A subterminal radial fissure eruption on Mt. Etna

The author describes a subterminal radial fissure eruption of Mt. Etna that took place in February 1964. A sketchmap and a chemical analysis of the lava are given.

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Zusammenfassung Der Verfasser beschreibt den subterminalen Radialspaltenausbruch des Ätnas im Februar 1964 und fügt eine chemische Analyse der geförderten Lava bei.

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Résumé L'auteur décrit une éruption subterminale de l'Etna qui a eu lieu sur une fissure radiale en février 1964. Une carte de la région sommitale de l'Etna et une analyse chimique de la lava sont ajoutées.

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Riassunto L'autore descrive l'eruzione radiale verificatasi al cono terminale dell'Etna nel Febbraio del 1964. Dà inoltre un rilevamento schematico della zona del cratere centrale ed il chimismo della roccia.

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, 1964 .

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday

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This study is a part of the researches, carried out with the funds of the National Research Council of the Italy under the direction of Prof. A.Ritmann.     

Buoyancy-controlled eruption of magmas at Mt Etna

Buoyancy controls the ability of magma to rise, its ascent rate and the style of the eruptions. Geophysical, geological and petrological data have been integrated to evaluate the buoyancy of magmas at Mt Etna. The density difference between host rocks and magmas is mainly related to the amount of H₂O dissolved in the magma and to the bubble‐liquid separation processes. In the depth interval 22–2 km b.s.l. highly hydrated (H₂O ～ 3%) basaltic magmas or mixtures of bubbles + liquid have positive buoyancy and rise rapidly. Conversely, bubble‐depleted liquids, with an intermediate H₂O content (～ 1.5%), having neutral buoyancy, will spread out and form magmatic reservoirs at different depths until cooling/crystallization further modify composition and density. These different processes account for the magma compositions, location of magmatic reservoirs as determined by geophysical methods, and the complex eruptive cycles (slow effusions, fire fountains and Plinian eruptions) that have been observed in the history of the volcano.     

The 1983 Mount Etna eruption: thermochemical and dynamical inferences

Thermochemical calculations and laboratory phase equilibration experiments on lavas of the 131 day 1983 Mt. Etna flank eruption of 0.1 km³ were undertaken to investigate possible systematic variations in inferred melt-phenocryst equilibration conditions as a function of time. The 1983 Mt. Etna lavas are multiply saturated; plagioclase, clinopyroxene and olivine, the dominant phenocrysts, occur in the ratio 1:1/2:1/4. Melts (glasses) plot close to the plagioclase saturated olivine-clinopyroxene low pressure cotectic on a Walker-O'Hara diopside-forsterite-silica diagram suggesting equilibration of melt and phenocrysts in a high level magma reservoir. Total pressures, temperatures and dissolved H₂O concentrations were calculated using the isoactivity method of Carmichael and coworkers based on about 300 elelctron microprobe analyses of coexisting olivine, clinopyroxene and plagioclase phenocrysts, microphenocrysts and groundmass microlites for samples collected 6, 46 and 125 days after the start of the eruption. Total pressures (P _t), temperatures and H₂O contents based on representative olivine-clinopyroxene pairs are 140 MPa, 1105°C, 2.4 wt% H₂O; 255 MPA, 1112°C, 1.0 wt% H₂O and 85 MPa, 1096°C, 1.8 wt% H₂O respectively for the early (283), middle (I83) and late (L83) samples. Corresponding equilibration depths are in the range 3 to 10 kilometers. Plagioclase feldspar phenocrysts, while showing more evidence of disequilibrium, provide compatible estimates of P _t and T when analysis is restricted to the low anorthite mode of the plagioclase frequency-composition histograms: 133 MPa and 1115°C; 260 MPa and 1117°C and 103 MPa and 1104°C, repectively for 283, I83 and L83. The pre-eruptive (i.e., in situ) temperature-pressure gradient calculated from olivine-clinopyroxene equilibria is 10.6 K/kbar. This compares well with independent estimates of the temperature-pressure derivative of the (pseudo) invariant point composition (10 to 12 K/kbar) in both model (e.g., diopside-forsterite-anorthite, Presnall et al. 1978) and natural (e.g., Walker et al. 1979; Grove et al. 1982) systems. Apparently, magma within the Etna reservoir was in a quasiequilibrium state buffered by its multiply-saturated character immediately preceding eruption. The temporal variation of computed P _t, T and H₂O concentrations for melt-phenocryst equilibrium agrees well with predictions based on simulations of the withdrawal of magma from a body zoned with respect to dissolved H₂O provided the temporal record of magma discharge is taken into account. Discharge varied by a factor of about 100 during the sample collection interval. The intermediate P _t but high H₂O content inferred for sample 283 reflects the withdrawal of H₂O enriched magma during an early phase of high average discharge of about (350 m³/s) before evaculation isochrons became quasistationary. The high P _t and relatively dry I83 magma reflects the deepening of the evacuation isochrons after 50 days of intermediate discharge with the development of quasi-stationary isochrons in time and space. Sample L83 from day 125 near the end of the eruption reflects the shoaling of evacuation isochrons (hence low P _t and relatively high H₂O content) associated with the observed low (0.5 m³/s) discharge. Our results show that thermochemical modeling efforts provide important opportunities for testing the predictions of magma with-drawal simulations.     

