Average Effusion Rate Research Articles

Rheological properties and ages of lava flows on Alba Mons, Mars

We present the results of our studies of the rheological properties and ages on Alba Mons lava flows. Previous studies have shown that the rheology of lava flows, such as yield strength and viscosity, as well as external parameters affecting flow characteristics, such as effusion rate and eruption duration can be derived by remotely sensed dimension measurements. The calculations made in this study are based on images obtained from the Context Camera (CTX) on board of NASA's Mars Reconnaissance Orbiter (MRO) as well as data from the Mars Orbiter Laser Altimeter (MOLA) on NASA's Mars Global Surveyor (MGS). Absolute model ages (AMAs) have been derived by performing crater size-frequency distribution (CSFD) measurements on CTX images. The main objectives of this investigation are not only the calculation of the rheological properties and ages, but also the analysis of possible correlations between the rheologies, ages and distances of the flows to the volcano's center. A comparison of the rheological properties of the investigated lavas with those reported for flows on other planetary bodies was also performed. The aim of the study is a better insight into the history of one of the largest volcanoes in the Solar System. We provide new information about the formation and composition of the volcano's lava flows, whether they changed over time, and until when Alba Mons has been active.In total, 29 lava flows close to the summit of Alba Mons were mapped and measured. The calculated yield strengths are 2.79 × 103 Pa on average, ranging from 4.00 × 102 Pa to 9.61 × 103 Pa. We found effusion rates between 46 m3 s−1 and 1528 m3 s−1 with an average effusion rate of 636 m3 s−1. The derived viscosity is on the order of 1.04 × 107 Pa s with values between 1.27 × 105 Pa s and 1.0 × 108 Pa s. The lavas have erupted over a period of 11 to 223 days, with an average eruption duration of 68 days. The rheological properties derived in this investigation are very similar to those of other volcanic regions on Mars, especially Elysium Mons and Elysium Planitia. However, they differ from calculated rheologies of previous studies of venusian lavas and lunar lava flows. Based on the derived yield strength and viscosity values, the composition of the lava flows can be inferred as basaltic/basaltic-andesitic, which is analog to terrestrial basaltic or andesitic a'a lava flows. Significant correlations between the rheological properties and the distance to Alba Mons' center have not been found. AMAs could be derived for 14 of the 29 lava flows investigated, because 15 lava flows have either reached the state of equilibration or have too few impact craters to derive reliable model ages. The AMAs range from 130 ± 40 Ma to 810 ± 60 Ma. The results indicate a late volcanic activity at Alba Mons. Correlations between the AMAs and distance to the volcano's center, as well as between the rheological properties and AMAs were not observed.

Episode 4 (2019\u20132020) Nishinoshima activity: abrupt transitions in the eruptive style observed by image datasets from multiple satellites

In December 2019, a new activity started at Nishinoshima volcano in the southern part of the Izu–Ogasawara arc, Japan. This is now referred to as Episode 4 of a series of activities that began in 2013. We analyzed the eruption sequence, including erupted volume and effusion rate, based on combined observations of thermal anomalies by Himawari-8 and topographic changes by ALOS-2. The total eruption volume during Episode 4 was ~ 132 × 106 m3, and the average effusion rate over the entire period was 0.51 × 106 m3 day−1 (5.9 m3 s−1), which was two to three times higher than that of Episode 1. Episode 4 had three stages. In Stage 1, effusive activity was dominant, and most of the lava erupted from a northeast vent at the foot of the pyroclastic cone to cover the northern half of the island. The average effusion rate was estimated to be 0.46 × 106 m3 day−1 (5.3 m3 s−1). In Stage 2, an intensive lava fountain with a high discharge rate developed, and it increased the size of the pyroclastic cone rapidly. The effusion rate temporarily reached 2.6 × 106 m3 day−1 (30 m3 s−1). Pyroclastic rocks accounted for 45–88% of the total erupted volume in this stage. Lava flows with rafted cone material were generated, and those possibly caused by intensive spatter falls on the slope were also formed. These lavas flowed down the southern half of the island. In Stage 3, continuous phreatomagmatic eruptions released ash and spread it over a wide area. The high effusion rate and the drastic change in the activity style in Episode 4 can be explained by deep volatile-rich magma being supplied to a shallower magma chamber prior to Episode 4. When the volatile-rich magma reached a shallow part of the conduit in Stage 2, fragmentation occurred due to rapid volume expansion to eject large amounts of magma and form the intensive lava fountain. Observations by satellite-borne ultraviolet–visible image sensors detected a rapid increase in SO2 emissions in response to the intensive lava-fountain activity. The less-differentiated nature of the ash fragments collected during Stage 2 may reflect the composition of the volatile-rich magma. Large-scale discolored-seawater areas appeared during the late period of Stage 1, which may have been caused by ascent of the volatile-rich magma.Graphical

