Jean-Luc Le Pennec

Ignimbrites of Cappadocia (Central Anatolia, Turkey): petrology and geochemistry

Abstract In Cappadocia (Central Anatolia, Turkey), high-K calc-alkaline volcanic rocks have a volume of at least 1000 km3 and cover an area of about 40,000 km2. Rhyolitic to dacitic ignimbrites and two andesitic lava flows were erupted from Upper Miocene (11.2 Ma) to Quaternary times, in relation to the collision of the Arabian and Eurasian plates. K-rich rhyolitic and locally Na-rich dacitic ignimbrites are commonly intercalated with lacustrine sediments and, more rarely, with andesitic lava flows. Each ignimbrite exhibits its own mineralogical association and trace-element chemistry that enable stratigraphic correlations (i.e., Rb and Sr or Fe, Mg, Mn and Ti contents of biotite). Geochemical data (major, trace elements and Sr–Nd isotopes) show that the origin of the studied volcanic units can be related to fractional crystallisation of a mantle-derived magma. However, crustal contamination is also thought to be a major process that evolved through time. From Miocene (11.2 Ma) to Quaternary times, the ignimbritic rocks exhibit a drastic decrease of 87 Sr / 86 Sr ratio which may be linked with the transition from collisional to extensional tectonics.

Interpretation of anisotropy of magnetic susceptibility fabric of ignimbrites in terms of kinematic and sedimentological mechanisms: An Anatolian case-study

Abstract A study of the Neogene low-aspect-ratio Kizilkaya ignimbrite (in the calc-alkaline Central Anatolian Volcanic Province, Turkey) has been carried out using the anisotropy of magnetic susceptibility (AMS) technique on 46 sites. IRM, hysteresis loops and Curie temperature analysis indicate that magnetite is the main carrier of the magnetic signal in the ignimbrite. The shape of the AMS ellipsoids varies widely from site to site and the anisotropy is commonly low: we review the data processing used in the literature on the AMS of ignimbrites to calculate principal directions or to evaluate the quality of the results, and we develop various simple techniques (data filtering and contouring procedures) in order to more accurately define the axis or symmetry of the AMS fabric. There is no simple relationship between the shape of the AMS ellipsoids and the shape of the petrofabric ellipsoids. Similarly, there is no direct relationship between magnetic and kinematic axes. Our results show that the source of the measured magnetic signal in these rocks is a complex result of various contributions including shape anisotropy of free and fixed magnetic crystals (shape AMS), crystallization of magnetic grains on all surfaces and discontinuities during cooling of the ignimbrite (distribution anisotropy), and alteration by hydrothermal and meteoric water. Particular attention is paid to the interpretation of the AMS stereoplots departing from the “standard” fabric of sedimentary rocks formed under unidirectional flow. In order to explain these “non-standard” AMS fabrics, we discuss their possible origin: (1) in analytical and mineralogical terms (instrumental artifact, pollution by xenoclasts, mineralogical inversion of the susceptibility axes, secondary mimetic fabric); and (2) in terms of sedimentological mechanisms (rheology, depositional processes). In the present state of knowledge, we conclude that, due to the physico-chemical complexity of the magnetic source, the AMS signal should be used and interpreted with care when attempting to decipher the sedimentological or rheological processes occurring in moving pyroclastic flows. On the other hand, AMS has it greatest potential when used in conjunction with other techniques, and when a large number of sampling sites and specimens is used.

Estimating rates of decompression from textures of erupted ash particles produced by 1999–2006 eruptions of Tungurahua volcano, Ecuador

Persistent low- to moderate-level eruptive activity of andesitic volcanoes is difficult to monitor because small changes in magma supply rates may cause abrupt transitions in eruptive style. As direct measurement of magma supply is not possible, robust techniques for indirect measurements must be developed. Here we demonstrate that crystal textures of ash particles from 1999 to 2006 Vulcanian and Strombolian eruptions of Tungurahua volcano, Ecuador, provide quantitative information about the dynamics of magma ascent and eruption that is difficult to obtain from other monitoring approaches. We show that the crystallinity of erupted ash particles is controlled by the magma supply rate (MSR); ash erupted during periods of high magma supply is substantially less crystalline than during periods of low magma supply. This correlation is most easily explained by efficient degassing at very low pressures (<<50 MPa) and degassing-driven crystallization controlled by the time available prior to eruption. Our data also suggest that the observed transition from intermittent Vulcanian explosions at low MSR to more continuous periods of Strombolian eruptions and lava fountains at high MSR can be explained by the rise of bubbles through (Strombolian) or trapping of bubbles beneath (Vulcanian) vent-capping, variably viscous (and crystalline) magma.

