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  • 1.
    Benediktsdottir, Asdis
    et al.
    Univ Iceland, Inst Earth Sci, Nord Volcanol Ctr, Sturlugata 7, IS-101 Reykjavik, Iceland.;Univ Iceland, Dept Earth Sci, Sturlugata 7, IS-101 Reykjavik, Iceland..
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Brandsdottir, Bryndis
    Univ Iceland, Sci Inst, Inst Earth Sci, Reykjavik, Iceland..
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ambient noise tomography of Eyjafjallajokull volcano, Iceland2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 347, p. 250-263Article in journal (Refereed)
    Abstract [en]

    We present a shear-velocity model for the Eyjafjallajokull stratovolcano, based on ambient seismic noise tomography applied to seven months of data from six permanent stations and -10 temporary seismic stations, deployed during and after the 2010 volcanic unrest. Vertical components of noise were cross correlated resulting in 30 robust phase-velocity dispersion curves between 1.6 and 6.5 s in period, displaying a +/- 20% variation in phase velocity beneath the volcano. The uneven distribution of noise sources, evaluated using signal-to-noise ratios, was estimated to cause less than 2% error in most curves. Sensitivity kernels showed resolution down to 10 km and the lateral resolution of the resulting phase-velocity maps was about 5 km. The model reveals east-west oriented high-velocity anomalies due east and west of the caldera. Between these a zone of lower velocity is identified, coinciding with the location of earthquakes that occurred during the summit eruption in April 2010. A shallow, southwest elongated low-velocity anomaly is located 5 km southwest of the caldera. The limited depth resolution of the shear-velocity model precludes detection of melt within the volcano.

  • 2. Berghuijs, Jaap F.
    et al.
    Mattsson, Hannes B.
    Magma ascent, fragmentation and depositional characteristics of “dry” maar volcanoes: Similarities with vent-facies kimberlite deposits2013In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 252, p. 53-72Article in journal (Refereed)
    Abstract [en]

    Several maar craters within the Lake Natron-Engaruka monogenetic volcanic field (LNE-MVF) of northern Tanzania show compelling evidence for magmatic fragmentation and dry deposition. This is in contradiction of the common belief that most maars are formed through the explosive interaction between ascending magma and ground- or surface water. We here present a detailed study on the eruptive and depositional characteristics of the Loolmurwak and Eledoi maar volcanoes, two of the largest craters in the LNE-MVF, focusing on high-resolution stratigraphy, sedimentology, grain size distribution, pyroclast textures and morphologies, bulk geochemistry and mineral chemistry. At both maars, ejected material has been emplaced by a combination of pyroclastic surges and fallout. Indicators of phreatomagmatic fragmentation and wet deposition, such as impact sags, accretionary lapilli, vesiculated tuffs and plastering against obstacles, are absent in the deposits. juvenile material predominantly occurs as fluidal-shaped vesicular melt droplets and contains no glass shards produced by the breakage of bubble walls. The Eledoi deposits comprise a large amount of inversely graded beds and lenses, which result from grain flow in a dry depositional environment. Preferential deposition of fine material toward the northern side of its crater can be related to effective wind winnowing in a dry eruption plume. This large variety of observations testifies to the dominance of magmatic fragmentation as well as dry deposition at the Loolmurwak and Eledoi maars, which is in line with what has been found for other structures in the LNE-MVF but contrasts with current ideas on maar formation. We infer that a volatile-rich, olivine melilitic magma was formed by small amounts of partial melting at upper mantle depths. With minimum average ascent rates of 5.3 m s(-1) for Loolmurwak and 26.2 m s(-1) for Eledoi, this magma rapidly moved toward the surface and exsolved a substantial amount of volatiles, sufficiently large to drive magmatic fragmentation. Both eruptions were pulsating in intensity and relatively short-lived, with estimated durations of 23 and 10 h for Loolmurwak and Eledoi, respectively. The depositional characteristics of these maars, including the abundant occurrence of mantle xenoliths in the deposits, as well as their envisaged mode of emplacement show a strong similarity to the often poorly preserved vent-facies of kimberlitic diatremes. Therefore, future research on well-preserved melilititic maar-diatreme deposits may provide valuable insights into kimberlite emplacement processes. (C) 2012 Elsevier B.V. All rights reserved.

  • 3. Bosshard-Stadlin, Sonja A.
    et al.
    Mattsson, Hannes B.
    Keller, Joerg
    Magma mixing and forced exsolution of CO2 during the explosive 2007-2008 eruption of Oldoinyo Lengai (Tanzania)2014In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 285, p. 229-246Article in journal (Refereed)
    Abstract [en]

    Oldoinyo Lengai is probably most famous for being the only active volcano on Earth which is erupting natrocarbonatitic magma. However, the mildly explosive natrocarbonatitic activity is alternating with highly explosive, nephelinitic eruptions of which the most recent episode occurred in September 2007 (and lasted until April 2008). Here we present petrographic observations, mineral chemistry as well as major- and trace element analyses of samples covering the evolution of the eruption with time. In the early phases of the eruption, the phenocryst assemblages are dominated by the carbonate minerals nyerereite and gregoryite, but as the eruption progresses the mineralogy becomes dominated by silicate minerals like nepheline, pyroxene, garnet, alumoakermanite, combeite and wollastonite. The observed major- and trace element variations during the 2007-2008 eruption indicate mixing between a natrocarbonatitic magma and a combeite-wollastonite-bearing nephelinitic magma (CWN), with higher portions of natrocarbonatite in the early stages of the eruption. Euhedral and uncorroded clinopyroxene crystals are abundant in the late 2007 deposits but quickly start to break-down and corrode as the eruption continues, indicating that the natrocarbonatite and the CWN are not in fact conjugate magmas derived from a single magma reservoir, but must have evolved separately in the crust from the point of immiscibility. When these magmas interact beneath the volcano, a hybrid silicate magma forms (where clinopyroxene is no longer stable) and the composition of this hybrid causes the overall solubility of CO2 in the system to decrease drastically. This results in rapid exsolution of CO2 (g) which is allowed to expand during ascent, and we conclude that this is most likely the reason behind the unexpected vigor in the explosive eruptions of Oldoinyo Lengai. This massive release of CO2 during ascent may also explain the petrographic features of the pyroclasts as these are dominated by near-spherical droplets with moderate vesicularities, indicative of being transported in a hot gas-stream/jet in the upper conduit and forming an aerosol-type spray. (C) 2014 Elsevier B.V. All rights reserved.