The 2017 Nishinoshima eruption: combined analysis using Himawari-8 and multiple high-resolution satellite images

Nishinoshima volcano suddenly resumed eruptive activity in April 2017 after about 1.5 years of dormancy since its previous activity in 2013–2015. Nishinoshima is an uninhabited isolated island. We analyzed the eruption sequence and the eruptive process of the 2017 eruption (17 April–10 August: 116 days) by combining high-temporal-resolution images from Himawari-8 and high-spatial-resolution images from the ALOS-2, Landsat-8, and Pleiades satellites. We used these data to discuss how temporal variations in the lava effusion rate affected the flow formations and topographical features of the effused lava. The total effused volume was estimated to be 1.6 × 107 m3, and the average effusion rate was 1.5 × 105 m3/day (1.7 m3/s). Based on variations in the thermal anomalies in the 1.6-μm band of Himawari-8, which roughly coincided with that of the lava effusion rate estimated by ALOS-2, the activity was segmented into five stages. In Stage 1 (17–30 April: 14 days), the lava effusion rate was the highest, and lava flowed to the west and southwest. Stage 2 (1 May–5 June: 36 days) showed a uniform decrease in flow, and lava flowed to the southwest and formed the southwestern lava delta. During Stage 3 (6–15 June: 10 days), the lava effusion rate increased in a pulsed manner, the flow direction changed from southwestward to westward, and a narrow lava flow effused from the southern slope of the cone. In Stage 4 (16 June–31 July: 46 days), the lava effusion rate decreased and lava flowed westward through lava tubes, enlarging the western lava delta. Around the end of July, lava effusion mostly stopped. Finally, in Stage 5 (1–10 August: 10 days), explosive eruptions occurred sporadically. The variation in lava effusion rate seemed to play an important role in forming different flow patterns of lava on Nishinoshima. In Stages 1 and 3, lava flowed in multiple directions, while in Stages 2 and 4, it flowed in single direction, probably because the effusion rate was lower. A pulsed increase in the lava effusion rate during Stage 3 caused new breaks and disturbances of the lava passages near the vents, which resulted in changes in flow directions. Differences in the size of lava lobes between the southwestern and western deltas are also considered to result from differences in the lava effusion rate.

Observation of the eruption sequence and formation process of a temporary lava lake during the June–August 2015 Mt. Raung eruption, Indonesia, using high-resolution and high-frequency satellite image datasets

From June to August 2015, Mt. Raung located at the east end of Java, Indonesia, erupted effusively with Strombolian activity at the intra-caldera cinder cone. The effused lava from the base of the cone entirely buried the caldera floor with a lava bed and ultimately formed a temporary lava lake. In this study, details of the eruption sequence and formation process of the temporary lava lake are analyzed based on a series of high-resolution images combined with the high-frequency thermal observation results of Himawari-8. The volume of effused lava was also estimated using a digital elevation model (ASTER GDEM) based on the lava bed bathymetry in the caldera and lava bed distribution at several points over the course of the eruption. The total effusion volume was estimated to be 7.5 × 107 m3, and the average effusion rate was 1.4 × 106 m3/day. The activity consisted of four stages. The Precursory Stage was characterized by low-level thermal anomalies, which are thought to result from high-temperature spots at the bottom of several pits formed in the buried vent, probably due to magma head partial exposure or gas emission. Pulses 1 and 2 were effusive stages that formed the lava bed. The pattern recognized in the time-series variation of thermal anomalies was confirmed to reflect the lava effusion rate using the volumes estimated in this study. The lava bed is divided into the Lower and Upper lava groups, which were generated in Pulses 1 and 2, respectively, except for the fine lobes formed around the margin of the Lower lava group. The Terminal Stage also showed low-level thermal anomalies, most likely caused by gas emissions. The intra-caldera cone grew to approximately 100 m higher than its initial height by mid-August and then collapsed into the conduit at the end of the Terminal Stage. The lava bed increased in thickness through lobe multi-layering and later through the addition of lava directly from the vent into the lava bed interior. Over the course of the lava bed growth, the bed outline nearly matched the caldera contours. Uplift of the lava bed surface caused lateral fluidization of the bed, and molten lava was extruded at the bed margins, forming fine lobes. This process enlarged the zone of fine lobes around the lava bed. Thus, the lava bed growth resulted in the appearance of a lava lake. In the middle of Pulse 1, a small amount of hot material was thrown onto the slope beyond the caldera rim. In the future, the caldera floor uplift caused by this activity (approximately 49 m) will increase the risk of erupted material ejection outside the caldera. In a comparison with the 2013–15 Nishinoshima eruption, which was a similar effusive eruption, Mt. Raung was shown to have predominantly large lobes over most of the lava bed, while in Nishinoshima, narrow medium-scale lobes occupied the majority of the surface. This may be attributed to the high effusion rate of lava for Mt. Raung, which was approximately seven times higher on average than the lava effusion rate for Nishinoshima.