Eruptive parameters and dynamics of the April 2015 sub-Plinian eruptions of Calbuco volcano (southern Chile)

We conducted geological and petrological analyses of the tephra fallout and pyroclastic density current (PDC) products of the 22-23 April 2015 Calbuco eruptions. The eruptive cycle consisted of two sub-Plinian phases that generated > 15 km height columns and PDCs that travelled up to 6 km from the vent. The erupted volume is estimated at 0.38 km3 (non-DRE), with approximately 90% corresponding to tephra fall deposits and the other 10% to PDC deposits. The erupted products are basaltic-andesite, 54-55 wt.% SiO2, with minor amounts of andesite (58 wt.% SiO2). Despite the uniform composition of the products, there are at least four types of textures in juvenile clasts, with different degrees of vesicularity and types and content of crystals. We propose that the eruption triggering mechanism was either exsolution of volatiles due to crystallization, or a small intrusion into the base of the magma chamber, without significant magma mixing or with a magma compositionally similar to that of the residing magma. In either case the triggering mechanism generated convection and sufficient overpressure to promote the first eruptive phase. The start of the eruption decompressed the chamber, promoting intense vesiculation of the remaining magma and an increase in eruption rate towards the end of the eruption.

The milling factory: Componentry-dependent fragmentation and fines production in pyroclastic flows

In order to decipher the mobility of hazardous small-volume pyroclastic flows (PFs), we investigate here the role of clast fragmentation and production of fine particles (<2 mm) in controlling PF dynamics. We report a high-resolution textural data set (i.e., grain size distribution, clast componentry, and clast morphology) of andesitic scoriaceous PF deposits from Tungurahua volcano (Ecuador) based on a unique method and data processing procedure. The strong relationship shown between clast size and componentry in PF deposits is quantified through a componentry versus size index (CSI), which varies between −1 and +1 (all components in the coarse or fine fractions, respectively). Our results show that the incorporated old lava clasts (CSI of −0.33) control early grinding of the pristine juvenile bombs (CSI of −1), which produce smaller scoriaceous andesitic fragments (CSI of 0.26). These latter interact in the granular assemblage and release fine particles through the abrasion processes, as supported by the morphological data. The textural maturity of the deposits reveals that the “milling factory” works before sedimentation begins (i.e., on steep upper slopes) and that there is some balance between fines production and loss. The process evident here tends to redistribute stress into the moving flow and produces fines that contribute to fluidization of it. The milling factory is thus likely to enhance the mobility of geophysical flows, which has strong implications for modeling and hazard assessment purposes.

Fragmental lava versus welded ignimbrite on Mount Etna: arguments inferred from crystal preferred orientation

Abstract Structural and textural similarities of some silicic lavas with strongly welded tuffs often lead to alternative interpretations of silicic rocks with doubtful origin. Such silicic “lavas” associated with pyroclastic deposits of benmoreitic composition were described on Mount Etna. In the first part of this paper we discuss the stratigraphic and lithologic arguments that lead to correlate these products with the “Cratere Ellittico” caldera. In a second part, we focus on the emplacement mechanism of these foliated “lava-like” flows and we reinterpret them as the result of welding and local rheomorphism of pyroclastic flow deposits. To discriminate between the two processes (fragmentation of a flowing lava or flattening of a fragmental deposit) we make a finite-strain analysis deduced from crystal orientation density distribution functions (DDF) in selected planes of the samples. Results of strain ratios show good agreement with previous values obtained by others on welded pyroclastic flow or welded air-fall deposits. We conclude that the “Cratere Ellittico” caldera is a major explosive event in the evolution of Etna. On the southwestern flank of the volcano the deposits of this eruption arc known as the Biancavilla-Montalto ignimbrite.

