V22A-01
History of the magmatic feeding system of the Campi Flegrei caldera (Italy)
The definition of the magmatic feeding system of active volcanoes in terms of architecture, composition, crystallization time-scale, relationships between composition of the erupted magmas and structural position of the vents, and magma processes, is of paramount importance for volcanic hazards evaluation. Investigations aimed at defining the Campi Flegeri magmatic system, include detailed mineralogical, geochemical and isotopic analyses (Sr, Nd, Pb, Th,U). The magmatic feeding system of the Campi Flegrei caldera is characterized by deep and shallow magma reservoirs. In the deep reservoirs (20-10 km depth) mantle- derived magmas differentiated and were contaminated by continental crust. In the shallow reservoirs isotopically distinct magmas, further differentiated, contaminated, and mixed and mingled before eruptions. These processes generated isotopically distinct components, variably interacting with the different structural elements of the Campi Flegrei caldera through time. The relationships between the structural position of the eruption vents, during the last 15 ka of activity, and the isotopic composition of the magmas erupted at the Campi Flegrei caldera allow us to reconstruct the architecture of the magmatic feeding system and to infer the chemical and isotopic composition of the magma feeding a future eruption, according to vent position.
V22A-02
The Campi Flegrei Deep Drilling Project ‘CFDDP': Understanding the Magma-Aquifers Interaction at Large Calderas
Campi Flegrei caldera is a good example of the most explosive volcanism on the Earth, a potential source of
global catastrophes. Alike several similar volcanic areas (Yellowstone and Long Valley, USA; Santorini, Greece;
Iwo Jima, Japan, etc.) its volcanic activity, i.e. eruptions and unrests, is dominated by physical mechanisms
involving the strict interaction between shallow magma sources and geothermal systems. Furthermore, just like
similar areas, it should be characterised by very large shallow magma chambers, filled by residual magma left
after the ignimbritic caldera forming eruptions. However, neither the physical mechanisms of magma-water
interaction, nor the evidence for such large magma chamber, have been ever clear enough to be used for detailed
volcanological interpretation and eruption forecast. The CFDDP project aims to understand, for the first time, the
location and rehology of large residual magma chambers and the mechanisms of interaction between magma
and aquifer systems to generate eruptions and unrests. CFDDP is then structured as a large multidisciplinary
project, with a main volcanological aim and with a further goal to launch a geothermal energy exploitation project
in the area. A larger goal of the CFDDP project is to establish at Campi Flegrei, a densely urbanised area in a
developed western country, a natural laboratory to study volcanic risk, environmental issues, monitoring
technologies, geothermal energy exploitation.
http:www.lab-ov.it
V22A-03
Evolution and hazards of an inactive compound shield volcano: Cofre de Perote (eastern Mexico)
Cofre de Perote is located at the northernmost end of the Cofre de Perote-Citlaltépetl volcanic range that forms the eastern boundary of the Trans-Mexican Volcanic Belt. This range comprises an important physiographic feature separating the Gulf coastal plains from the Altiplano, with a difference in relief of more than 1,000 m causing regional unstable conditions that have favored sector collapse of the volcanoes towards the coast. In contrast to the typical stratovolcanoes of the Mexican Volcanic Belt, Cofre de Perote volcano is dominated by the emplacement of andesistic and dacitic lava flows with very minor proportions of pyroclastic deposits, which gives it the morphology of a shield volcano with gentle slopes. The lava flows erupted through different vents like a compound volcano instead of a single conduit. The summit area of Cofre de Perote volcano is characterized by a prominent set of scarps that as a group show a spectacular horseshoe shape that may be linked to repetitive flank failures. So far, at least two main debris avalanche deposits have been confirmed on the eastern lower slopes of Cofre de Perote towards the Gulf of Mexico. There are five major evolutionary stages that characterize the growth of this compound volcano including: 1) emplacement of a multiple vent dome complex that forms the basal structure of Cofre around 1.3-1.9 Ma; 2) construction of the lower structure of the Cofre compound volcano by at least two main upper vents at 400 ky; 3) building of the summit dome-like lavas through various vents at 240 ky; 4) A large number of Pleistocene-to- Holocene monogenetic cones, likely related to regional volcanism, were erupted through the flanks of the Cofre de Perote edifice; 5) long after the volcano ceased its activity, repetitive sector collapse events occurred causing the truncation of the original volcano shape at 40 and 10 ky. These events show that the upper parts of the volcanic edifice collapsed in the Late-Pleistocene-Holocene, apparently unrelated to any volcanic activity of the central edifice. Therefore, other triggers such as earthquake shaking or unusual heavy rainfall must be considered as the initiating cause.
