Summit Zone Research Articles

Multi-Annual Changes in Heat Stress Occurrence and Its Circulation Conditions in the Polish–Saxon Border Region

Heat stress is one of the most critical factors affecting human life. In Central Europe, its influence is noticeable, especially in the Polish–Saxon region, which is a very popular tourist region also inhabited by a high number of elders. The main goal of this paper was to assess multi-annual changes in heat stress occurring in the region, considering the frequency of heat days, the UTCI (Universal Thermal Climate Index), and circulation conditions. The research showed that all the thermal and biothermal indices in this region significantly increased during 1971–2019 in the lowlands, the mountain foreland, and the lower mountain zone. In terms of the UTCI, a negative trend for cold stress frequency was noticed in the entire region in favor of an increase in a tendency toward thermoneutral conditions and heat stress. This concerns especially strong and very strong heat stress (UTCI > 32 °C), in which positive trends were observed for most of the stations located in the lower hypsometric zones. The results also showed that heat stress mainly occurs on days with anticyclonic circulation. Analysis of selected cases of heat waves in the 21st century indicated that the lower hypsometric zones are characterized by a very high UTCI, while the summit zone is free from heat stress occurrence.

Periglacial landforms in the high Drakensberg, Southern Africa: morphogenetic associations with rock weathering rinds and shrub growth patterns

ABSTRACT Here, we aim to establish possible morphogenetic associations between periglacial phenomena (sorted circles and turf-/stone-banked lobes) by examining (a) rock surface weathering rinds by thin section microscopy and SEM-EDX analysis, and (b) alpine shrub growth patterns of Helichrysum trilineatum supported by AMS 14C age determination. The study area is the Mafadi–Njesuthi summit zone, one of the highest alpine regions of southern Africa. Rock weathering results indicate that clasts in large sorted circle centres experience high chemical weathering rates on both exposed (upper) and non-exposed (lower) surfaces, whereas clasts in lobes and very large apparently ‘relict’ sorted circles have been inactive for a prolonged period of time, based on strongly contrasting weathering rates on both exposed and non-exposed clast surfaces. Large sorted circles originating from a previous (possibly Last Glacial Maximum or late-glacial) colder period are still marginally or episodically active, thus restricting plant succession on these landforms. Even where very large sorted circles and turf-/stone-banked lobes are inactive, their legacy continues, as expressed in today’s surface morphology, and still influence ground abiotic conditions through the generation of microrelief and microclimate which in turn have impacts on ecosystem patterns including the distribution of H. trilineatum. Such shrub growth patterns are influenced by microscale site morphology, associated abiotic controls, and ongoing seasonal cryogenic activity. Both active and inactive periglacial landforms on highest summits of the Drakensberg represent microscale environments with distinct fine-scale abiotic and geomorphic settings, which in turn are manifested through differences in plant ecology and rock surface weathering, respectively.

Asynchrony among insect pollinator groups and flowering plants with elevation

Mountains influence species distribution through differing climate variables associated with increasing elevation. These factors determine species niche ranges and phenology. Although the distribution patterns of some specific insect groups relative to elevation have been determined, how differing environmental conditions across elevation zones differentially influence the phenology of various insect groups is largely unknown. This is important in this era of rapid climate change. We assess here how species composition and seasonal peaks in abundance among different insect pollinator groups and flowering plants differ across four floristically distinct elevation zones up a sentinel mountain subject to strong weather events. We sampled insect pollinators in four major groups (bees, wasps, beetles and flies) over two spring seasons. Pollinator species composition across all elevation zones tracks flowering plant species composition. In terms of abundance, beetles were the dominant group across the three lower zones, but declined greatly in the summit zone, where flies and bees were more abundant. Bee abundance peaked earlier than the other groups across all four elevation zones, where there were significant peaks in abundance. Bee abundance peaked earlier than flowering plants at the middle zone and slightly later than flowering plants at the base zone, suggesting a mismatch. We conclude that, while elevation shapes species distribution, it also differentially influences species phenology. This may be of great significance in long-term assessment of species distribution in sensitive mountain ecosystems.