Mass budget partitioning during explosive eruptions: insights from the 2006 paroxysm of Tungurahua volcano, Ecuador

How and how much the mass of juvenile magma is split between vent-derived tephra, PDC deposits and lavas (i.e. mass partition) is related to eruption dynamics and style. Estimating such mass partitioning budgets may reveal important for hazard evaluation purposes. We calculated the volume of each product emplaced during the August 2006 paroxysmal eruption of Tungurahua volcano (Ecuador) and converted it into masses using high-resolution grainsize, componentry and density data. This dataset is one of the first complete descriptions of mass partitioning associated with a VEI 3 andesitic event. The scoria fall deposit, near-vent agglutinate and lava flow include 28, 16 and 12 wt. % of the erupted juvenile mass, respectively. Much (44 wt. %) of the juvenile material fed Pyroclastic Density Currents (i.e dense flows, dilute surges and co-PDC plumes), highlighting that tephra fall deposits do not depict adequately the size and fragmentation processes of moderate PDC-forming event. The main parameters controlling the mass partitioning are the type of magmatic fragmentation, conditions of magma ascent, and crater area topography. Comparisons of our dataset with other PDC-forming eruptions of different style and magma composition suggest that moderate andesitic eruptions are more prone to produce PDCs, in proportions, than any other eruption type. This finding may be explained by the relatively low magmatic fragmentation efficiency of moderate andesitic eruptions. These mass partitioning data reveal important trends that may be critical for hazard assessment, notably at frequently active andesitic edifices. This article is protected by copyright. All rights reserved.

Magnetic fabric of ignimbrites: a case study from the Central Anatolian Volcanic Province

Abstract The magnetic fabric of the Pliocene Kızılkaya ignimbrite in the Central Anatolian Volcanic Province has been investigated by anisotropy of magnetic susceptibility (AMS) and isothermal remanent magnetization (AIRM). Seven sections were sampled at various stratigraphic heights within the devitrified portion of the ignimbrite. The magnetic mineralogy is complex: titanomagnetite occurs as magmatic grains, and as inclusions in other phenocryst and glass shards; an oxidized phase and hematite occur in deposit levels affected by alteration processes. The disturbance produced by lithic and pumice clasts has been reduced by discarding the specimens that deviate more than ±1σ from the site mean value of the density. The AMS fabric varies along each individual section. Neither the AMS magnetic lineation nor the magnetic foliation plunge clearly define a common area as the vent location. The AIRM fabric of low-coercivity minerals, mainly represented by free titanomagnetite grains of magmatic origin, is consistent between sites and the inferred flow directions converge on a region near Derinkuyu, in the Nevsehir plateau, previously reported as the Kızılkaya ignimbrite source area. This study shows that systematic use of the remanent fabric improves the results given by AMS and aids the identification of the primary magnetic fabric related to the ignimbrite emplacement dynamics.

Managing decision-making with certainty of threat

Several factors, such as emotion and uncertainty of the outcome, influence decision-making. We assessed decision-making during a risky event (natural hazard, focusing here on two types of volcanic threats) by manipulating the certainty of lethal threat in an information campaign. We hypothesized that the reduction of uncertainty of lethal threat in an information campaign would improve behavior through more suitable choices by reducing the use of emotional choices. In the scenario that occurred in a familiar place, participants who received information with uncertainty of lethal threat presented more emotional and comfortable choices, such as staying at home, rather than detached ones, such as leaving the area. These were either appropriate (for volcanic ash cloud) or inappropriate (for pyroclastic flow). The certainty hypothesis was partially validated, as certainty influenced the quality of choice in the scenarios that took place at home. Furthermore, participants in the volcanic disaster context presented less suitable decisions compared to those in the neutral context, which was discussed in terms of the presence of emotions, such as fear of volcanic eruption. Our results highlight the importance of controlling the comforting emotional aspect of the home environment in any information communication.

Influence of uncertainty on framed decision-making with moral dilemma

In cases of impending natural disasters, most events are uncertain and emotionally relevant, both critical factors for decision-making. Moreover, for exposed individuals, the sensitivity to the framing of the consequences (gain or loss) and the moral judgments they have to perform (e.g., evacuate or help an injured person) constitute two central effects that have never been examined in the same context of decision-making. In a framed decision-making task with moral dilemma, we investigated whether uncertainty (i.e., unpredictably of events) and a threatening context would influence the framing effect (actions framed in loss are avoided in comparison to the ones framed in gain) and the personal intention effect (unintentional actions are more morally acceptable in comparison to intentional actions) on the perceived moral acceptability of taking action. Considering the impact of uncertainty and fear on the processes underlying these effects, we assumed that these emotions would lead to the negation of the two effects. Our results indicate that the exposure to uncertain events leads to the negation of the framing effect, but does not influence the moral acceptability and the effect of personal intention. We discuss our results in the light of dual-process models (i.e. systematic vs. heuristic), appraisal theories, and neurocognitive aspects. These elements highlight the importance of providing solutions to cope with uncertainty, both for scientists and local populations exposed to natural hazards.