  • 4. Bosshard-Stadlin, Sonja A.
    et al.
    Mattsson, Hannes B.
    Stewart, Carol
    Reusser, Eric
    Leaching of lava and tephra from the Oldoinyo Lengai volcano (Tanzania): Remobilization of fluorine and other potentially toxic elements into surface waters of the Gregory Rift2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 332, p. 14-25Article in journal (Refereed)
    Abstract [en]

    Volcanic ash leachate studies have been conducted on various volcanoes on Earth, but few have been done on African volcanoes until now. Tephra emissions may affect the environment and the health of people living in this area, and therefore we conducted a first tephra (ash and lapilli sized) leachate study on the Oldoinyo Lengai volcano, situated in northern Tanzania. The recent explosive eruption in 2007-2008 provided us with fresh samples from the first three weeks of the eruption which were used for this study. In addition, we also used a natrocarbonatitic sample from the activity prior to the explosive eruption, as the major activity at Oldoinyo Lengai is natrocarbonatitic. To compare the leaching process affecting the natrocarbonatitic lavas and the tephras from Oldoinyo Lengai, the 2006 natrocarbonatitic lava flow was resampled 5 years after the emplacement and compared to the initial, unaltered composition. Special interest was given to the element fluorine (F), since it is potentially toxic to both humans and animals. A daily intake of fluoride (F-) in drinking water of >1.5 mg/l can lead to dental fluorosis, and higher concentrations lead to skeletal fluorosis. For this reason, a guideline value for fluoride in drinking water was set by the WHO (2011) to 1.5 mg/l. However, surface waters and groundwaters in the Gregory Rift have elevated fluoride levels of up to 9.12 mg/l, and as a consequence, an interim guideline value for Tanzania has been set at 8 mg/l. The total concentration of fluorine in the samples from the natrocarbonatitic lava flow is high (3.2 wt%), whereas we observed a significant decrease of the fluorine concentration (between 1.7 and 0.5 wt%) in the samples collected three days and three weeks after the onset of the explosive 2007-08 eruption. However, the total amount of water-extractable fluoride is lower in the natrocarbonatitic lavas (319 mg/l) than in the nephelinitic tephra (573-895 mg/l). This is due to the solubility of the different F-bearing minerals. In the natrocarbonatites, fluorine exists predominantly in fluorite (CaF2), and in the early tephra as Na-Mg bearing salts such as neighborite (NaMgF3) and sellaite (MgF2). All these three minerals have very low solubility in water (16-130 mg/l). The later nephelinitic tephras contain surface coating of villiaumite (NaF), which is highly soluble (42,200 mg/l) in water and can thus release the fluoride more readily upon contact with water. Although there is still the need for further data and a more precise study on this topic in Tanzania, we can already draw a first conclusion that the intake of water during or directly following the deposition of the tephra is not advisable and should be avoided, whereas the release of fluoride from the lava flow has less influence on the river waters. 2017 Elsevier B.V. All rights reserved.

  • 5.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    New insights in the mechanics of sill emplacement provided by field observations of the Njardvik Sill, Northeast Iceland2008In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 173, no 3-4, p. 280-288Article in journal (Refereed)
    Abstract [en]

    Sills are concordant sheet-like bodies of magma. Their mechanics of emplacement is an important but still not fully understood topic. The well-exposed basaltic Njardvik Sill in the extinct Tertiary Dyrfjöll Volcano in Northeast Iceland offers exceptionally clear insights into the mechanism of sill emplacement. The sill is multiple and consists of at least 7 units (sills) all of which were emplaced along a sharp contact between a rhyolitic intrusion and adjacent basaltic lava flows. Each sill unit was supplied with magma from an inclined sheet. The contacts between the sheets and the sill units are very clear and show that the sill units are much thicker than their feeder sheets. Since the Njardvik Sill consists of separate units, it obviously did not evolve into a homogeneous magma body. Nevertheless, the abrupt change in dip and thickness from inclined sheets to horizontal sills at this particular locality indicates that the earlier sills were influencing the stress field in their vicinity during the subsequent sheet injections. The local stresses around the newly formed sill units forced each of the subsequently injected sheets to change into sills. The Njardvik Sill can be followed laterally in a coastal section for 140 m until it ends abruptly at a fault that cuts the sill. Using these field observations as a basis, a numerical model shows how an inclined sheet opens up the contact between the felsic intrusion and the basaltic lava pile, along which the sill emplacement takes place. The results suggest that sill emplacement is primarily the result of stress rotation at contacts between layers of contrasting mechanical properties. There, the orientation of the maximum principal compressive stress σ1 is horizontal. Hence, such contacts can represent interfaces along which sill emplacement is encouraged. Once a sill has been emplaced, it extends the stress field with a horizontal orientation of σ1. Consequently, inclined sheets and dykes injected near the sill will be deflected into sills. The injection frequency of further sill units controls if the sill can grow into a larger magma body by mixing of the newly supplied with the initially injected magma. In case of the Njardvik Sill, the injection frequency was low, so subsequently emplaced sill units can be distinguished.