Slope movement crisis on the east flank of Mt. Etna volcano: Models for eruption triggering and forecasting

Benchmarks installed on the upper eastern flank of Mt. Etna in 1982 have subsided continually since then, with the rate of subsidence twice accelerating prior to eruptions. The first of these eruptions was in December 1985, and the second in September 1989. This pattern of accelerating downslope movement has also been observed prior to landslides, and recent work applying knowledge of the failure of materials has shown that analysis of the inverse rate of these movements can be used to predict the time of failure. Post-eruption analyses of geodetic or seismic data from volcanoes has shown that in several cases, this approach could have been used to forecast eruptions weeks in advance. Applying the same principles to the accelerating subsidence on Mt. Etna's eastern flank prior to the eruptions of 1985 and 1989 shows that rough estimates of eruption dates could have been obtained several months in advance. These observations also suggest a speculative eruption-triggering mechanism involving an interplay between slope creep deformation and extensional weakening over the zone of intrusion.     

Lava flow simulations using discharge rates from thermal infrared satellite imagery during the 2006 Etna eruption

Techniques capable of measuring lava discharge rates during an eruption are important for hazard prediction, warning, and mitigation. To this end, we developed an automated system that uses thermal infrared satellite MODIS data to estimate time-averaged discharge rate. MODIS-derived time-varying discharge rates were used to drive lava flow simulations calculated using the MAGFLOW cellular automata model, allowing us to simulate the discharge rate-dependent spread of lava as a function of time. During the July 2006 eruption of Mount Etna (Sicily, Italy), discharge rates were estimated at regular intervals (i.e., up to 2 times/day) using the MODIS data. The eruption lasted 10 days and produced a ~3-km-long lava flow field. Time-averaged discharge rates extracted from 13 MODIS images were utilized to produce a detailed chronology of lava flow emplacement, demonstrating how infrared satellite data can be used to drive numerical simulations of lava flow paths during an ongoing eruptive event. The good agreement between simulated and mapped flow areas indicates that model-based inundation predictions, driven by time-varying discharge rate data, provide an excellent means for assessing the hazard posed by ongoing effusive eruptions.     

Seismological constraints on the 2018 Mt. Etna (Italy) flank eruption and implications for the flank dynamics of the volcano

3D earthquake locations, focal mechanisms and stress tensor distribution in a 16‐month interval covering the 2018 Mt. Etna flank eruption, enabled us to investigate the relationship between magma intrusion and structural response of the volcano and shed light on the dynamic processes affecting the instability of Mt. Etna. The magma intrusion likely caused tension in the flanks of the volcano, leading to significant ground deformation and redistribution of stress on the neighbouring faults at the edge of Mt. Etna's unstable sector, encouraging the ESE sliding of the eastern flank of the volcano. Accordingly, FPSs of the post‐eruptive events show strike slip faulting mechanisms, under a stress regime characterized by a maximum compressive σ₁, NE‐SW oriented. In this perspective, any flank eruption could temporarily enhance the sliding process of both the southern and eastern flanks of the volcano.     

Large dyke intrusion and small eruption: The December 24, 2018 Mt. Etna eruption imaged by Sentinel‐1 data

On December 24th, Mt. Etna volcano underwent a seismic crisis beneath the summit and upper southern flank of the volcano, accompanied by significant ash emission. Eruptive fissures opened at the base of summit craters, propagating SE‐wards. This lateral eruption lasted until December 27th. Despite the small eruption, seismic swarm and ground deformation were very strong. Sentinel‐1 interferograms show a wide and intense ground deformation with some additional features related to volcano‐tectonic structures. We inverted DInSAR data to characterise the magma intrusion. The resulting model indicates that a large dyke intruded but aborted its upraise at about the sea level; however, this big intrusion stretched the edifice, promoting the opening of the eruptive fissures fed by a shallower small dyke, and activating also several faults. This model highlights that a big intrusion beneath a structurally complex volcano represents a main issue even if the eruption is aborted.     