Intensity of heat stress in 2015 and 2018 summer seasons in the region of the Lower Silesia (Poland)

Abstract The main goal of this paper was to assess the intensity of heat stress in Lower Silesia, Poland, during selected weather events characterized by high air temperatures. The complex impact of weather on the thermal load of the human organism is presented using the Universal Thermal Climate Index (UTCI). The analysis was carried out for the 2015 and 2018 summer seasons and compared with the multiannual period of 1971–2018. It was based on meteorological data from the IMGW-PIB stations of Wrocław, Jelenia Góra and Śnieżka. In order to examine how heat conditions affect UTCI in different geographical regions, stations located at different altitudes and representing the lowlands, the lower mountain zone and the summit zone of the Sudetes Mountains were considered. The research showed that during the most extreme thermal events, UTCI values in the lowlands and the lower mountain zones can be among the highest heat stress classes. In the summit zone, the maximum UTCI values are usually classed as ‘no thermal stress’.

Mesoscale atmospheric circulation controls of local meteorological elevation gradients on Kersten Glacier near Kilimanjaro summit

Abstract. Elevation gradients of meteorological variables in mountains are of interest to a number of scientific disciplines and often required as parameters in modeling frameworks. Measurements of such gradients on glaciers, however, are particularly scarce and strongly skewed towards the midlatitudes and valley glaciers. This article adds a tropical perspective and presents 4 years of overlapping measurements at 5603 and 5873 m on Kersten Glacier, Kilimanjaro (East Africa), between 2009 and 2013. Mean gradients in near-surface air temperature (T), water vapor pressure (VP), and snow accumulation (ACC) per 100 m elevation are −0.75 ∘C, −0.16 hPa, and -114±16 mm w.e. yr−1, respectively. An intriguing feature is a strong diurnal cycle of the T and VP gradients, which are (depending on season) 2–4 times larger between early and late morning than in the hours of weak gradients. The ACC decrease with elevation, furthermore, is mostly the result of a lower recorded frequency of ACC events at the upper measurement site and not due to contrasting amounts at the two altitudes during events. A novel facet of our study is the linking of measured on-glacier gradients to a high-resolution atmospheric modeling data set, which reveals the importance of the mesoscale atmospheric circulation. A thermally direct circulation is established over the mountain in response to diabatic surface heating or cooling with upslope flow during the day and downslope flow in the night. This persistent circulation communicates heat and moisture changes in the lowlands to the higher elevations during morning and early afternoon, which is evident in the advection patterns of potential temperature and VP, and shapes the time variability in gradients as recorded by our weather stations on the glacier. A few local processes seem to matter as well (glacier sublimation, turbulent heat fluxes), yet they show a secondary influence only during limited time windows. Atmospheric model data also demonstrate that declining moist entropy and water vapor fluxes in the summit zone favor formation of the negative ACC gradient. The results extend the empirical basis of elevation gradients in high mountains, in particular over glacier surfaces, by the unusual case of a slope glacier on an equatorial, free-standing massif. Our measurement–model link, moreover, demonstrates an approach for future studies to put observations of elevation gradients more systematically in a multiscale process context.

Genecology and ecophysiology of the maintenance of foliar phenotypic polymorphisms of Leptospermum recurvum (Myrtaceae) under oscillating atmospheric desiccation in the tropical‐subalpine zone of Mount Kinabalu, Borneo

AbstractWe investigated genecology and ecophysiological mechanisms of the polymorphism of leaf trichome density of Leptospermum recurvum Hook. f. (Myrtaceae) in the deglaciated summit zone above 3,000 m a.s.l. of Mt. Kinabalu, Borneo. Various phenotypes with variable foliar trichome densities occurred sympatrically in the same population, and the composition of coexisting phenotypes varied substantially among populations. We conducted a common garden experiment by sowing seeds from multiple maternal trees of different leaf trichome densities. We found a significant relation between pubescence of maternal trees and offspring, which indicated that leaf trichome density had a genetic basis. Microsatellite analysis revealed that there was no barrier to gene flows among phenotypes or among populations, and very low neutral genetic differentiation among populations with high gene flows for both directions of phenotypes. The soils in the sites dominated by pubescent trees were significantly more desiccated than in the sites dominated by glabrous trees during a short drought. Glabrous trees had a significantly greater mortality rate than pubescent trees after an intensive El Niño drought (13.7 vs. 3.9%) in the same sites where both phenotypes occurred sympatrically. Pubescent individuals demonstrated a significantly greater photosynthetic water‐use efficiency than glabrous individuals. El Niño droughts could cause large difference in soil moisture among sites and that a greater desiccation stress removed glabrous phenotypes as one end of divergent natural selection to form pubescent populations. These results implied that the process shaping the phenotypic polymorphisms involved strong gene flows combined with ongoing divergent selection.