  • 6.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    New insights into the mechanics of sill emplacement provided by field observations of the Njardvik Sill, Northeast Iceland2008In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 173, no 3-4, p. 280-288Article in journal (Refereed)
    Abstract [en]

    Sills are concordant sheet-like bodies of magma. Their mechanics of emplacement is an important but still not fully understood topic. The well-exposed basaltic Njardvik Sill in the extinct Tertiary Dyrfjöll Volcano in Northeast Iceland offers exceptionally clear insights into the mechanism of sill emplacement. The sill is multiple and consists of at least 7 units (sills) all of which were emplaced along a sharp contact between a rhyolitic intrusion and adjacent basaltic lava flows. Each sill unit was supplied with magma from an inclined sheet. The contacts between the sheets and the sill units are very clear and show that the sill units are much thicker than their feeder sheets. Since the Njardvik Sill consists of separate units, it obviously did not evolve into a homogeneous magma body. Nevertheless, the abrupt change in dip and thickness from inclined sheets to horizontal sills at this particular locality indicates that the earlier sills were influencing the stress field in their vicinity during the subsequent sheet injections. The local stresses around the newly formed sill units forced each of the subsequently injected sheets to change into sills. The Njardvik Sill can be followed laterally in a coastal section for 140 m until it ends abruptly at a fault that cuts the sill. Using these field observations as a basis, a numerical model shows how an inclined sheet opens up the contact between the felsic intrusion and the basaltic lava pile, along which the sill emplacement takes place. The results suggest that sill emplacement is primarily the result of stress rotation at contacts between layers of contrasting mechanical properties. There, the orientation of the maximum principal compressive stress σ1 is horizontal. Hence, such contacts can represent interfaces along which sill emplacement is encouraged. Once a sill has been emplaced, it extends the stress field with a horizontal orientation of σ1. Consequently, inclined sheets and dykes injected near the sill will be deflected into sills. The injection frequency of further sill units controls if the sill can grow into a larger magma body by mixing of the newly supplied with the initially injected magma. In case of the Njardvik Sill, the injection frequency was low, so subsequently emplaced sill units can be distinguished.

  • 7. Clarke, H.
    et al.
    Troll, V. R.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Carracedo, J.C.
    Phreatomagmatic to strombolian eruptive activity of basaltic cinder cones: Montana Los Erales, Tenerife, Canary Islands2009In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 180, p. 225-245Article in journal (Refereed)
  • 8.
    Dingwell, Adam
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Rutgersson, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Estimating volcanic ash hazard in European airspace2014In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 286, p. 55-66Article in journal (Refereed)
    Abstract [en]

    The widespread disruption of European air traffic in late April 2010, during the eruption of Eyjafjallajökull,showed the importance of early assessment of volcanic hazard from explosive eruptions. In this study, wefocus on the short-term hazard of airborne ash from a climatological perspective, focusing on eruptions onIceland. By studying eruptions of different intensity and frequency, we estimate the overall probability that ashconcentration levels considered hazardous to aviation are exceeded over different parts of Europe.

    The method involves setting up a range of eruption scenarios based on the eruptive history of Icelandic volcanoes,and repeated simulation of these scenarios for 2 years' worth of meteorological data. Simulations are conducted using meteorological data from the ERA-Interim reanalysis set, which is downscaled using the Weather Researchand Forecasting (WRF) model. The weather data are then used to drive the Lagrangian particle dispersion model FLEXPART-WRF for each of the eruption scenarios. A set of threshold values, commonly used in Volcanic Ash Advisories, are used to analyze concentration data from the dispersion model.

    We see that the dispersion of ash is highly dominated by the mid-latitude westerlies and mainly affect northern UK and the Scandinavian peninsula. The occurrence of high ash levels from Icelandic volcanoes is lower over con-tinental Europe but should not be neglected for eruptions when the release rate of fine ash (<16 μm) is in theorder of 107 kg s−1 or higher.

    There is a clear seasonal variation in the ash hazard. During the summer months, the dominating dispersiondirection is less distinct with some plumes extending to the northwest and Greenland. In contrast, during thewinter months, the strong westerly winds tend to transport most of the emissions eastwards. The affected area of a winter-time eruption is likely to be larger as high concentrations can be found at a further distance downwind from the volcano, effectively increasing the probability of hazardous levels of ash reaching the European continent.

    The concentration thresholds for aviation, which were adopted after the Eyjafjallajökull eruption in 2010, havestrong influence on the hazard estimates for weaker eruptions but is less important for larger eruptions; thusash forecasts for weaker eruptions are likely more uncertain in comparison to larger eruptions.

  • 9. Donoghue, E.
    et al.
    Troll, V.R.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Harris, C.
    OHalloran, A.
    Perez-Torado, J.F.
    Walter, T.R.
    Low-temperature hydrothermal alteration of intra-caldera tuffs, Tejeda caldera, Gran Canaria: mineralogical and isotopic changes.2008In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 176, p. 551-564Article in journal (Refereed)
  • 10.
    Gudmundsson, Olafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Benediktsdottir, Asdis, Gudmundsson, Olafur, Brandsdottir, Bryndis, Tryggvason, Ari, 2017.  Ambient noise tomography of Eyjafjallajökull volcano, Iceland, J. Volc. Geoth. Res. 347, 250-263.2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Jornal of Volcanology and Geothermal Research, ISSN 0377-0273, Vol. 347, p. 250-263Article in journal (Refereed)
  • 11.
    Gudmundsson, Olafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Jeddi, Zeinab, Gudmundsson, Olafur, Tryggvason, Ari, 2017.  Ambient-noise tomography of Katla volcano, south Iceland, J. Volc. Geoth. Res. 347, 264-277.2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Journal of Volanology and Geothermal Research, ISSN 0377-0273, Vol. 347, p. 264-277Article in journal (Refereed)
  • 12.
    Gudmundsson, Olafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Sgattoni, Giulia, Gudmundsson, Olafur, Einarsson, Pall, Lucchi, Federico, Li, Ka Lok, Sadeghisorkhani, Hamzeh, Roberts, Roland, Tryggvason, A, 2017.  The 2011 unrest at Katla volcano: characterization and interpretation of the tremor sources, J. Volc. Geoth. Res. 338, 63-78.2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Journal of volcanology and Geothermal Research, ISSN 0377-0273, Vol. 338, p. 63-78Article in journal (Refereed)
  • 13. Jaxybulatov, Kairly
    et al.
    Koulakov, Ivan
    Ibs-von Seht, Malte
    Klinge, Klaus
    Reichert, Christian
    Dahrén, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Troll, Valentin R.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Evidence for high fluid/melt content beneath Krakatau volcano (Indonesia) from local earthquake tomography2011In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 206, no 3-4, p. 96-105Article in journal (Refereed)
    Abstract [en]