V22A-04
Fragmentation Processes, Depositional Mechanisms and Lithification of Glassy Fragmental Rocks, Macquarie Island
Hyaloclastite breccia is interlayered with pillow-fragment breccia, pillow lava and basalt sheet flows on Macquarie Island (54 30' S, 158 54' E). These facies formed along the Proto-Macquarie Spreading Ridge (PMSR) ca. 10 Ma and have since been uplifted and exposed on the apex of the Macquarie Ridge Complex. Through a combination of field, microscopic and geochemical analyses, we investigate the submarine production, transportation, and deposition of basalt and sideromelane clasts within a spreading ridge environment, as well as the diagenetic consolidation of their accumulations into hyaloclastite breccia and pillow-fragment breccia rocks. The findings of this study indicate that hyaloclasts form predominantly by cooling-contraction granulation of pillow lava rinds while crystalline basalt clasts are derived from the fragmentation of pillows along concentric and radial cooling joints. Hyaloclastite breccia sediment is transported predominantly by short-lived grain flows, while pillow-fragment breccias form largely in-situ adjacent to source pillow lava. Talus accumulation of crystalline pillow fragments may contribute to the generation of clast-supported units. The above interpretations culminate in the production of a facies model: these rocks formed on the slopes or immediate vicinity of submarine pillow cones along the PMSR and homogenous geochemistry of hyaloclasts in any one stratigraphic section indicate they are supplied from a single magma reservoir. Scanning electron microscopy reveals that palagonite alteration rims on hyaloclasts lithify the sediment. The findings may be used as an analogue for the formation of hyaloclastite breccia and pillow-fragment breccia along past and present mid-oceanic ridges.
V22A-05
Analysis of Ballistics from the Vulcanian Eruptions of 1999 and 2003 of Volcán de Colima: Physical Modelling and Estimate of the Kinetic Energy.
The vulcanian-style explosions of Volcán de Colima have intensified since 1998 and are characterized by the
ejection of fragments of juvenile material and dome. After the effusive phase, which begun in November of 1998,
three explosions of larger magnitude occurred during 1999; then a further effusive phase started in 2001, which
was followed by three explosions in July and August 2003. The object of this study is to analyze the ballistic
projectiles of these explosive eruptions, to characterize them and propose physical models that can help to
understand the eruptive mechanisms. The study is assisting in the improvement of the continuous monitoring
strategy and risk analysis. The influence of the drag force within the atmosphere is considered since it modifies
the trajectories of the projectiles. It depends on factors like: the initial velocity, shape, ruggedness and density of
the fragments or blocks as well as the direction and velocity of the wind. Analysis shows initial sub-sonic
velocities (<1Mach) for an ideal sphere, which implies a drag coefficient of 0.5. The maximum and minimum
diameters of impact craters for the explosions of 1999 and 2003 were 10.2 and 0.15 meters, and 9.3 and 0.15
meters, respectively. For the explosions of 2003 diameters of ballistics were measured in the range 14cm to
1.43m; the maximum calculated height for the projectiles was 835m above the crater; the maximum range
measured in horizontal projection was 2.2 km and the kinetic energy liberated by the total mass was 4.21x107
joules. The kinetic energy liberated during the explosions of July and August 2003, is compared to the seismic
energy quantified with the broadband record.