Variation in chewing efficiency of Yakushima Japanese macaque (Macaca fuscata yakui).

Chewing efficiency plays an important role in the survival and distribution of primates. Yet, little is known about the intra-specific variation of chewing efficiency. The purpose of this study is to report the pattern of seasonal and regional variation in chewing efficiency among Yakushima Japanese macaques (Macaca fuscata yakui). Fecal samples of Yakushima Japanese macaques were collected from lowland, highland and summit areas in Yakushima between July 2015 and March 2016 (n = 236). Using sieving analysis, we compared fecal particle size (dMEAN) and proportion of finest particles p(0) between different geographical areas and seasons. Seasonally, in the lowland zone, there was a non-significant decrease in dMEAN during spring, while p(0) was significantly higher during summer than it was during winter and spring. Regionally, dMEAN was higher in the summit zone than it was in other areas during autumn, while p(0) was also higher in the summit zone. While seasonal variation in dMEAN can be explained by the reported difference in the proportions of food categories in diet between seasons, its influence is mitigated, possibly by the selective feeding of less mechanically challenging parts in each category. Regional variation in dMEAN and p(0) may be the results of bamboo consumption in this area. Combining our data with studies that focus on seasonal and regional variations of food properties or gut microbes might provide a better understanding of the relation between diet, chewing and digestion in Yakushima macaques.

Imbabura volcano (Ecuador): The influence of dipping-substrata on the structural development of composite volcanoes during strike-slip faulting

The geology and long-term structural development of Imbabura volcano (Ecuador) have been discerned from field surveys, as well as petrographical and geochemical studies. The volcano is composed of an ancient edifice (Imbabura I), which has experienced one major volcanic landslide directed N 330–340°. After the emplacement of that debris-avalanche deposit, the petrographically and geochemically different edifices Imbabura II-1 and Imbabura II-2 developed on the N and NE flank of Imbabura I, as well as in its summit zone. Finally the smaller Imbabura II-3 edifice, a dome-complex locally known as Huarmi Imbabura, was emplaced in the SSW flank of Imbabura I. The relatively complex structure of the edifice (i.e. flank collapse direction and spatial distribution of the major stratigraphical units) cannot be explained solely by volcano-tectonic interactions with the regional left-lateral Río Ambi strike-slip fault, which traverses the edifice from NNE to SSW. Instead, the fact that the whole Imbabura edifice stands on a substratum dipping to the NW (i.e. the NW flank of the neighbor Cubilche volcano) is proposed to be the main factor influencing the departure of Imbabura structure from previously known models of volcanic edifices interacting with strike-slip faults. This hypothesis was tested with scaled experiments, during which the dip-direction of the substratum was systematically changed with respect to the strike of the fault. The structures developed by the experimental prototypes are highly coherent with those observed at Imbabura volcano.

Topographic Maps of Mount Etna’s Summit Craters, updated to December 2015

ABSTRACTNew maps of the summit of Mount Etna volcano (1:5000–1:4000), derived from helicopter photogrammetry, thermal images and terrestrial laser scanner survey, are here presented. These maps indicate the main morpho-structural changes occurring during the powerful explosive and effusive eruptions involving the summit craters of Etna over the first two weeks of December 2015. The survey enabled identifying the proximal erupted volume (7.2 ± 0.14 × 106 m3) and the size and location of the vent causing the powerful explosive activity inside the Central Crater. Our survey also outlines the growth of a recent (2011–2015) summit cone on top of a former pit crater, named New SE-Crater. This new cone is by now comparable in size to the former SE-Crater. The shape and size of two small cinder cones that formed on the upper eastern flank of the summit zone in May–July 2014 are also shown. This approach can be used in fast and frequent monitoring of very active volcanoes.