    Within the KRAKMON project for multiparameter monitoring of Anak Krakatau volcano (Indonesia), a network of temporary stations was installed on the islands of the Krakatau complex as well as in the surrounding areas of the Sunda Strait, Sumatra and Java. The network was operated from June 2005 until January 2006. More than 700 local events were recorded during this experiment, and travel times from these events were used to perform a tomographic inversion for P and S velocities and for the Vp/Vs ratio. In this study, special attention was paid to the validation of the computed model based on different tests, such as inversion of independent data subsets and synthetic modeling. Although the network configuration and the distribution of the events are not favorable for high-quality tomographic imaging, we have obtained some important and robust features which give information about sources of volcanic activity in the Krakatau complex. The most interesting feature of this study is a zone of high Vp/Vs ratio beneath the Krakatau complex. At depths down to 4 km depth we observe anticorrelation of higher P- and lower S-velocities that leads to Vp/Vs ratio higher than 2. This is a probable indicator of the presence of partially molten and/or with high fluid content material with a composition corresponding to deeper layers. It is important that the anomaly of high Vp/Vs ratio beneath the Krakatau complex appears to be separated in two parts at a depth of 5-6 km. This fits to results of geobarometric analysis that presume the existence of several levels of magma chambers beneath Anak Krakatau.

  • 14.
    Jeddi, Zeinab
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ambient-noise tomography of Katla volcano, south Iceland2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 347, p. 264-277Article in journal (Refereed)
    Abstract [en]

    A shear-wave velocity model of subglacial Katla volcano, southern Iceland, has been developed using ambient seismic noise tomography based on data from a temporary network operating between May 2011 and August 2013 and permanent stations around the volcano. Phase-velocity dispersion curves were obtained using cross correlations of vertical components of 136 station pairs and non-linearly inverted for phase-velocity maps between 1.7 and 7.5 s. Local dispersion curves were inverted for shear-velocity variation with depth using a grid search imposing a fixed ice layer at the top. The resulting one-dimensional (1-D) velocity models were combined to obtain a pseudo three-dimensional (3-D) model with estimated lateral resolution of 8 km and depth resolution varying from close to 1 km near the surface to about 8 km at 10 km depth. Shear wave velocities are generally higher within the Katla central volcano than in its surroundings. The most striking feature of the model is a high-velocity anomaly beneath the caldera at >6 km depth interpreted to be due to cumulates resulting from differentiation of shallower magma intrusions and remelting of subsiding upper crust. No shallow low-velocity anomaly is resolved beneath the central caldera, but a low-velocity region is found at 2-4 km depth beneath the western half of the caldera. V-p/V-s ratios, estimated from average velocity-depth profiles from surface-wave data and higher frequency P-wave data, are anomalously high (>1.9) compared to average Icelandic crust, particularly in the top 2-3 km. This is argued not to be an artifact due to lateral refraction or topography. Instead, this anomaly could be explained as an artifact caused by velocity dispersion due to attenuation and a difference in frequency content, and possibly to a degree by the compositional difference between the transalkalic Fe-Ti basalts of Katla and average tholeiitic Icelandic crust.

  • 15.
    Juncu, Daniel
    et al.
    University of Iceland.
    Árnadóttir, Thóra
    University of Iceland.
    Geirsson, Halldor
    University of Iceland.
    Gudmundsson, Gunnar
    Icelandic Meteorological Office.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gunnarsson, G
    OR - Reykjavik Energy.
    Hooper, Andrew
    University of Leeds.
    Sigrun, Hreinsdottir
    GNS Science, New Zealand.
    Michalczewska, K
    University of Iceland.
    Injection-induced surface deformation and seismicity at the Hellisheidi geothermal field, Iceland2018In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097Article in journal (Refereed)
  • 16.
    Kalscheuer, Thomas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Commer, Michael
    Helwig, Stefan L.
    Hördt, Andreas
    Tezkan, Buelent
    Electromagnetic evidence for an ancient avalanche caldera rim on the south flank of Mount Merapi, Indonesia2007In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 162, no 1-2, p. 81-97Article in journal (Refereed)
    Abstract [en]

    Long-Offset Transient Electromagnetic (LOTEM) data and VIBROTEM data from the south flank of Mount Merapi on Java island, Indonesia, are interpreted with one-dimensional (1D) inversions as well as two-dimensional (2D) forward modelling. One-dimensional joint inversions of several components of the electromagnetic field with Occam's method reduce the number of equivalent models, which were derived from inversions of single components and fit the data to a similar misfit. The 1D results, together with results from other geophysical measurements, serve as the basic model for further 2D forward modelling. The final model depicts a layering that follows the topography of the strato-volcano. In the depth range of 500 m to 1000 m, the resistivity of the layers decreases rapidly downwards into a good conductor with resistivities below 10 Ohm*m. The deepest layer has a resistivity of 0.4 Ohm*m which is quantitatively explained with a combination of saline fluids and hydrothermally altered minerals. Furthermore, the final model supports a hypothesis from the interpretation of central-loop TEM (Transient Electromagnetic) data that there is a fault structure below the southern flank, approximately 7.3 km south of the summit. To the north of the fault, the top of the good conductor is lowered from a depth of 500 m to 1000 m. We propose that the fault structure coincides with an ancient avalanche caldera rim.