3D Lava flow mapping of the 17–25 May 2016 Etna eruption using tri-stereo optical satellite data

During basaltic eruptions, the average rate at which lava is erupted (effusion rate) is one of the most important factor controlling the evolution, growth and extent of the flow field. This has implication both for forecasting purposes as input parameter into physics-based numerical models, and for advancing knowledge on the shallow feeder system by constraining the mass supplied. Satellite remote sensing provides a means to estimate the average effusion rate by applying a direct conversion from the measured radiant heat loss by an active lava flow. This conversion relies on a set of parameters of lava (e.g. rock density, heat capacity, vesicularity, emissivity, etc.) and suffers of multiple sources of uncertainties and measurements errors, whose quantification is still an open problem. Here we constrain the lava volume emitted at Mt Etna on 17-25 May 2016, by using pre-eruptive and post eruptive digital elevation models (DEMs) obtained processing satellite images acquired by the Pleiades constellation, which provides images at 50 cm resolution in stereo and tri-stereo mode. The 3D processing of the tri-stereo Pleiades imagery (acquired on 24 December 2015 and 18 July 2016), performed using the free and open source MicMac photogrammetric library provides estimations of the distribution of thickness and the bulk volume emitted. The integration of multi-platform remote sensing products represents a new potential of merging capabilities to enable a more comprehensive response to effusive crises.

The emplacement of the active lava flow at Sinabung Volcano, Sumatra, Indonesia, documented by structure-from-motion photogrammetry

An effusive eruption at Sinabung Volcano in Indonesia began in December 2013. We use structure-from-motion (SfM) photogrammetric techniques to create digital elevation models (DEMs) of the active lava flow. We build DEMs from photographs taken during two separate time periods and from two separate low-cost handheld cameras and compare them with a pre-eruption DEM to assess the quality and accuracy of photogrammetric DEMs created using different cameras, calculate flow volume and long-term average effusion rate, and document changes in flow morphology. On September 22nd, 2014, the lava flow was 2.9km long and had a volume of 1.03±0.14×108m3, leading to an estimated time-averaged discharge rate of 4.8±0.6m3s−1. Differencing the photogrammetric DEMs shows that during the two-week field campaign, topographic changes of the flow occurred in zones along the flow front and on the upper flank, a finding supported by relatively high temperatures in corresponding thermal images. The deformation can be explained by active advance at the flow front and development of instabilities and collapse on the upper flanks. Large pyroclastic density currents associated with gravitational collapse of upper-flank instabilities in October 2014 and June 2015 were caused by lava growing over ridges that had initially confined the flow to a pre-existing channel. This work demonstrates the ability of SfM photogrammetry to measure or identify the lava flow volume, time-averaged discharge rate, flow emplacement rate and style, as well as the development of gravitational instabilities. Our results show the potential of SfM photogrammetry as a cost- and time-effective method of repeatedly measuring active volcanic features and monitoring hazards at Sinabung and during similar eruptions.

The role of unsteady effusion rates on inflation in long-lived lava flow fields

The emission of volcanic gases and particles can have global and lasting environmental effects, but their timing, tempo, and duration can be problematic to quantify for ancient eruptions where real-time measurements are absent. Lava flows, for example, may be long-lasting, and their impact is controlled by the rate, tempo, and vigor of effusion. These factors are currently difficult to derive from the geologic record but can have large implications for the atmospheric impact of an eruption. We conducted a set of analogue experiments on lava flow inflation aiming at connecting lava morphologies preserved in the rock record to eruption tempo and dynamics through pulsating effusion rates. Inflation, a process where molten material is injected beneath the crust of an active lava flow and lifts it upwards, is a common phenomenon in basaltic volcanic systems. This mechanism requires three components: a) a coherent, insulating crust; b) a wide-spread molten core; and c) pressure built up beneath the crust from a sustained supply of molten material. Inflation can result in a lava flow growing tens of meters thick, even in flow fields that expand hundreds of square kilometers. It has been documented that rapid effusion rates tend to create channels and tubes, isolating the active part of the flow from the stagnant part, while slow effusion rates may cause crust to form quickly and seize up, forcing lava to overtop the crust. However, the conditions that allow for inflation of large flow fields have not previously been evaluated in terms of effusion rate. By using PEG 600 wax and a programmable pump, we observe how, by pulsating effusion rate, inflation occurs even in very low viscosity basaltic eruptions. We show that observations from inflating Hawaiian lava flows correlate well with experimental data and indicate that instantaneous effusion rates may have been 3 times higher than average effusion rates during the emplacement of the 23 January 1988 flow at Kīlauea (Hawai‘i). The identification of a causal relationship between pulsating effusion rates and inflation may have implications for eruption tempo in the largest inflated flows: flood basalts.

