Stefano Branca

Etna 2004–2005: An archetype for geodynamically‐controlled effusive eruptions

[1]xa0The 2004–05 eruption of Etna was characterised by outpouring of degassed lava from two vents within Valle del Bove. After three months of eruption lava volumes were estimated to be between 18.5 and 32 × 106 m3, with eruption rate between 2.3 and 4.1 m3/s. Petrological analyses show that magma is resident in the shallow plumbing system, emplaced during the last South-East Crater activity. SO2 flux data show no increase at the onset of the eruption and SO2/HCl ratios in gas emitted from the eruptive fissure are consistent with a degassed magma. No seismic activity was recorded prior to eruption, unlike eruptions observed since the 1980s. The purely effusive nature of this eruption, fed by a degassed, resident magma and the fracture dynamics suggest that magmatic overpressure played a limited role in this eruption. Rather, lateral spreading of Etnas eastern flank combined with general inflation of the edifice triggered a geodynamically-controlled eruption.

Analysis of the 2001 lava flow eruption of Mt. Etna from three‐dimensional mapping

[1]xa0The 2001 Etna eruption was characterized by a complex temporal evolution with the opening of seven eruptive fissures, each feeding different lava flows. This work describes a method adopted to obtain the three-dimensional geometry of the whole lava flow field and for the reconstruction, based on topographic data, of the temporal evolution of the largest lava flow emitted from a vent located at 2100 m a.s.l. Preeruption and posteruption Digital Elevation Models (DEM) were extracted from vector contour maps. Comparison of the two DEMs and analysis of posteruption orthophotos allowed us to estimate flow area, thickness, and bulk volume. Additionally, the two-dimensional temporal evolution of the 2100 flow was precisely reconstructed by means of maps compiled during the eruption. These data, together with estimates of flow thickness, allowed us to evaluate emitted lava volumes and in turn the average volumetric flow rates The analysis performed in this paper provided, a total lava bulk volume of 40.1 × 106 m3 for the whole lava flow field, most of which emitted from the 2100 vent (21.4 × 106 m3). The derived effusion rate trend shows an initial period of waxing flow followed by a longer period of waning flow. This is in agreement not only with the few available effusion rate measurements performed during the eruption, but also with the theoretical model of Wadge (1981) for the temporal variation in discharge during the tapping of a pressurized source.

Eruptions of Mt. Etna during the past 3,200 years: A revised compilation integrating the historical and stratigraphic records

A critical re-examination of the original chronicles and previously published summaries of Etnean activity, within the context of recently available stratigraphic, mapping, and radiometric data, has made possible a detailed reconstruction of Etnas historical eruptions during the past three millennia. Our compilation, updated to the 2002 flank eruptions, lists only those events for which both age and the corresponding deposit are known, thereby eliminating inaccurate, uncertain, or poorly located volcanic events contained in the previous compilations. Not surprisingly, discrepancies between the historical and stratigraphic databases were noted for the events before 1600 AD, after which, however, the two databases are in good agreement. By excluding questionable eruptions, our compilation necessarily underestimated the number of pre-1600 events, when compared with that determined from historical sources. Our study indicates that, during the period from 1600 to 1975, the eruption frequency and lava output of Etna were relatively uniform; since 1975, however, both these parameters increased markedly. To understand the significance and possible implications of this increased activity during 1975-2000, a more complete reconstruction of Etnas prehistoric and historical eruptive history will be essential.

Intrusive mechanism of the 2002 NE‐Rift eruption at Mt. Etna (Italy) inferred through continuous microgravity data and volcanological evidences

[1]xa0In October 2002 a new eruption began at Etna. Lava flows were issued from two different fissure systems on the NE and S flanks of the volcano. A continuous microgravity sequence, acquired at a station on the N slope (2800 m a.s.l.), shows a marked decrease (about 400 μGal in less than one hour) about 4 hours before lava was first emitted from the eruptive fissures along the NE-Rift. This anomaly reversed soon afterward at a high rate. The strong gravity decrease is interpreted as the opening, by external forces, of a shallow fracture system 1 km W of the gravity station. Magma from the central conduit entered the new fracture system passively, and propagated through it towards lower portions of the NE-Rift. Both the arrangement of the new fracture system and the eruptive dynamics are in keeping with the inferred intrusive mechanism.