Analysis of Surface Growth in the Conifer Shoot Apical Meristem

Premise of research. The structure and organization of the conifer shoot apical meristem was investigated using confocal microscopy of whole-mount shoot apical meristems. This article presents detailed surface and internal patterns that provide an informative and comparative model of meristem growth that is able to account for the diversity and variation of conifer meristem form.Methodology. Confocal microscopy; light microscopy.Pivotal results. On the surface of the meristem, two zones with distinct cell division patterns were recognized. The first is the summit zone that covers the apex of the meristem dome, and the second is the peripheral zone that covers the flanks of the meristem. In the summit zone, successive anticlinal cell divisions are perpendicular to each other and produce isotropic growth. In the peripheral zone, surface cells divide with repetitive transverse divisions to form long axial chains that create packets with anisotropic growth. Only cells in the summit zone divide periclinally to contribute daughter cells to the underlying tissue. Those meristems with a small number of cells in the summit zone have a correspondingly small number of periclinal divisions of apical initial cells. Those meristems with larger summit zones and many wedge-shaped packets have a larger number of apical initials dividing periclinally.Conclusions. Conifer meristems have consistent cell division patterns that define the summit and peripheral zones. Apical initials are transient, and their number varies between meristems, depending on the size of the summit zone.

A study of the focal mechanisms of small volcanic earthquakes during the precursory period and the occurrence of the 2008–2009 eruption on Koryakskii Volcano

This is the first-ever study to determine the focal mechanisms for very small volcanic earthquakes (magnitudes approximately 1). We used a swarm of small volcanic earthquakes in the Koryakskii Volcano area during the seismic precursory period before the eruption (March to December 2008) and during the subsequent volcanic activity (December 28, 2008 through April 2009). Statistically significant estimates of mechanism parameters were found for the volcanic earthquakes in this swarm. The mechanisms were determined using the polarities of the first motions of P-waves. The resulting rupture planes strike along azimuths that are consistent with the fault plane of the nearly north–south zone where the 2008–2009 seismicity of Koryakskii Volcano occurred. It was shown that the earthquakes in and around the fault zone can be classified into those south of the Koryakskii summit (zone I) and north of it (zone II). The azimuths of rupture planes in zones I and II are significantly different (212° ± 12° and 5° ± 12°, respectively).

Vegetation and Climate of the Summit Zone of Mount Kinabalu in Relation to the Walker Circulation

Mountains of the tropical Pacific are influenced by synoptic-scale air subsidence, which causes a temperature inversion and a distinct dry meteorological condition above the inversion. The inversion appears at a lower altitude in the eastern Pacific where descending air of the Walker circulation prevails. On the other hand, if or how the alpine ecosystem of the tropical mountains of the western Pacific is influenced by dry synoptic-scale air subsidence is not well documented. We studied the vegetation and climate of the summit zone of Mount Kinabalu (4095 m) of Borneo. The leaf-size spectrum and physiognomy of forest community changed abruptly along the slope approximately at 3200 m from microphyll to leptophyll, suggesting that dry climatological conditions influence the vegetation above that altitude. Mean daily vapor pressure deficits (VPDs), estimated daily potential evapotranspiration (ET0), and the ratio of 30-day total ET0 to 30-day total rainfall increased drastically during El Niño and the magnitude of the increase was greater in the summit zone than in the montane zones. Increased VPDs during El Niño were linked with katabatic winds in the summit zone. We suggest that such irregular dry spells caused by synoptic-scale air subsidence in El Niño years can be a major factor for the formation of xeromorphic vegetation of the summit zone of Mount Kinabalu.