The 1928 eruption of Mount Etna (Italy): Reconstructing lava flow evolution and the destruction and recovery of the town of Mascali

Mount Etna in Sicily (Italy) shows >2500years of interactions between volcanic eruptions and human activity, and these are well documented in historical sources. During the last 400years, flank eruptions have had major impacts on the urban fabric of the Etna region, especially in 1651–54, 1669, 1923 and 1928, and it is the last of these which is the focus of this paper. A detailed field and historical reconstruction of the 1928 eruption is presented which allows three themes to be discussed: the evolution of the flow field, lava volume and average magma discharge rate trend; the eruption's human impact, particularly the destruction of the town of Mascali; and the recovery of the region with re-construction of Mascali in a new location. Detailed mapping of lava flows allowed the following dimensions to be calculated: total area, 4.38×106m2; maximum length, 9.4km; volume, 52.91±5.21×106m3 and an average effusion rate of 38.5m3s−1. Time-averaged discharged rates are calculated allowing the reconstruction of their temporal variations during the course of the eruption evidencing a high maximum effusion rate of 374m3s−1. These trends, in particular with regard to the Lower Fissure main phase of the eruption, are in accordance with the ‘idealized discharge model’ of Wadge (1981), proposed for basaltic eruptions driven by de-pressurization of magma sources, mainly through reservoir relaxation (i.e. elastic contraction of a magma body). The eruption took place when Italy was governed by Mussolini and the fascist party. The State response both, during and in the immediate aftermath of the eruption and in the years that followed during which Mascali was reconstructed, was impressive. This masked a less benign legacy, however, that can be traced for several subsequent decades of using responses to natural catastrophes to manufacture State prestige by reacting to, rather than planning for, disasters.

Magma emission rates from shallow submarine eruptions using airborne thermal imaging

The effusion rate is the most important parameter to gather when a volcanic eruption occurs, because it controls the way in which a lava body grows, extends and expands, influencing its dimensional properties. Calculation of lava flow volume from thermal images collected by helicopter surveys has been largely used during the last decade for monitoring subaerial effusive eruptions. However, due to the depths where volcanic activity occurs, monitoring submarine volcanic eruptions is a very difficult task. The 2011–2012 submarine volcanic eruption at El Hierro, Canary Islands, has provided a unique and excellent opportunity to monitor eruptive processes occurring on the seabed. The use of a hand-held thermal camera during daily helicopter flights allowed us to estimate for the first time the daily and total erupted magma volumes from a submarine eruption. The volume of magma emitted during this eruption has been estimated at 300Mm3, giving an average effusion rate of ~25m3s−1. Thermal imagery by helicopter proved to be a fast, inexpensive, safe and reliable technique of monitoring volcanic eruptions when they occur on the shallow seabed.

The large and destructive 1669 AD eruption at Etna volcano: reconstruction of the lava flow field evolution and effusion rate trend

The 1669 AD flank eruption was the most destructive event on Etna volcano in historical times (∼700 BC) and provided, because of the presence of numerous quarries and subsurface data, the opportunity for a unique case study in which we directly measured the thickness of the lava field. Moreover, analysis of historical documents allowed reconstruction of the temporal evolution of the lava field and estimation of the average effusion rate. One hundred and thirty-eight thickness measurements, acquired from field surveys and subsurface data, allowed us to divide the lava field into 12 zones of homogenous mean thickness and to calculate a total lava volume of (607 ± 105) × 106 m3, corresponding to an average effusion rate of 58 ± 10 m3/s. This new volume differs by −24 % up to +64 %, from previously published values. The temporal evolution of the cumulative volume and average effusion rate were reconstructed for the first fourteen days, from field data and analysis of historical records. A short initial phase was characterized by a rapid increase in effusion rate, which reached a peak of ∼640 m3/s after 3 days. This was followed by a longer phase in which the flow rate decreased. The first 14 days were crucial for the development of the lava field, and in this time it covered 72 % of its final area and produced most of the damage. Thereafter, the growth of a complex lava tube network promoted lava field lengthening to the city of Catania, 17 km away from the vent. Effusion rate trends like those of the 1669 eruption can be adopted for future investigations aimed at assessing the effects of similar events on Etna’s most highly urbanized area and at other effusive basaltic volcanoes.