Geological Evolution of Etna Volcano

New stratigraphic and structural data obtained during recent geological surveys have allowed us to subdivide the almost continuous evolution of Etnean volcanism into four main phases. The oldest phase (Basal Tholeiitic) corresponds to a long period of dispersed fissure-type volcanism with tholeiitic affinity, from about 580 up to 260 ka. This phase represents the northward extension of the Plio-Pleistocene Hyblean volcanism to the Etnean region. The second phase (Timpe) started about 220 ka when eruptive activity was mainly concentrated on the Ionian coast along the NNW-SSE oriented Timpe fault system. In this area the occurrence of fissure-type eruptions formed a small shield volcano and the passage between tholeiitic to alkaline volcanics occurred. The third phase (Valle del Bove Centers) is marked by a main westward shift of the feeding system in the Valle del Bove (VdB) area forming some nested volcanic centers. The earliest volcanic edifices recognized are Tarderia and Rocche. Afterward the volcanism was mainly concentrated on the southwestern side of the VdB with the formation ofTrifoglietto volcano. Local shifting of the feeder caused the formation of three different volcanic centers: Giannicola, Salifizio and Cuvigghiuni. Finally, in the fourth phase (Stratovolcano) the definitive stabilization of the plumbing system led to the construction of the main stratovolcano, Ellittico, which forms the bulk of the present edifice. Four caldera-forming Plinian eruptions, occurring at about 15 ka, marked the end of Ellittico activity. During the Holocene, persistent basaltic volcanic activity formed the Mongibello volcano, whose products cover at least 85% of the Mount Etna area.

The dem of mt. etna: geomorphological and structural implications

AbstractA Digital Elevation Model (DEM) of Mt. Etna is presented; it has altimetric and planimetric resolution of 1 m and 5 m, respectively, and covers an area of about 120 km . This 3-D view of Mt. Etna allowed both recognition and location of the main morphostructural and volcano-tectonic features of the volcano. A slope map has been generated from the DEM; on the basis of slope distributions and surface textures, five acclivity domains have been recognized. The largest domain, south of the summit craters, reflects the occurrence of old plateau lavas, distinct from central volcanoes which built the present Etnean volcanic system. Interaction between the central volcanoes, with their summit calderas and failed slopes, produced the other recognised domains. Furthermore, newly identified relevant morphostructural lines are discussed.

40Ar/39Ar isotopic dating of Etna volcanic succession

Since the 1970s, about 50 radio-isotopic ages have been determined on Etna volcanics using different techniques: Th-U and K/Ar.Unfortunately, these ages cannot be readily used to constrain the new stratigraphic setting of the volcano, because of the uncertainty in sample locations or, sometimes, the large errors affecting the calculated ages. For this reason a program of radio-isotopic dating applying the 40Ar/39Ar incremental heating technique to date the groundmass of basaltic samples has been carried out from 2002.Forty samples (22 of which are of new publication) were collected from key outcrops on Etna volcano, selected on the basis of their stratigraphic position, while one sample was collected from the Hyblean plateau volcanics. We have obtained reliable results from all volcanics analysed from 542 ka up to 10 ka with the MSWDs (Mean Square of Weighted Deviates) ranging from 0.03 up to 1.7 excluding IS sample (MSWD = 6.28). These new results allow us to: i) assign an age to 19 of the 25 lithostratigraphic units defined in the new geological map of Etna volcano; ii) clarify the uncertain stratigraphic position of isolated volcanic units; iii) constraint the temporal hiatus that matches the main unconformities; iv) outline the lapse of time between the end of the Hyblean volcanism and the beginning of eruptive activity in the Etna region.

Geological and geomorphological evolution of the Etna volcano NE flank and relationships between lava flow invasions and erosional processes in the Alcantara Valley (Italy)