The 18.7 ka phreatomagmatic flank eruption on Etna (Italy): relationship between eruptive activity and sedimentary basement setting

This paper documents a phreatomagmatic flank eruption that occurred 18 700 ± 100 a BP , on the lower north-eastern slope of Etna during the Ellittico volcano activity, which produced fall and surge deposits. This type of eruption is connected to a sedimentary basement ridge at Etna. The interaction between the rising magma and the shallow groundwater hosted in the volcanic pile overlying the impermeable sediments resulted in phreatomagmatic instead of strombolian activity. Three eruptive phases are distinguished based on field and analytical data: (i) an explosive phreatomagmatic opening, (ii) a main phase producing coarse lithic-rich fallout and a strombolian deposit, and (iii) the final pulsating surge-forming phase. The discovery of this phreatomagmatic flank eruption, which occurred at lower altitude, raises important issues for previous hazard assessments at Etna.     

A new dyke intrusion style for the Mount Etna May 2008 eruption modelled through continuous tilt and GPS data

After a recharge phase that began in 2007, on 13 May 2008, a new eruption started on Mt. Etna volcano. The final intrusion was very fast, accompanied by a violent seismic swarm and marked by ground deformation recorded at permanent tilt and GPS stations. The violence of the eruptive event generated concern that the eruptive fissures might propagate downslope towards populated areas. The ground deformation modelling explains both the mechanism of the intrusion as well as the attempt of the dyke to propagate in the shallower part of the northern sector of the volcano. We show that the 2008 intrusion was characterized by a mechanism, which is new and different to the ones modelled in previous eruptions, following the path of the central conduit in the first part of the intrusion (below 1.6 km) and then breaking off towards the east in the last shallow part.     

Geodetic techniques applied to the study of the Etna volcano area (Italy)

Volcanic behaviour of Mt. Etna is due to the complex interaction between both the local and the regional stress field involving the eastern Sicily. Eruptions could trigger (be triggered?) during crust extension and/or compression, which are strictly linked with the dynamics of the lower mantle. In this study, very long baseline interferometry (VLBI) space geodesy technique has been used in order to study Etna volcano’s activity by means of the crustal deformations between Noto and Matera (located on the African and the Eurasian Plates, respectively). By analysing VLBI data, we obtained the behaviour of the baseline which crosses the Etnean area, from 1990 December to 2003 March, representing the time variations of the distance between the two geodetic stations; the linear trend of the baseline shows a general increasing, pointing out an extension of the crust between them. A detailed analysis of the Noto-Matera baseline allows the identification of three parts of the VLBI curve in the considered period. In the first part of the curve (from 20/12/90 to 09/02/94), VLBI data are rather poor and therefore no reliable consideration about correlation between crust movements and volcanic and seismicity activity has been made. In the second part of the curve (from 09/02/94 to 04/09/00), VLBI data are more frequent and show slightly fluctuations in the distance. Increasing in the extension and compression were observed in the central and in the final part of this period. In the third period (from 04/09/00 to 25/03/03), VLBI data are very sparse even if the time series was quite long; therefore, to fill gaps in the information, we analysed global positioning system (GPS) data. GPS technique performs continuous observations, and we were able to highlight both extensions and compressions in detail. Comparisons between the trend of Noto-Matera baseline length variations, volcanic activity and seismicity in the Etna area show the complexity of the development over time and space of the phenomenology determined by a deep cause which can be traced, in our opinion, to the interaction between the asthenospheric mantle, deep crust and surface crust. Therefore, we state that crustal distension and compression are determined by the lower pulsating mantle.     

Some considerations on the recent tectonic stress field of China

Based on the improved hydrofracturing method, the in-situ principal stresses are determined in the north, northeast, south and the northwest of China, and the results of many fault-plane solutions, crustal deformations, geological and geophysical observations are used to discuss the distribution of subcrustal stress in space, the difference between seismically active regions and stable regions, and the variations of the stress field near the source region before and after the Tangshan earthquake. Finally, the characteristics of intraplate tectonic stress fields in China are explained in relation with the combined actions of the movement of the major Indian Ocean, Pacific and Philippine Sea plates.     