Photogrammetric monitoring of lava dome growth during the 2009 eruption of Redoubt Volcano

The 2009 eruption of Redoubt Volcano, Alaska, began with a phreatic explosion on 15 March followed by a series of at least 19 explosive events and growth and destruction of at least two, and likely three, lava domes between 22 March and 4 April. On 4 April explosive activity gave way to continuous lava effusion within the summit crater. We present an analysis of post-4 April lava dome growth using an oblique photogrammetry approach that provides a safe, rapid, and accurate means of measuring dome growth. Photogrammetric analyses of oblique digital images acquired during helicopter observation flights and fixed-wing volcanic gas surveys produced a series of digital elevation models (DEMs) of the lava dome from 16 April to 23 September. The DEMs were used to calculate estimates of volume and time-averaged extrusion rates and to quantify morphological changes during dome growth.Effusion rates ranged from a maximum of 35m3s−1 during the initial two weeks to a low of 2.2m3s−1 in early summer 2009. The average effusion rate from April to July was 9.5m3s−1. Early, rapid dome growth was characterized by extrusion of blocky lava that spread laterally within the summit crater. In mid-to-late April the volume of the dome had reached 36×106m3, roughly half of the total volume, and dome growth within the summit crater began to be limited by confining crater walls to the south, east, and west. Once the dome reached the steep, north-sloping gorge that breaches the crater, growth decreased to the south, but the dome continued to inflate and extend northward down the gorge. Effusion slowed during 16 April–1 May, but in early May the rate increased again. This rate increase was accompanied by a transition to exogenous dome growth. From mid-May to July the effusion rate consistently declined. The decrease is consistent with observations of reduced seismicity, gas emission, and thermal anomalies, as well as declining rates of geodetic deflation or inflation. These trends suggest dome growth ceased by July 2009. The volume of the dome at the end of the 2009 eruption was about 72×106m3, more than twice the estimated volume of the largest dome extruded during the 1989–1990 eruption. In total, the 2009 dome extends over 400m down the glacial gorge on the north end of the crater, with a total length of 1km, width of 500m and an average thickness of 200m.

Quantitative analysis of the 1981 and 2001 Etna flank eruptions: a contribution for future hazard evaluation and mitigation

Lava flows produced during Etna flank eruptions represent severe hazards for the nearby inhabited areas, which can be protected by adopting prompt mitigation actions, such as the building of diversion barriers. Lava diversion measures were attempted recently during the 1983, 1991-93, 2001 and 2002 Etna eruptions, although with different degrees of success. In addition to the complexity of barrier construction (due to the adverse physical conditions), the time available to successfully slow the advance of a lava flow depends on the lava effusion rate, which is not easily measurable. One method to estimate the average lava effusion rate over a specified period of time is based on a volumetric approach; i.e. the measurement of the volume changes of the lava flow over that period. Here, this has been compared to an approach based on thermal image processing, as applied to estimate the average effusion rates of lava flows during the 1981 and 2001 Etna eruptions. The final volumes were measured by the comparison of pre-eruption and post-eruption photogrammetric digital elevation models and orthophotographs. Lava volume growth during these eruptions was estimated by locating the flow-front positions from analyses of scientific papers and newspapers reports, as well as from helicopter photographs. The analyses of these two eruptions contribute to the understanding of the different eruptive mechanisms, highlighting the role of the peak effusion rate, which represents a critical parameter for planning of mitigation actions and for hazard evaluation.