  • 17.
    Li, Ka Lok
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Abril, Claudia
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Reykjavic Univ, Sch Sci & Engn, Reykjavik, Iceland.
    Gudmundsson, Gunnar B.
    Iceland Meteorol Off, Reykjavik, Iceland.
    Seismicity of the Hengill area, SW Iceland: Details revealed by catalog relocation and collapsing2019In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 376, p. 15-26Article in journal (Refereed)
    Abstract [en]

    The spatial distribution of seismicity in the Hengill region, SW Iceland, is analyzed by relocation and collapsing. The Hengill region is a diffuse triple junction with volcano-tectonic activity associated with rifting, tectonic activity on a transecting transform and induced seismicity due to drilling and injection of fluid into geothermal fields. The Icelandic Meteorological Office has compiled 114,000 events over a 20-year period within an area of approximately 600 km(2). The events in their catalog are relocated by application of empirical travel-time tables using a non-linear location strategy. The relocations are then redone applying a Bayesian inversion using the catalog event density as a prior. Finally, they are collapsed using the same catalog density as an attractor. We show that this catalog processing reproduces details of the spatial pattern of seismicity that independently emerges from relative relocations of a small subset of the catalog events (swarm activity). In particular, the predominant faulting orientations are reproduced in different parts of the region and the depth distribution of events resembles that obtained by dense deployments in the area. Its depth extent varies between 5 and 7 km in the northern part of the region, where volcanic processes dominate, and between 7 and 8 km in the southern part, where tectonic deformation is predominant. Induced seismicity is shallower than adjacent natural seismicity. An intriguing lineation emerges in the lateral distribution of inferred depth to the brittle-ductile transition in the northern volcanic part of the region, which is parallel to the strike of the fissure swarms in the area. Associating this transition with an isotherm (650 degrees C), the Hengill volcanic system and its fissure swarm appear to be considerably cooler than the Hromundartindur system. This may relate to a recent intrusion into the latter or more efficient cooling in the Hengill fissure swarm due to deeper penetrating permeability. In both cases this has potential consequences for geothermal exploitation in the area.

  • 18. Mancini, Andrea
    et al.
    Mattsson, Hannes B.
    Bachmann, Olivier
    Origin of the compositional diversity in the basalt-to-dacite series erupted along the Heidarspordur ridge, NE Iceland2015In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 301, p. 116-127Article in journal (Refereed)
    Abstract [en]

    The Heidarspordur ridge, located in the Northern Volcanic Zone of Iceland, was formed approximately 9000 years ago by a volcanic episode known as Ludent Fires. The episode produced a broad spectrum of different magma types, forming approximately 50 small scoria cones and two larger craters (Ludent and Hraunbunga). The bulk compositions cluster in five distinct groups: (1) olivine basalts, (2) Fe-Ti basalts, (3) basaltic icelandites, (4) icelandites, and (5) dacites. Major and trace element trends, together with mineral chemistry and isotopic ratios, suggest that the dominant process involved in generating the evolved magmas was crystal fractionation occurring at variable depth. An origin by polybaric differentiation is confirmed by MELTS modeling. Magma mixing played a dominant role in the formation of the basaltic icelandites. Additionally, the Fe-Ti basalts, which erupted shortly after the dacites and used approximately the same vent area, display unusually high concentrations of Fe, Ti, P, and Sr. Their composition is best explained by some pyroxene-dominated fractionation (prior to Fe-Ti oxide stability), and by entrainment of some crystal cumulate material at shallow depth, mostly left over from the silicic differentiation stage. Textural and chemical features of the minerals (e.g., presence of glomerocrysts, two populations of plagioclase in these basalts) support this interpretation of evolved cumulate remobilization. Fe-Ti basalts with the same field, compositional and textural characteristics have also been erupted in the nearby but magmatically independent Krafla Volcanic System, suggesting that a similar differentiation trend occurs also in this larger central volcano. (C) 2015 Elsevier B.V. All rights reserved.

  • 19.
    Mattsson, H B
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Hoskuldsson, A
    Eruption reconstruction, formation of flow-lobe tumuli and eruption duration in the 5900 BP Helgafell lava field (Heimaey), south Iceland2005In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 147, no 1-2, p. 157-172Article in journal (Refereed)
    Abstract [en]

    The 5900 BP Helgafell eruption started with an intense fire-fountaining phase that deposited scoria and spatter near the vent. Spatter accumulated, agglutinated, and was subsequently emplaced. as clastogenic flows. The intense initial phase was followed by effusive emplacement of tube-fed pahoehoe. This type of lava emplacement makes up the bulk of the lava field, and contains abundant inflation features such as tumuli and pressure-ridges. We measured the depths of 358 inflation clefts in tumuli and the thicknesses of the upper-vesicular crust in flow-lobes from the Helgafell lava field. Measurements of altitude between a point-source of lava tubes and the depth of inflation clefts in tumuli suggest that the magmastatic pressure affects initial tumuli formation. The overall growth of tumuli is, however, more dependent on effusion rate and collapse/clogging of individual tubes than on the magmastatic head within tubes. Three areas with abundant tumuli are recognized in the Helgafell lava field, each of which was active for at least 47 days. The average time for tumuli formation is 32 days. Using the crustal measurements of inflation features we test the hypothesis that the total eruption duration can be estimated for a pre-historic volcanic eruption. We chose the Helgafell lava field to test this hypothesis because it has (1) abundant inflation features that are easily measured, and (2) the majority of lava flows emplaced during the eruption are well exposed. Our result shows that the duration of the Helgafell eruption can be estimated to 11-12 months, yielding an average volumetric effusion rate for the Helgafell eruption of 0.05-0.06 km(3)/month. This result is very similar to the effusion rates for two known eruptions in the same volcanic system (i.e. the 1963-1967 Surtsey and the 1973 Eldfell eruptions, with effusion rates of 0.02 and 0.04 km(3)/month, respectively). Given that all three eruptions occurred within the same volcanic system and that they have similar volumetric effusion rates, we suggest that our estimated eruption duration for Helgafell is reasonable. Our results, building on the suggestion by Hon et al. (1994) [Hon, K., Kauhikaua, J., Denlinger, R., MacKay, K., 1994. Emplacement and inflation of pahoehoe sheet flows: observations and measurements of active lava flows on Kilauea Volcano, Hawaii. Geol. Soc. Amer. Bull. 106, 351-370] that the duration of inflation can be estimated for individual flows by measuring inflation features, show that by combining abundant and measureable inflation features with a well-established stratigraphy the duration of an entire pre-historic eruption can be estimated. (c) 2005 Elsevier B.V. All rights reserved.