An experiment of muon radiography at Mt Etna (Italy)

Interactions of conduit geometry with gas–liquid flows control volcanic activity, implying that the evaluation of volcanic hazards requires quantitative understanding of the inner structure of the volcano. The more established geophysical imaging techniques suffer from inherent ambiguity, may require spatially dense measurements in active areas and may not provide sufficient spatial resolution in the uppermost part of the conduit system. It is thus desirable to develop new imaging techniques allowing a better spatial resolution of a volcano's upper feeding system, with reduced ambiguity and a low level of risk for operators. Muon particles can be utilized to image the internal density distribution of volcanic structures. The principle of muon radiography is essentially the same as X-ray radiography, except for substituting penetrating particles in place of photons. Muons are more attenuated by higher density parts inside the target and thus information about its inner structure are obtained from the differential muon absorption. We report on a muon-imaging experiment that was conducted at Mt Etna in 2010. The target structure was one of the summit craters of the volcano. This experiment was performed using a muon telescope suitably designed to withstand the harsh conditions in the summit zone of a high volcano. We found a marked difference between synthetic and observed attenuation of muons through the target. This discrepancy is likely due to the bias on the observed flux, arising from false muon tracks. They are caused by low-energy particles that, by chance, hit simultaneously the two matrixes of the telescope, leading to detection of a false positive. We separated the useful from the unwanted signal through a first-order model of the background noise. The resulting signal is compared with the corresponding synthetic flux. Eventually, we found regions of higher-and lower-than-expected muon flux, that are possibly related to inner features of the target crater.

Micrometeorological conditions and surface mass and energy fluxes on Lewis Glacier, Mt Kenya, in relation to other tropical glaciers

Abstract. The Lewis Glacier on Mt Kenya is one of the best-studied tropical glaciers, but full understanding of the interaction of the glacier mass balance and its climatic drivers has been hampered by a lack of long-term meteorological data. Here we present 2.5 yr of meteorological data collected from the glacier surface from October 2009 to February 2012. The location of measurements is in the upper portion of Lewis Glacier, but this location experiences negative annual mass balance, and the conditions are comparable to those experienced in the lower ablation zones of South American glaciers in the inner tropics. In the context of other glaciated mountains of equatorial East Africa, the summit zone of Mt Kenya shows strong diurnal cycles of convective cloud development as opposed to the Rwenzoris, where cloud cover persists throughout the diurnal cycle, and Kilimanjaro, where clear skies prevail. Surface energy fluxes were calculated for the meteorological station site using a physical mass- and energy-balance model driven by measured meteorological data and additional input parameters that were determined by Monte Carlo optimization. Sublimation rate was lower than those reported on other tropical glaciers, and melt rate was high throughout the year, with the glacier surface reaching the melting point on an almost daily basis. Surface mass balance is influenced by both solid precipitation and air temperature, with radiation providing the greatest net source of energy to the surface. Cloud cover typically reduces the net radiation balance compared to clear-sky conditions, and thus the frequent formation of convective clouds over the summit of Mt Kenya and the associated higher rate of snow accumulation are important in limiting the rate of mass loss from the glacier surface. The analyses shown here form the basis for future glacier-wide mass and energy balance modeling to determine the climate proxy offered by the glaciers of Mt Kenya.

Ground‐based InSAR reveals conduit pressurization pulses at Stromboli volcano

AbstractAt Stromboli volcano (Southern Italy), only few minor precursors (gas output) have been identified for ‘major’ Strombolian explosions. We use ground‐based interferometric synthetic aperture radar (GBInSAR) technology to monitor the displacement rate of the summit area of Stromboli. We analysed the 2009–2011 period. The analysis of the displacement rate has been performed by dividing the summit zone monitored by the GBInSAR into three regions, corresponding to the edge of the crater area (1 and 2), and to the slope of the NE crater (3). Pulses of rapid expansion of sector 3, of variable duration and amplitude, appear in coincidence with periods of intense activity that include lava flows and major explosions. We associate this expansion with the pressurization of the magma column that is recharged by deep‐derived gas, promoting the onset of ‘major explosion‐dominated’ activity.