Focal parameters of seismic sources during the 1981 and 1983 eruption at Mt. Etna volcano (Sicily,Italy)

In this study 50 seismic events, preceding and accompanying the eruptions occurring in 1981 and 1983, have been considered. Seismic moments, fault radii, stress drops and seismic energies have been calculated using Brune’s model (J Geophys Res 75:4997–5009, 1970; J Geophys Res 76:5002, 1971); site, anelastic attenuation along the propagation path, geometrical spreading and interaction with the free surface effects are taken into account. For each event we have also estimated the equivalent Wood–Anderson magnitude (MWAeq) (Scherbaum and Stoll in Bull Seism Soc Am 73:1321–1343, 1983); relations among all these source parameters have been determined. Furthermore, the hypothesis of self-similarity (Aki in J Geophys Res 72:1217–1231, 1967) is not verified for events with seismic moments <10¹² N-m: in fact the relationship between log-stress drop and log-moment is linear up to a moment of 10¹² N-m (events of 1981 eruption), while for higher moments (events of 1983 eruption) the slope of the regression line is not significantly different from zero. We suppose that such a behaviour is related to a heterogeneous medium with barriers on the faults. Finally, the main conclusion is that eruptions of 1981 and 1983 differ from one another both in eruptive and seismic aspects; analysis of seismic energies indicates an increase in Mt. Etna’s activity, confirmed by studies performed on the following lateral eruption of 1991–1993 (Patanè et al. in Bull Volcanol 47:941–952, 1995), occurring on the same structural trend.     

Geodetic control on recent tectonic movements in the central Mediterranean area

During the Neogene and Quaternary, the western Mediterranean geodynamics was apparently dominated by the nearly eastward migration of the Apenninic arc and the associated opening (spreading) of the back-arc basin (Tyrrhenian Sea). However, during the last 5 My, the collision of the arc with the Apulian platform led to a dramatic change in the tectonic setting of the area. As geological processes require a long period of time to register the displacements of the different blocks, it is indispensable to take into account the present-day motion given by space geodesy data analysis in order to better constrain the geological models.Geodetic motions were derived from Global Positioning System (GPS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) observations collected from different networks. All the geodetic solutions have been computed and combined at the Centre of Space Geodesy (CGS), at Matera, Italy.The geodetic results show a NNE motion of the Adriatic plate with a small component of counter-clockwise rotation, in good agreement with the geological and geophysical observations.In the southern Tyrrhenian area, the lengthening of the Matera–Cagliari baseline should imply that convergence cannot be considered as the driving mechanism for the Apenninic subduction process. The estimated motion of Noto is in quite good agreement with the estimated motion of the African plate.     

对造山带古地理学在盆地构造——古地理重建中的若干思考

造山带古地理学在盆地构造--古地理重建中的若干思考的核心，是以活动论的构造观指导盆地分析。本文强调以发展的、动态演化的思路来研究盆地。现存的一个盆地，可能只是地史期间大盆地的残余（或残留）部分；或是原来彼此分开和独立的几个盆地内的沉积记录组成的沉积物拼盘。一个盆地内现存的次级构造单元可能由年轻的造山运动形成；另一种可能性则是原来的次级单元已被后来的构造-热事件模糊或遮蔽了。在盆地的建造过程中，隆起（包括次级隆起）的时起时伏十分常见；相应地，盆地间的时分时合也十分常见。在泛大陆旋回与大盆地发育间有着内在联系，即：大盆地发育受邻侧的洋盆演化和造山过程制约，故“盆”“山”间的耦合分析是正确认识盆地动态演化的钥匙。体现在构造--古地理重建中，造山带古地理学倡导原地的古地理重建与非原地的古地理重建相结合且以后者为主。非原地的古地理重建可通过定量的（如基于古地磁资料的重建）和定性的两条途径实现。文章讨论了定性重建中的5个要点：1）时间上，要结合“反序”的研究，2）空间上，要结合“反转构造”的研究，3）强调构造复位在古地理重建中的意义，4）盆地建造阶段中的改造事件所起的承前启后作用，和5）盆地动力学与造山动力学的结合研究。     

Some considerations concerning seismic geomorphology and paleoseismology

Seismic geomorphology studies landforms which developed in connection with earthquakes. Among them, two different end members may be distinguished: 1) seismo-tectonic landforms, including surface faults and fractures, land uplift and subsidence at different scales, surface bulges, elongate ridges, and any other permanent ground deformations directly related to tectonic stress, and 2) seismo-gravitational landforms, such as landslides, deep-seated gravitational slope deformations, sinkholes, and fissures due to sediment compaction or liquefaction and sand blows, connected with both seismic shaking and gravitational stress.A clear-cut distinction between the two categories of landforms is not always easy to make (and in many instances not really useful), while there are, in many cases, ground effects that might be (and should be) considered as simultaneous combinations of seismo-tectonic and seismo-gravitational processes. This applies especially to surface fracturing and faulting which could be the combined result of tectonic stress, stress produced by seismic shaking, and gravitational stress.The objective of this paper is to review selected case histories mainly from Italy and the Mediterranean region, in order to show the importance of a comprehensive study of earthquake-generated landforms for understanding the seismicity level of the area under investigation. We argue that in earthquake prone areas, seismic landforms often constitute typical patterns (seismic landscapes) whose recognition, mapping and paleoseismological analysis may help in the evaluation of seismic hazards.     