  • 20.
    Mattsson, H B
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Hoskuldsson, A
    Hand, S
    Crustal xenoliths in the 6220 BP Saefell tuff-cone, south Iceland: Evidence for a deep, diatreme-fonning, Surtseyan eruption2005In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 145, no 3-4, p. 234-248Article in journal (Refereed)
    Abstract [en]

    Three types of xenoliths were collected from a 80 in high vertical section of the Refell tuff-cone on Heimaey, Iceland. These types are: alkali basaltic (Type I), sedimentary (Type II) and cpx-bearing basalts (Type III). They are distinct in hand specimen, thin section and geochemistry. Near the base of the eruptive succession only juvenile material and Type I xenoliths are present. The middle section has Type II xenoliths in addition to Type I. Xenoliths of Type III first appear in the uppermost xenolith-rich horizons, where they are found together with all other types. Juvenile Swfell clasts are present in various amounts throughout the investigated section. A downward migration of explosion foci during the eruption is established based on the distribution of the different types of xenoliths in the tuff-cone (i.e. the appearance of Type I-Type II-Type III xenoliths with increasing stratigraphic height) in combination with the observed relations in basement stratigraphy beneath Heimaey. In the beginning of the eruption a combination of high eruption rate and abundant seawater resulted in shallow-seated explosions depositing only juvenile material and Type I xenoliths. The downward migration of explosion foci is suggested to be the result of lowered influx of seawater (due to the formation of a cone that reached 50-75 in a.s.l.) in combination with lowered eruption rate. As the basement stratigraphy beneath Heimaey is well documented by a 1565 in deep drill-hole, the depth of the explosions can be constrained to vary between less than 170 in in the beginning of the eruption and exceeding 820 in in the final phases. This is much deeper than previously reported for any Surtseyan tuff-cone forming eruption and consistent with the downward penetration of a diatreme. (c) 2005 Elsevier B.V. All rights reserved.

  • 21.
    Mattsson, H B
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Oskarsson, N
    Petrogenesis of alkaline basalts at the tip of a propagating rift: Evidence from the Heimaey volcanic centre, south Iceland2005In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 147, no 3-4, p. 245-267Article in journal (Refereed)
    Abstract [en]

    Heimacy is the volcanic centre in the Vestmannaeyjar Volcanic System, situated at the tip of Iceland’s propagating Eastern Volcanic Zone (EVZ). The Heimaey lavas are slightly alkaline, ne-normative, basalts with plagioclase, olivine and Ti-magnetite as phenocryst phases. Inversion modelling using REE suggests that a primary magma is generated by fractional melting over a depth interval of 100-65 km beneath Heimaey of a source similar to that of MORB. The primary magma fractionated 31% olivine and clinopyroxene en route to ponding at the base of the crust (30-35 km depth). Tectonic events as a result of the extensional stress exerted by the southwards propagating EVZ causes magmas to be emplaced into different levels of the crust where they evolve by fractional crystallization independently of each other (i.e. polybaric fractionation). During residence in a parental magma chamber at the mantle/crust boundary the fractionating assemblage is dominated by olivine and clinopyroxene, whereas olivine and plagioclase dominates the fractionating assemblage during residence in crustal magma chambers. The most evolved magma composition can be related to a parental Heimaey melt by 73% fractional crystallization of predominantly plagioclase, clinopyroxene and olivine. The residence times in crustal magma chambers are short as indicated by the absence of equilibrium phenocryst assemblages (e.g. lack of cpx-phenocrysts). Oxygen and radiogenic isotopes suggests that no crustal contamination occurred, and that the role of magma mixing is insignificant in the evolution of the Heimaey lavas. The polybaric evolution of the Heimacy lavas in small, isolated, magma chambers at different levels in the crust fits well with a southward propagation of the EVZ, with the rift-tip currently located beneath the Vestmannaeyjar Volcanic System. (c) 2005 Elsevier B.V. All rights reserved.

  • 22. Mattsson, Hannes
    et al.
    Armann, Höskuldsson
    Geology of the Heimaey volcanic centre, south Iceland: early evolution of a central volcano in a propagating rift?2005In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 127, p. 55-71Article in journal (Refereed)
  • 23. Mattsson, Hannes B.
    Rapid magma ascent and short eruption durations in the Lake Natron-Engaruka monogenetic volcanic field (Tanzania): A case study of the olivine melilititic Pello Hill scoria cone2012In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 247, p. 16-25Article in journal (Refereed)
    Abstract [en]

    The Pello Hill scoria cone displays widely different characteristics dependent on which side of the cone is studied. This follows as a direct result of a strong depositional asymmetry (i.e., the crater rim varies between 14 and 111 m in height), preferentially depositing material to the northwestern side of the construct. This is interpreted to reflect sedimentation from a sustained eruption plume (with prevailing winds at the time of the eruption coinciding with the direction of maximum deposition). The scoria deposits on this side of the cone form relatively fine-grained and well-sorted deposits which are laterally continuous over distances of 10’s of meters. To all other sides of the vent, deposits are characterized by coarse-grained lenticular deposits rich in mantle xenoliths, occasionally showing inverse grading (consistent with an origin as ballistic ejecta and slight reworking down-slope as grain-flows). The pyroclastic textures are dominated by a moderate vesicularity (similar to 40 vol. %) in combination with smooth, fluidal, outer surfaces. Two other features that stands out in comparison with “normal” scoria cones, these are defined by the absence of: (i) inward dipping layers into the crater area, and (ii) no agglutination/welding features can be found in any of the exposed outcrops. Calculated magma ascent rates yields values between 8.5 and 36.0 m s(-1), which is similar to that previously reported for kimberlitic magmas. Building on these ascent rates, and the volume of the pyroclastic construct (similar to 5 x 10(6) m(3) DRE), the eruption duration is estimated to less than 6 hours (for any vent area larger than 28 m(2)). Therefore, it may not have taken longer time than 9 hours from the time the magma started to ascend from upper-mantle depth (90 km) and the point at which the eruption halted. Overall, the olivine melilititic Pello Hill scoria cone displays many characteristics that can also be found in the vent-facies deposits of kimberlite eruptions. Therefore, further detailed studies of well-preserved, CO2-rich, olivine melilitite eruptions (such as the provided by the landforms within the LNE-MVF and Pello Hill) could provide a tool to understand the emplacement dynamics of the vent-facies of kimberlitic eruptions. (C) 2012 Elsevier B.V. All rights reserved.