The evolution of the Sciara del Fuoco subaerial slope during the 2007 Stromboli eruption: Relation between deformation processes and effusive activity

Focusing on the Island of Stromboli, this research investigates whether airborne remote sensing systems, such as those based on digital photogrammetry and laser scanner sensors, can be adopted to monitor slope deformation and lava emplacement processes in active volcanic areas. Thanks to the capability of extracting accurate topographic data and working on flexible time schedules, these methods can be used to constrain the regular and more frequent measurements derived from satellite observations. This work is dedicated to the monitoring of Stromboli's volcanic edifice which is beneficial when obtaining quantitative data on the geometry of deformation features and the displaced (failures and landslides) and emplaced (lava flows) volumes. In particular, we focus on the capability of extracting average effusion rates from volume measurements that can be used to validate or integrate satellite-derived estimates. Since 2001, a number of airborne remote sensing surveys, namely Digital Photogrammetry (DP) and Airborne Laser Scanning (ALS), have been carried out on Stromboli's volcano to obtain high resolution Digital Elevation Models (DEM) and orthophotos with sub-meter spatial resolution and a time schedule suitable for monitoring the morphological evolution of the surface during the quiescent phases. During the last two effusive eruptions (2002–2003 and 2007) the surface modifications, created on the Sciara del Fuoco slope and on the crater area as a consequence of effusive activity, were quantified and monitored using the same methodologies. This work, which is based on the results obtained from the multi-temporal quantitative analysis of the data collected from 2001 to 2007, mainly focuses on the 2007 eruption but also accounts for analogies and differences regarding the 2002–2003 event. The 2007 eruption on the Sciara del Fuoco slope from 27 February until 2 April, produced a compound lava field including a lava delta on the shoreline, discharging most of the lava into the sea. The comparison of the 2007 DEMs with a pre-eruption surface (2006 LIDAR survey) allowed for the evaluation of the total lava volume that accumulated on the subaerial slope while two syn-eruption DEMs were used to calculate the average effusion rates during the eruption. Since the evolution of a lava field produced during an eruption can be seen as a proxy for the magma intrusion mechanism, hypotheses are formulated on the connection between the lava discharge and the instabilities suffered by the slope.

Emplacement of the 1907 Mauna Loa basalt flow as derived from precision topography and satellite imaging

An eruption in January of 1907, from the southwest rift zone of Mauna Loa, produced a substantial lava flow field. Satellite images and Differential Global Positioning System (DGPS) survey data, along with observations and photographs from the field, are combined to provide a new perspective on the 1907 eruption. Boundaries of the flow field from the satellite data, combined with field measurements of flow thickness, indicate an area of 25.1 km 2 and a volume of 86.6 million m 3. The eastern lobe of the flow field covers an area of 13.1 km 2, with a volume of 55.0 million m 3, and was emplaced with an average effusion rate of 119 m 3/s (at least, for the upper portion of the lobe). Ten DGPS topographic profiles across the eastern lobe aid in distinguishing the characteristics of, and transitions between, the zones identified during the emplacement of the 1984 Mauna Loa flow. Several subdivisions have been built directly on top of or adjacent to the 1907 lava flow. The strong likelihood of future eruptions from the Mauna Loa southwest rift zone makes these housing developments of particular importance for assessments of potential volcanic hazards.

The magma budget of Volcán Arenal, Costa Rica from 1980 to 2004

Using topographic data collected by radar interferometry, stereo-photogrammetry, and field survey we have measured the changing surface of Volcán Arenal in Costa Rica over the period from 1980 to 2004. During this time this young volcano has mainly effused basaltic andesite lava, continuing the activity that began in 1968. Explosive products form only a few percent of the volumetric output. We have calculated digital elevation models for the years 1961, 1988 and 1997 and modified existing models for 2000 and 2004. From these we have estimated the volume of lava effused and coupled this with the data presented by an earlier study for 1968–1980. We find that a dense rock equivalent volume of 551 M m 3 was effused from 1968 to 2004. The dense rock equivalent effusion rate fell from about 2 m 3 s − 1 to about 0.1–0.2 m 3 s − 1 over the same period, with an average rate of about 0.5 m 3 s − 1 . Between 1980 and 2004, the average effusion rate was 0.36 m 3 s − 1 , a similar rate to that measured between 1974 and 1980. There have been two significant deviations from this long-term rate. The effusion rate increased from 1984 to 1991, at the same time as explosivity increased. After a period of moderate effusion rates in the 1990s, the rate fell to lower levels around 1999.