Abstract In this paper, the interrelationships between volcanic activity and fluvial events in the Alcantara Valley are investigated. Based on the correlation between the stratigraphy of the NE flank of Mt Etna and subsurface data, the geological and geomorphological evolutions of the valley are reconstructed. New 1:10xa0000 scale geological mapping shows that the bulk of this sector of the volcano is made up of the Ellittico volcano lava flows, though they are widely covered by the products of the eruptive activity of the last 15 ka. The present-day morphological setting of the Alcantara Valley is the result of two main evolutionary phases initiated during the activity of the Ellittico volcano. Only one lava flow invasion of the valley floor occurred in the first phase. This phenomenon was followed by a long period of erosional processes leading to the entrenchment of the drainage pattern and the erosion of the Ellittico lava flow. About 20–25 ka ago, an important change in the frequency of the lava flow invasions into the valley occurred associated with the final stage of the Ellittico volcano activity marking the beginning of the second phase. During this phase, volcanic processes became predominant with respect to other morphogenetic processes in the Alcantara Valley. Lava flows coming from the NE flank of the Ellittico volcano caused a radical modification of the morphological setting of this area, even though only one lava flow emitted by an eruptive fissure located within the valley partially filled the riverbed. During the eruptive activity of the last 15 ka, the complete filling of the Alcantara Valley floor occurred. In particular, between 15 and 7 ka, a lava flow originated from the Mt Moio scoria cone filled the valley floor for a distance of about 9 km. Following a short period of erosion, an eruptive fissure located within the valley generated a 20–21-km-long lava flow that was channelled along the full extent of the Alcantara Valley and stretches for about 3 km offshore in the Ionian sea. In the last 7 ka, lava flows originating from the NE-Rift zone produced only temporary damming of the riverbed without any important contribution to the filling of the Alcantara Valley.

How accurate is “paleomagnetic dating”? New evidence from historical lavas from Mount Etna

[1]xa0In the last years, paleomagnetism has been increasingly used to provide emplacement ages of loosely dated volcanics. Dating is achieved by comparison of paleomagnetic directions with a given reference curve of the paleosecular variation (PSV) of the geomagnetic field. Recently, a debate has developed on the achievable precision (the α95 value) of the paleomagnetic directions and hence on the accuracy that “paleomagnetic dating” can yield. At 39 different sites from Etna we paleomagnetically investigated 13 flows (four “test flows” with known age, and nine loosely dated flows), emplaced between 122 B.C. and 1865 A.D. We systematically drilled 12 cores per flow spaced in three (far from each other) sites and demagnetized one specimen per core by alternating field cleaning. Results from the four test flows yield age windows effectively encompassing the respective true flow ages, when dating based on Bayesian statistics at a 95% confidence level is adopted. We find α95 values for the flow mean directions ranging between 3.3° and 5.7° (4.5° on average), which translate into accuracies of age determinations of 136–661 years (307 years on average). Such dating uncertainty is likely underestimated, as we disregarded several kinds of errors that might affect both the fidelity of paleomagnetic recording and the PSV reference curve. The strong magnetization of both the underlying terrain and the cooling flow itself and mineral magnetic variations across the flows are the most likely sources for the scatter characterizing the recording process of the magnetic field in volcanic rocks.

An example of river pattern evolution produced during the lateral growth of a central polygenic volcano: the case of the Alcantara river system, Mt Etna (Italy)

Abstract Analysis of numerous geognostic soundings and geophysical investigation data, indicates the presence of a wide depression, identifiable as the ancient valley of the Alcantara River, below the northern flank of Mt Etna. Surface and subsurface data were used to reconstruct the complex stratigraphic succession filling the paleo-Alcantara valley, consisting of superimposed lava flows, interlayered with thick lentiform clastic bodies. The evolution of the Alcantara river system was defined with reference to the different phases of growth of Mt Etna. The first correlation between volcanic activity and the drainage system refers to the initial phases of alkaline volcanism (170–100 ka). During this period, the southern watershed of the paleo-Alcantara valley acted as a morphological barrier to the products of the various volcanoes which gradually grew in the sector now occupied by the Valle del Bove. The paleo-Alcantara riverbed was sporadically invaded by lava flows generated by peripheral activity of the central edifice. A radical modification of the hydrographic pattern of the region began about 45–40 ka as a consequence of the growth of the Ellittico volcano, the main eruptive centre of the Etnean edifice. The gradual expansion of this volcano led, over a period between about 30 and 25 ka, to the northward deviation of the paleo-Alcantara riverbed. The consequent erosion of a section of the watershed with the adjacent hydrographic basin of the paleo-S. Paolo river resulted in the capture of the latter. From then on, the hydrographic pattern of the area gradually took on the characteristics it has today. A water circulation system developed within the thick volcanic pile filling the paleo-Alcantara valley, with the formation of an aquifer, which is today one of the most important hydro-structures of the whole volcanic edifice and is, in part, exploited in the eastern side of Sicily.