  • 24. Mattsson, Hannes B.
    Textural variation in juvenile pyroclasts from an emergent, Surtseyan-type, volcanic eruption: The Capelas tuff cone, Sao Miguel (Azores)2010In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 189, no 1-2, p. 81-91Article in journal (Refereed)
    Abstract [en]

    Here I present textural data (i.e., vesicularity, vesicle size distributions (VSD), plagioclase crystallinity, crystal size distributions (CSD), combined with fractal analyses of particle outlines) from a natural succession of alternating fall and surge deposits in the emergent Capelas tuff cone (Azores). The textural variation in the Capelas succession is surprisingly small considering the wide variety of fragmentation processes, vent activity and emplacement mechanisms that are characteristic of emergent eruptions. The plagioclase crystal content varies between 24 and 33 vol.% CSD analyses of plagioclase show near-linear trends with a slight increase in time for the smallest crystal sizes (with surge deposits having more groundmass plagioclase when compared with fall deposits). This is consistent with crystallization induced by degassing and decompression at lower eruption rates. The vesicularities of the Capelas pyroclasts are more variable (18 to 59 vol.%), with VSDs displaying kinked trends characteristic of coalescence. This is especially evident in the fall deposits, and consistent with being formed in continuous uprush (jetting) with an overall shallow fragmentation level within the conduit. Bubble coalescence can also be identified in the surge deposits, although to a much lesser extent. The amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization (i.e., plagioclase) in the Capelas deposits. A relatively broad range of fractal dimensions (with average D-box = 1.744 and sigma = 0.032) for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at Capelas. In addition to this, the fact that the fractal dimensions for both the fall and surge end-members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges. These results are consistent with emergent eruptions, building tuff cones, to be a relatively low-energy phreatomagmatic landform (e.g., at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes). (C) 2009 Elsevier B.V. All rights reserved.

  • 25. Mattsson, Hannes B.
    et al.
    Hoskuldsson, Armann
    Contemporaneous phreatomagmatic and effusive activity along the Hverfjall eruptive fissure, north Iceland: Eruption chronology and resulting deposits2011In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 201, no 1-4, SI, p. 241-252Article in journal (Refereed)
    Abstract [en]

    The 2500 BP Hverfjall eruption in northern Iceland produced pyroclastic and effusive deposits of widely different characteristics along the length of the eruptive fissure. The southern half of the fissure was located in a shallow lake, whereas the northern part of the fissure extended onto dry land. This specific setting, with overlapping periods of activity at the different vents, resulted in various mingling features between fine-grained phreatomagmatic deposits and lava flows. Here we reconstruct the course of events during this eruption based on field observations and granulometric analyses of the resulting deposits. The eruption can be divided into three main phases depending on the main depositional characteristics and vent locations. The initial phase is dominated by phreatomagmatic fall deposits which are attributed to an overall high eruption rate in a shallow lacustrine setting. The second phases involved opening of two new vents on dry ground, and deposition of scoria and lava flows. The third and final phase of the eruption is associated with a lowering of the eruption rate in the southernmost vent, with a shift in the activity from continuous uprush and fall deposits to discrete explosions and emplacement of base surges. These surges display features consistent with drying up with increasing distance from the vent, suggesting that their dynamics changed during emplacement. Most wet surges were channelized within a preexisting graben structure close to the vent, but some more dilute (i.e., dry) surges were able to flow over this obstacle and continued to flow for more than 5 km away from the vent and 100 m uphill before stopping. (C) 2010 Elsevier B.V. All rights reserved.

  • 26. Mattsson, Hannes B.
    et al.
    Tripoli, Barbara A.
    Depositional characteristics and volcanic landforms in the Lake Natron-Engaruka monogenetic field, northern Tanzania2011In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 203, no 1-2, p. 23-34Article in journal (Refereed)
    Abstract [en]

    The Lake Natron-Engaruka monogenetic volcanic field (LNE-MVF) is situated in the East African Rift of northern Tanzania, where it comprises approximately 200 vents scattered over an area of 2500 km(2). Similar to most other monogenetic volcanic fields in the world, the landforms of the LNE-MVF are characterized by a wide array of morphologies, such as maar-diatreme volcanoes, tuff cones and tuff rings, scoria cones and spatter cones. However, in contrast to most other MVFs (which are basaltic in composition) the magmas erupted within the LNE-MVF are predominantly of olivine-melilititic to nephelinitic compositions. Here we show by field observations, granulometric analyses and morphological studies of particle shapes from a large selection of different landforms from the LNE-MVF that there are some crucial differences compared to the more common basaltic equivalents. These differences are reflected in both eruption dynamics and fragmentation mechanisms as well as the overall characteristics of the deposits. Landforms within the LNE-MVF that resemble those produced by phreatomagmatic eruptions in morphology, display strong evidence for dry fragmentation and also dry deposition. Therefore, to reconcile the observed depositional characteristics with eruptive processes within the LNE-MVF we propose a hypothetical eruption scenario. Partial melting of a carbonate-bearing mantle source produces small volume volatile-rich melilititic melts (rich in phlogopite and amphibole). These volatile-rich, and mantle xenolith-bearing, magmas ascend rapidly from the mantle to the surface without being subject to significant degassing. The volatile-rich nature of these melilitite magmas, in combination with a significant exsolution of CO2 during decompression (ascent) can explain the dry characteristics of these apparent “phreatomagmatic” landforms. (C) 2011 Elsevier B.V. All rights reserved.