Postglacial eruptive history of the Western Volcanic Zone, Iceland

New field observations, age constraints, and extensive chemical analyses define the complete postglacial eruptive history of the 170‐km‐long Western Volcanic Zone (WVZ) of Iceland, the ultraslow‐spreading western boundary of the south Iceland microplate. We have identified 44 separate eruptive units, 10 of which are small‐volume eruptions associated with the flanking Grímsnes system. Overall chemical variations are consistent with very simplified models of melting of a source approximating primitive mantle composition. The 17 eruptions in the first 3000 years of postglacial time account for about 64% of the total postglacial production and are incompatible‐element depleted compared to younger units, consistent with enhanced melting as a consequence of rebound immediately following deglaciation. Steadily declining eruption rates for the last 9000 years also correlate with changes in average incompatible element ratios that appear to reflect continued decline in melting extents to the present day. This result is not restricted to the WVZ, however, and may herald a decline in melting throughout all of western Iceland during later postglacial time. Lavas from the northern part of the WVZ are depleted in incompatible elements relative to those farther south at all times, indicating either a long‐wavelength gradient in mantle source composition or variations in the melting process along axis. We find no evidence in the postglacial volcanic record for current failure of the WVZ, despite evidence for continued propagation of the eastern margin of the microplate. The dominance of lava shields in the eruptive history of the WVZ contrasts with the higher number of fissure eruptions in other Icelandic volcanic zones. WVZ shields represent long‐duration, low‐effusion rate eruptions fed by recharge magma arising out of the mantle. Average effusion rate is the key variable distinguishing shield and fissure eruptions, both within the WVZ and between different volcanic zones. High effusion rate, large‐volume eruptions require the presence of large crustal magma reservoirs, which have been rare or absent in the WVZ throughout postglacial time.

Effusive Activity at Mount Etna Volcano (Italy) During the 20th Century: A Contribution to Volcanic Hazard Assessment

Mount Etna is an open conduit volcano, characterised by persistent activity, consisting of degassing and explosive phenomena at summit craters, frequent flank eruptions, and more rarely, eccentric eruptions. All eruption typologies can give rise to lava flows, which represent the greatest hazard by the volcano to the inhabited areas. Historical documents and scientific papers related to the 20th century effusive activity have been examined in detail, and volcanological parameters have been compiled in a database. The cumulative curve of emitted lava volume highlights the presence of two main eruptive periods: (a) the 1900–1971 interval, characterised by a moderate slope of the curve, amounting to 436 × 106 m3 of lava with average effusion rate of 0.2 m3/s and (b) the 1971–1999 period, in which a significant increase in eruption frequency is associated with a large issued lava volume (767 × 106 m3) and a higher effusion rate (0.8 m3/s). The collected data have been plotted to highlight different eruptive behaviour as a function of eruptive periods and summit vs. flank eruptions. The latter have been further subdivided into two categories: eruptions characterised by high effusion rates and short duration, and eruptions dominated by low effusion rate, long duration and larger volume of erupted lava. Circular zones around the summit area have been drawn for summit eruptions based on the maximum lava flow length; flank eruptions have been considered by taking into account the eruptive fracture elevation and combining them with lava flow lengths of 4 and 6 km. This work highlights that the greatest lava flow hazard at Etna is on the south and east sectors of the volcano. This should be properly considered in future land-use planning by local authorities.

Volumetric characteristics of lava flows from interferometric radar and multispectral satellite data: the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Gal�pagos

We have used a suite of remotely sensed data, numerical lava flow modeling, and field observations to determine quantitative characteristics of the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galapagos Islands. Flank lava flow areas, volumes, instantaneous effusion rates, and average effusion rates were all determined for these two eruptions, for which only limited syn-eruptive field observations are available. Using data from SPOT, TOPSAR, ERS-1, and ERS-2, we determined that the 1995 Fernandina flow covers a subaerial area of 6.5×106 m2 and has a subaerial dense rock equivalent (DRE) volume of 42×106 m3. Field observations, ATSR satellite data, and the FLOWGO numerical model allow us to determine that the effusion rate declined exponentially from a high of ~60–200 m3 s-1 during the first few hours to <5 m3 s-1 prior to ceasing after 73 days, with a mean effusion rate of 4–16 m3 s-1. Integrating the ATSR-derived, exponentially declining effusion rate over the eruption duration produces a total (subaerial + submarine) DRE volume of between 27 and 100×106 m3, the range in values being due to differing assumptions about heat loss characteristics; only values in the higher part of this range are consistent with the independently derived subaerial volume. Using SPOT, TOPSAR, ERS-1, and ERS-2 data, we determine that the 1998 Cerro Azul flow is 16 km long, covers 16 km2, and has a DRE volume of 54×106 m3. FLOWGO produces at-vent velocity and effusion rate values of 11 m s-1 and ~600 m3 s-1, respectively. The velocity value agrees well with the 12 m s-1 estimated in the field. The mean effusion rate (total DRE volume/duration) was 7–47 m3 s-1. Dike dimensions, fissure lengths, and pressure gradients along the conduit based on magma chamber depth estimates of 3–5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude, the range being due to uncertainty in the magma viscosity, dike dimensions, and pressure gradient between magma chamber and vent. Although somewhat consistent with mean effusion rates from other techniques, their wide range makes them less useful. The exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions. Our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption, as well as time constants of exponential decay during the eruption. The results and theoretical relationships combine to indicate that at both volcanoes probably 25–30% of the volumetric increase in the magma chamber erupted as lava onto the surface. In both eruptions the lava flow volumes are less than 1% of the magma chamber volume.

