a case study of the Altiplano-Puna Volcanic Complex of the Central Andes (APVC). The timing, pattern, and volumes of ignimbrite volcanism in the APVC reveal: 1) Pulsing of the ignimbrite eruptions with an approximate two million year period; 2) A trend to larger volume eruptions climaxing at about 4 Ma; 3) Migration and focusing of activity toward the central part of the APVC with time; and 4) Markedly diminished activity since 4 Ma. These observations suggest that the ignimbrite flare-up is result of progressive thermal (and mechanical) maturation of the crustal column due to intrusion and batholith formation and attendant effects on lithosphere strength. The progressive erosion of crustal strength results in failure of the crust and catastrophic eruption. Examination of available data from other large silicic volcanic provinces through space and time reveal a general pattern similar to that shown by the APVC. This suggests a consistency of process consisting of thermal preparation, catastrophic response, and relaxation in the development of these large volcanic fields. The possibility that the largest ignimbrite eruptions (>1000 km3) are triggered by mechanical failure of a thermally-weakened, low aspect ratio, roof requires a change in paradigm. In this model the roof fails as its tensile strength is exceeded and founders into the magma due to a density inversion produced by concentration of the gas phase into the uppermost part of the magma chamber. Large vents open quickly and catastrophic mass eruption rates are achieved immediately. Fissures and ring faults may progressively unzip allowing dense eruptive curtains or fountains to collapse quickly to generate dense pyroclastic flows with high energy. This type of mechanism may help explain the paucity of classic Valles-type caldera structures and the growing evidence for eruption of large ignimbrites from complex nested sources and along faults or fissures.
V33A-02 13:50h
The Size and Frequency of the Largest Explosive Volcanic Eruptions on Earth
An analysis of the size and frequency of the largest known explosive eruptions on Earth reveals 42 known eruptions with erupted masses in excess of 1015 kg (magnitude 8 or larger) in the past 40 million years. This places the minimum frequency of eruptions of magnitude 8 and larger at ~1.4 Ma-1, in two pulses over the past 40 Ma. On the basis of the activity of the past 13.5 Ma, there is at least a 75% probability of an eruption of this scale occurring within the next 1 Ma. There is a 1% chance of an eruption of this scale in the next 460 - 7200 years. While the impact of any individual large magnitude eruption would be considerable, the time-averaged impact (i.e. erupted mass × frequency) of the very largest eruptions is small, due to their rarity. The long-term time-averaged erupted mass flux from magnitude 8 and 9 eruptions is one to two orders of magnitude less than for magnitude 7 eruptions, involving between 1014 and 1015 kg of magma. Comparison of the energy release by volcanic eruptions with that due to asteroid impacts suggests that on timescales of < 100,000 years, energetic volcanic eruptions are considerably more frequent than impacts of a similar energy yield and destructiveness. This has important implications for our understanding of risk from extreme events.
http://www-volcano.geog.cam.ac.uk/database/
V33A-03 14:10h
Evidence of fast Magma Rising in Explosive Eruption of Neapolitan Areas: a Comparative Analytical and Numerical Approaches.
Results of comparative analytical studies (crystal and bubble size distribution and number density and glass inclusions and matrix glass volatile content) of rock samples from effusive and explosive eruptions of Campi Flegrei and Somma Vesuvio volcanoes, and numerical simulation based on the inferred pre-eruptive conditions have provided constraints on vesiculation processes, shallow magma rising dynamics and timing. Results from high magma ascent rate typical of highly explosive eruptions indicate that degassing occurs under diffusion regime. In this case vesiculation is not accompanied by microlite formation, being microlite growth likely inhibited by short decompression time. Consistently with simulation the features of the explosive rocks of the Neapolitan area indicate that magma rose in very short time, in the order of a few hours.
V33A-04 14:30h
Field monitoring of CO2 and H2O emissions at Vulcano island by high-resolution laser spectroscopy
The concentration of CO2 and H2O have been monitored during a four-days campaign in Vulcano (Eolian Archipelago, Sicily), by means of a portable spectrometer based on a semiconductor diode laser operating at a wavelength of 2 Ym. The spectrometer was able to work both in open-path configuration and as an accumulation chamber. In-situ measurements were made at different heights from the ground and in several sites. In particular, we analyzed fumaroles in Porto Levante's beach, and on the volcano crater, as well as carbon dioxide emissions in the valley at the foot of the volcano. Variations of gas concentrations and their ratio were continuously recorded in these sites over different time scales.