Spatial variations of soil weathering processes in a tropical mountain environment: The Baturité massif and its piedmont (Ceará, NE Brazil)

Strong spatial variations in soil weathering processes were identified in the uplands and piedmont zones of the Baturité massif (Ceará, NE Brazil), a medium-elevation tropical mountain characterized by a steep ecocline between humid climate of the summit zone and semi-arid conditions of the erosional piedmont. With the combined help of field surveys and laboratory analyses, involving micromorphological observations as well as geochemical and mineralogical investigations, this study demonstrates a close spatial correlation between soil weathering processes and present-day water balance (WBI) variations: (i) in the humid massif (WBI > 500 mm·y− 1), monosiallitization is exclusive in soil and upper saprolite horizons and leads to the neoformation of 1:1 clay minerals (well-crystallized kaolinite) with minor amounts of gibbsite; (ii) in the subhumid peripheral area of the massif (50 < WBI < 500 mm·y− 1), monosiallitization coexists with bisiallitization in the soil profiles, producing a mixture of 1:1 (kaolinite) and 2:1 (illite) clay minerals along with lower weathering intensity; (iii) in the semiarid piedmont (0 < WBI < 50 mm·y− 1), bisiallitization becomes the dominant weathering pathway conducive to grus formation and fersiallitic pedogenesis, with the most vulnerable primary minerals weathered into high-charge, 2:1 clay minerals (smectite and illite). Given that soils and weathering mantles are integrators of “average” bioclimatic conditions that prevail at timescales of 1 to 10 Myr, the distribution of soil weathering signatures in the study area is probably reflective of inevitable, long-term (Quaternary to Neogene) climatic fluctuations around median values close to the present ones. Paleoclimatic legacies inherited from the Paleogene lateritization event are rare, because of the denudation pulse occurring in the Late Tertiary and favoring soil stripping to the paleoweathering front on the erosional piedmont.

Eruptive and earthquake activities related to the 2000 eruption of Mount Cameroon volcano (West Africa)

Mount Cameroon is an active volcano located in the Gulf of Guinea, west of Central Africa. After the March–April 1999 eruption on the SW flank, another eruption of the volcano occurred in 2000. It took place from three sites on the southwest flank and near the summit. The first eruptive site was located 500 m to the southwest of the summit, at 3900 m altitude. Activity on this site was mainly explosive with no lava flow. The second site was located between 3220 and 3470 m altitude. Lava was emitted along NNE–SSE fissures from this site and flew towards Buea, the main city of the area, stopping ~ 4 km from the first houses. The last site was located in the south western flank at 2750 m altitude. The lava ejected from an old cone near the first 1999 eruptive site was divided into two branches, for a total length of around 1 km. The location of active volcanic cones in 1999 and 2000 seems to be linked to the local tectonics. The pre-eruptive period was characterized by a seismic swarm which may be a precursor recorded in March 2000 by an analogue seismic station. The main shock was a magnitude 3.2 event, and was felt by the population in Ekona town located on the eastern flank. It had a Modified Mercalli intensity of III–IV. When the eruption started, a temporary network of short period 3-component seismic stations was set up around the volcano to improve the monitoring of seismic activity. The co-eruptive period from late May to September was characterized by sequences of earthquake swarms, volcanic tremor and a family of earthquakes having similar waveform and appearing regularly in August and early September. Some of the earthquakes were felt by the population in Buea and its environments. The largest seismic event recorded had a magnitude of 4. During the post-eruptive period from mid-September to December, seismicity returned to its background level of 1–3 earthquakes per 3 days. Hypocenter locations reveal a linear narrow structure under the summit zone which could represent the magmatic conduit of the volcano. The frequency/magnitude relationship revealed a b-value of 1.43 higher than those previously determined, but more representative of volcanic media. Seismic energy release was gradual after the 2000 eruption started.

Formation of the April 2007 caldera collapse at Piton de La Fournaise volcano: Insights from GPS data