The July?August 2001 eruption of Mt. Etna (Sicily)

The July–August 2001 eruption of Mt. Etna stimulated widespread public and media interest, caused significant damage to tourist facilities, and for several days threatened the town of Nicolosi on the S flank of the volcano. Seven eruptive fissures were active, five on the S flank between 3,050 and 2,100 m altitude, and two on the NE flank between 3,080 and 2,600 m elevation. All produced lava flows over various periods during the eruption, the most voluminous of which reached a length of 6.9 km. Mineralogically, the 2001 lavas fall into two distinct groups, indicating that magma was supplied through two different and largely independent pathways, one extending laterally from the central conduit system through radial fissures, the other being a vertically ascending eccentric dike. Furthermore, one of the eccentric vents, at 2,570 m elevation, was the site of vigorous phreatomagmatic activity as the dike cut through a shallow aquifer, during both the initial and closing stages of the eruption. For 6 days the magma column feeding this vent was more or less effectively sealed from the aquifer, permitting powerful explosive and effusive magmatic activity. While the eruption was characterized by a highly dynamic evolution, complex interactions between some of the eruptive fissures, and changing eruptive styles, its total volume (~25×106 m3 of lava and 5–10×106 m3 of pyroclastics) was relatively small in comparison with other recent eruptions of Etna. Effusion rates were calculated on a daily basis and reached peaks of 14–16 m3 s-1, while the average effusion rate at all fissures was about 11 m3 s-1, which is not exceptionally high. The eruption showed a number of peculiar features, but none of these (except the contemporaneous lateral and eccentric activity) represented a significant deviation from Etna's eruptive behavior in the long term. However, the 2001 eruption could be but the first in a series of flank eruptions, some of which might be more voluminous and hazardous. Placed in a long-term context, the eruption confirms a distinct trend, initiated during the past 50 years, toward higher production rates and more frequent eruptions, which might bring Etna back to similar levels of activity as during the early to mid seventeenth century.

A simple explanation for the space-based calculation of lava eruption rates

Knowing how lava effusion rates vary during basaltic eruptions can be of great significance when trying to provide preliminary forecasts regarding how far lava will flow. However, problems exist in accurately determining effusion rates using conventional field-based techniques. To ameliorate this problem Harris et al. [J. Geophys. Res. 102 (1997), 7985–8003; Bull. Volcanol. 59 (1997), 49–64; J. Volcanol. Geotherm. Res. 102 (2000), 237–269] developed a method for determining effusion rates using infrared satellite data, and showed how the method could be used to provide realistic estimates of effusion rates, repeatedly during several eruptions at Kilauea (Hawai’i) Krafla (Iceland), Etna and Stromboli (Italy). Harris et al. [J. Geophys. Res. 102 (1997), 7985–8003; Bull. Volcanol. 59 (1997), 49–64] indicate that their method allows instantaneous lava effusion rates to be determined thermodynamically by equating the amount of heat lost by an active lava flow (derived from the satellite data) to the amount of heat liberated by the cooling mass of lava. The purpose of this paper is to provide a simpler, alternative explanation. We find that rather than being used to calculate heat loss, Harris et al. [J. Geophys. Res. 102 (1997), 7985–8003; Bull. Volcanol. 59 (1997), 49–64; J. Volcanol. Geotherm. Res. 102 (2000), 237–269] actually use the satellite data to estimate the area of active lava present within the satellite’s field of view at the movement of data acquisition. Thus, changes in the effusion rates they present can only be proportional to changes in this area. The active flow areas were then multiplied by a constant, the value of which is obtained from a crude approximation of the lava flows heat balance. Crucially, the absolute value of this term falls within the range of an empirically derived parameter that was found by Pieri and Baloga [J. Volcanol. Geotherm. Res. 30 (1986),29–45] to explain strong linear correlations between eruption rate (i.e. the time-averaged effusion rate) and lava flow area for 34 historic Hawaiian flows. As a result, we find that the method of Harris et al. [J. Geophys. Res. 102 (1997), 7985–8003] does not yield instantaneous effusion rates, but instead provides a valid and useful way to estimate average effusion rates (i.e. the eruption rate) from measurements of flow area.