Rhyolitic pumice in the basaltic pyroclasts from the 1971 eruption of Teneguía volcano,Canary Islands

Fragments of acid pumice have been found among the basic pyroclastics thrown out by the Teneguía volcano during its explosive phases. The presence of these pumice fragments brings up the problem of their origin and their relationships to the mechanism of the eruption.The composition of the analysed samples coincides exactly with the low temperature trough in petrogeny's residua system, albits-orthoclase-silica. The samples can be classified as rhyolites and trachytes.The authors propose that the pumice was produced by the fusion of the acid phase in a subvolcanic complex located beneath the island of La Palma.     

造山带古地理学——重建区域构造古地理的若干思考

以活动论构造观为指导,尽可能客观地复原地史期间海陆面貌和盆山格局的关键是在给定时间约束之后,于研究区现存的地质记录中发现业已消失了的和后期才出现的构造古地理单元。文章基于大量实例论述了可能导致构造古地理单元消失和后期出现的各种情况,如大陆地块的解体、沉没、俯冲、剥蚀、冲断-推（滑）覆作用、走滑作用、碎裂和被卷入年轻造山带等,特别提出东亚大陆边缘地区自侏罗纪末开始的移置地体拼贴增生是该区的一大特点。构造古地理单元消失和后期出现的发生大都与巨型断裂带（尤其是作为构造边界的巨型断裂带）发育有关。陆内阶段的断裂带活动对盆山格局的变化、盆地发育、沉积演化及其关键地质事件有明显的控制作用,还应注意断裂带解体阶段出现的新生构造对古地理重建的制约。简言之,造山带古地理学的发展是推动构造古地理学前进的关键之一,在恢复古特提斯多岛洋的构造古地理面貌、重建全球古大陆演化尤其是造山带和经受造山运动改造地区的古大陆演化,有着积极的不可或缺的作用。     

Case studies on the origin of basalt: III. Petrogenesis of the Mauna Ulu eruption,Kilauea, 1969–1971

During the Mauna Ulu flank eruption on Kilauea, Hawaii, the concentrations in the lavas of the minor elements K, P, Na and Ti, and the incompatible trace elements (analyzed by isotope dilution) K, Rb, Cs, Ba, Sr, and the REE (except Yb) decreased monotonically and linearly with the time (or date) of the eruption. At the same time, the concentrations of the major elements and of Yb, and the ratios of K/Rb, K/Cs, Ba/Rb, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd remained constant. Most of the scatter in the raw concentration data is removed by a simple correction for olivine (plus chromite) fractionation previously established by Wright et al. (1975). These results are explained by simple equilibrium partial melting of a uniform source. The degree of melting increased by about 20% of the initial value during the course of the eruption. The trace element data are inverted by the method originated by Minster and Allègre (1978) and simplified by Hofmann and Feigenson (1983). The source has the following element (or isotope) ratios: K/Rb=501±7, Ba/Rb=14.0±0.5, Rb/Cs=95±7, Rb/Sr=0.0193 (+0.0045, –0.0090), (Ce/Ba)_CN= 1.1±0.1, (Sr/Ba)_CN=1.19 (+0.30, –0.19), ⁸⁷Sr/⁸⁶Sr=0.703521±0.000016, and ¹⁴³Nd/¹⁴⁴Nd=0.512966±0.000008. The REE pattern of the source has a nearly flat or slightly negative slope (=relative LREE enrichment) between Ce and Dy and a strongly positive slope between Dy and Yb. However, this relative HREE enrichment is poorly constrained by the analytical data, is highly model dependent and may not be a true source feature. The Yb concentration in the source is particularly poorly constrained because it is essentially constant in the melts. On the other hand, this special feature demonstrates that Yb must be buffered by a mineral phase with a high partition coefficient for Yb, namely garnet. The calculated clinopyroxene/garnet ratio in the source is roughly equal to one. In contrast, the source of Kohala volcano had previously been found to contain little or no garnet.