http:www.ucol.mx/volcan
V22A-06
Volcán de Colima's recent eruptive phase: an experimental synthesis
Volcán de Colima's most eruptive phase of lava dome growth and explosions was investigated through an array of rheological apparatus mimicking physical processes. Samples were chosen which represent the full range of materials present in the conduit and the lava dome; this was ascertained by thorough field-based density distribution campaigns. The viscosity of dome and conduit margin material, i.e., bubble-poor, crystal-rich lavas was investigated using a uniaxial deformation apparatus. The strain-rate dependence of viscosity presents a shear thinning rheology which favours the ascent through a conduit as a plug flow. Yet the lower viscosity of Colima's lava (relatively less viscous than other dome growing lavas) suggest that micro-fracturing may be lesser at the range of strain rates estimated along the conduit margin. Fragmentation and permeability measurements on inner conduit material with a high porosity were achieved through a shock-tube apparatus. For instance, the threshold for fragmentation of 24 % porosity samples required <11 MPa of overpressure, whereas denser samples with 15 % pores fragmented with only 8 MPa. This large threshold at higher porosity is explained by a higher permeability. In fact Colima samples generally exhibit a comparatively high permeability, giving rise to a low explosivity. The results help constrain the ongoing eruptive phase. A relatively dense, yet permeable magma with low pore pressure and buoyancy slowly ascends through the conduit. Slow ascent and relatively low viscosity in turn lessen the occurrence of micro-cracking in the magmas and thus the detection of seismic activity during effusive extrusion periods. Faster effusion episodes, like that of 2004, have been accompanied by far more seismicity with a swarm of LF events occurring prior to the onset of dome growth, resulting from faster ascent. The magma was relatively degassed so did not produce a large explosion. The slow ascent and high permeability further allow significant volatile loss which prevents rapid pressure accumulations necessary for large eruptions; the activity is rather characterized by small explosions daily. The overall level of activity has been increasing since the onset of the eruptive phase in 1998 and our results suggest that progressive sealing and closing of the conduit and lowering of the permeability could lead to much stronger explosive events.
V22A-07
Geophysical Investigations of Soufriere Volcano Crater, St Vincent, West Indies: Where is the Lake?
Soufriere volcano in St Vincent, West Indies, is one of the most active volcanoes of the Eastern Caribbean arc with eruptions in 1812, 1902, 1971-72 and the latest in 1979. Typical eruption style involved dome building phases alternated with more explosive events such as 1902 when over than 1500 people perished due to pyroclastic flows. Between eruptions, the active crater has always been filled by a deep water lake and this lake, although disappearing during the eruptions, always replenished afterwards within a few years. However, since the last eruption in 1979, the crater remained dry. This work addresses the following questions: Is there any trapped water underground below the crater floor? How much? What is the implication of the presence or not of water in terms of future eruptions dynamics and associated volcanic hazards? We present here the results from a recent resistivity survey on the crater floor, around the degassing dome. Data inversion on several 2D profiles shows a shallow groundwater body below the surface and allows estimate of the volume of water trapped in near surface. The estimated amount of water currently trapped in near-surface is much smaller than it has been since historical times. These results thus raise issues about potential for increased explosivity and associated volcanic hazards when Soufriere volcano will next erupt since water vs. magma mass ratio may be much less than it has been for the past hundreds of years.
V22A-08
Seismic Pattern Recognition Techniques to Predict Eruptions at the Popocatepetl, Mexico, Volcano
Using pattern recognition techniques, we formulate a simple prediction rule for a retrospective prediction of the two last largest eruptions of the Popocatépetl (Popo), Mexico, volcano that occurred on 23 April-30 June 1997 (Eruption 1; VEI~2-3) and 11 December 2000-23 January 2001 (Eruption 2; VEI~3-4). Times of Increased Probability (TIP) were estimated from the seismicity recorded by the local seismic network from 1 January 1995 to 31 December 2005. We consider a TIP as a cluster of seismic events which occurs in the Popo region in a temporal window several days (or weeks) prior to large volcanic activity providing sufficient time to organize an effective alert strategy. The best predictions are obtained when averaging seismicity rate over a 5-day window with a threshold value of 11 events and declaring an alarm for 45 days. A TIP was issued about one a half months before Eruption 1. Another TIP was detected about five days before Eruption 2. According to our objectives, in both cases, the observed TIPs would have allowed the development of an effective civil protection strategy. Thus, under our model considerations, the two eruptive events were successfully predicted. However, two "false alarm cases" were also issued by our algorithm. An analysis of the epicentral and depth distribution of the local seismicity used by our prediction rule reveals that successful TIPs were issued from microearthquakes that took place below and towards SE of the crater. On the contrary, the seismicity that issued the two observed "false alarms" occurred only below the summit of the volcano. We conclude that recording of precursory seismicity below and SE of the crater together with detection of TIPs as described here, are an important tool to predict future large eruptions at Popo.