On April 5th 2007, a major collapse of the Dolomieu summit crater occurred at Piton de La Fournaise volcano (La Réunion Island). To constrain the origin and the dynamics of the collapse, we modelled continuous GPS displacements preceding and accompanying this event. Our results reveal that the Dolomieu collapse formed by a “piston-like” mechanism, in which the fast draining of the shallow magma reservoir at the beginning of the March 30th–May 1st 2007 eruption caused a gravity-driven downward displacement of its roof. During the five days preceding the collapse, summit deflation was caused by volume change through the fractured rock column above the shallow magma chamber due to the closure of voids or small-scale cavities or the upward stoping and migration of an underground cavity. Since 2000, weakening of the rock column by repeated refilling and draining of the shallow magma chamber favoured its destabilization. On April 5th, 7 h before the onset of the collapse, the deflation source was located at ~ 300 m depth revealing the initiation of the collapse at very shallow depth. The sudden collapse on April 5th was followed by continuous cyclic subsidence until April 6th. Each cycle was characterized by a progressive inward deflation of the summit zone and ended by sudden summit outward displacements caused by stress relaxation following a collapse event. Subsidence of the stoped column in the magma reservoir during the collapse of the crater on April 5th and 6th acted as a piston thus increasing the eruption rate. The collapse occurred in only 24 h by successive events and increased the depth of the Dolomieu crater by 340 m. After April 6th, only minor readjustments of the summit cone occurred with slight summit deflation and small landslides of the caldera walls.

Review of Microgravity Observations at Mt. Etna: A Powerful Tool to Monitor and Study Active Volcanoes

Microgravity observations at Mt. Etna have been routinely performed as both discrete (since 1986) and continuous (since 1998) measurements. In addition to describing the methodology for acquiring and reducing gravity data from Mt. Etna, this paper provides a collection of case studies aimed at demonstrating the potential of microgravity to investigate the plumbing system of an active volcano and detect forerunners to paroxysmal volcanic events. For discrete gravity measurements, results from 1994–1996 and 2001 are reported. During the first period, the observed gravity changes are interpreted within the framework of the Strombolian activity which occurred from the summit craters. Gravity changes observed during the first nine months of 2001 are directly related to subsurface mass redistributions which preceded, accompanied and followed the July-August 2001 flank eruption of Mt. Etna. Two continuous gravity records are discussed: a 16-month (October 1998 to February 2000) sequence and a 48-hour (26–28 October, 2002) sequence, both from a station within a few kilometers of the volcano's summit. The 16-month record may be the longest continuous gravity sequence ever acquired at a station very close to the summit zone of an active volcano. By cross analyzing it with contemporaneous discrete observations along a summit profile of stations, both the geometry of a buried source and its time evolution can be investigated. The shorter continuous sequence encompasses the onset of an eruption from a location only 1.5 km from the gravity station. This gravity record is useful for establishing constraints on the characteristics of the intrusive mechanism leading to the eruption. In particular, the observed gravity anomaly indicates that the magma intrusion occurred “passively” within a fracture system opened by external forces.

A data sequence acquired at Mt. Etna during the 2002–2003 eruption highlights the potential of continuous gravity observations as a tool to monitor and study active volcanoes

Abstract A 2.5-month long gravity sequence, encompassing the starting period of the 2002–2003 Etna eruption and coming from a summit station only 1 km away from the new fractures, is presented and discussed. The sequence comprises four hours-long anomalies that have a great chance to reflect mass redistributions linked to the ensuing activity. In particular, the start of the eruptive activity on the northeastern flank was marked by a gravity decrease as strong as about 400 μGal, which reverted soon afterwards. This strong decrease/increase anomaly is interpreted as the opening, by tectonic forces, of a fracture system along the Northeastern Rift of Mt. Etna, followed by an intrusion of magma from the central conduit to the new fractures. They were used by the intruding magma as a path to the eruptive vents at lower elevations. Afterwards, on three occasions, in November and December 2002, 6–12 h-lasting gravity decreases, with amplitude ranging between 10 and 30 μGal, were observed simultaneously with increases in the amplitude of the volcanic tremor from four seismic stations. A correlation analysis, between the gravity signal and the overall spectral amplitude of each tremor sequence is performed over the 7 November–9 December period. A marked anti-correlation is found over each contemporaneous gravity decrease/tremor increase, while, over the rest of the investigated period, the correlation is negligible. Accordingly, a joint source is inferred to have acted during the occurrence of the three common anomalies. On the grounds of some volcanological observations spanning the period covered by our analysis, we propose the temporary accumulation of a gas cloud at some level within the plumbing system of the volcano to have acted as a joint source. The present work is a further evidence of the potential of continuous gravity observations as a tool to monitor and study active volcanoes and encourages their employment in spite of the difficulty of running spring gravimeters in a continuous fashion under the adverse conditions normally encountered on the summit zone of an active volcano.