  • 27.
    Saubin, Elodie
    et al.
    Univ Canterbury, Dept Geol Sci, Ilam Rd, Christchurch 8041, New Zealand.
    Kennedy, Ben
    Univ Canterbury, Dept Geol Sci, Ilam Rd, Christchurch 8041, New Zealand.
    Tuffen, Hugh
    Univ Lancaster, Lancaster Environm Ctr, Lancaster, England.
    Villeneuve, Marlene
    Univ Canterbury, Dept Geol Sci, Ilam Rd, Christchurch 8041, New Zealand.
    Davidson, Jonathan
    Univ Canterbury, Dept Geol Sci, Ilam Rd, Christchurch 8041, New Zealand.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Comparative field study of shallow rhyolite intrusions in Iceland: Emplacement mechanisms and impact on country rocks2019In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 388, article id 106691Article in journal (Refereed)
    Abstract [en]

    Shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields. However, our knowledge of magma intrusions remains far from complete, and processes occurring at intrusion margins are poorly understood. In this field-based study, we characterise four shallows, dissected rhyolitic intrusions at three sites in Iceland (Njarovik-Dyrfjoll, Krafla and Husafell central volcanoes). We focus on the relationship between intrusion emplacement mechanisms and country rock response, employing scanline mapping of fractures and in-situ rock property measurements (hardness and permeability) along transects from the intrusion margins to damaged and undamaged country rocks. We identify various scenarios of shallow intrusion emplacement style, based upon their diverse geometry and lithofacies architecture. Additional information from rock properties and characteristics of fractures and vesicles, indicates that initial country rock properties strongly influence the emplacement style. We identify two discrete types of country rock response to magma injection. The matrix permeability of weak, porous and permeable lithologies (conglomerate and hyaloclastite) is reduced by >1 order of magnitude adjacent to intrusions due to pore occlusion. Stronger and denser, low-permeability lithologies (basalt and welded ignimbrite) undergo a decrease in hardness by a factor >2 related to an up to fivefold increase in fracture density, with no significant change in matrix permeability. Our observations highlight the importance of robust characterisation of the mechanical properties of caldera-filling or geothermal reservoir formations, for appropriate forecasting of magma mobility, geophysical data interpretation, and geothermal resources characterisation.

  • 28.
    Sgattoni, Giulia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. University of Bologna; University of Iceland.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Einarsson, Pall
    University of Iceland.
    Lucchi, Federico
    University of Bologna.
    Li, Ka Lok
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Sadeghisorkhani, Hamzeh
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    The 2011 unrest at Katla volcano: Characterization and interpretation of the tremor sources2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 338, p. 63-78Article in journal (Refereed)
    Abstract [en]

    A 23-hour tremor burst was recorded on July 8-9th 2011 at the Katla subglacial volcano, one of the most active and hazardous volcanoes in Iceland. This was associated with deepening of cauldrons on the ice cap and a glacial flood that caused damage to infrastructure. Increased earthquake activity within the caldera started a few days before and lasted for months afterwards and new seismic activity started on the southern flank. No visible eruption broke the ice and the question arose as to whether this episode relates to a minor subglacial eruption with the tremor being generated by volcanic processes, or by the flood. The tremor signal consisted of bursts with varying amplitude and duration. We have identified and described three different tremor phases, based on amplitude and frequency features. A tremor phase associated with the flood was recorded only at stations closest to the river that flooded, correlating in time with rising water level observed at gauging stations. Using back-projection of double cross-correlations, two other phases have been located near the active ice cauldrons and are interpreted to be caused by volcanic or hydrothermal processes. The greatly increased seismicity and evidence of rapid melting of the glacier may be explained by a minor sub-glacial eruption. A less plausible interpretation is that the tremor was generated by hydrothermal boiling and/or explosions with no magma involved. This may have been induced by pressure drop triggered by the release of water when the glacial flood started. All interpretations require an increase of heat released by the volcano.

  • 29.
    Sgattoni, Giulia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Univ Bologna, Dept Biol Geol & Environm Sci, Bologna, Italy; Univ Iceland, Inst Sci, Inst Earth Sci, Reykjavik, Iceland.
    Jeddi, Zeinab
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Einarsson, Pall
    Univ Iceland, Inst Sci, Inst Earth Sci, Reykjavik, Iceland.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lucchi, Federico
    Univ Bologna, Dept Biol Geol & Environm Sci, Bologna, Italy.
    Long-period seismic events with strikingly regular temporal patterns on Katla volcano’s south flank (Iceland)2016In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 324, p. 28-40Article in journal (Refereed)
    Abstract [en]

    Katla is a threatening volcano in Iceland, partly covered by the Myrdalsjokull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream at the southern margin of Myrdalsjokull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. We have not found any comparable examples in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank, we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a jokulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rainstorms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.

  • 30. Solgevik, Henrik
    et al.
    Mattsson, Hannes B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Hermelin, Otto
    Growth of an emergent tuff cone: Fragmentation and depositional processes recorded in the Capelas tuff cone, Sao Miguel, Azores2007In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 159, no 1-3, p. 246-266Article in journal (Refereed)
    Abstract [en]

    The Capelas tuff cone is an emergent Surtseyan-type tuff cone that erupted in shallow seawater off the coast of Sao Miguel, Azores. In this paper, we present a detailed stratigraphic study which is used to infer depositional processes and modes of fragmentation for the Capelas tuff cone deposits. The growth of the tuff cone can be divided into three stages based on variations in depositional processes that are probably related to differences in water/magma (W/M) ratios. The first stage corresponds well to wet Surtseyan-type activity where wet fallout is the dominant depositional process, with only minor representation of pyroclastic surge deposits. The second stage of the eruption is suggested to be the result of alternating wet and slightly drier periods of Surtseyan activity, with an overall lower W/M-ratio compared to the first stage. The drier Surtseyan periods are characterized by the presence of minor grain-flow deposits and undulating pyroclastic surge deposits that occasionally display relatively dry structures such as strongly grain-segregated layers and brittle behavior when impacted by ballistic ejecta. The first deposits of the second stage show an intense activity of pyroclastic surges but fallout, commonly modified by surges, is still the dominant depositional process during the second stage. The third stage represents a final effusive period, with the build-up of a scoria cone and ponded lava flows inside the tuff cone crater. Phreatomagmatic fragmentation, as seen by studies of the fine ash fraction (< 64 mu m), is dominant in the Capelas tuff cone. However, particles with shapes and vesicularities characteristic of magmatic fragmentation are abundant in proximal deposits and present in all investigated beds (in various amounts). Emergent Surtseyan-type tuff cones are characterized by a domination of fallout deposits, both wet and dry, where dry periods are characterized by the deposition of relatively dry falling tephra transforming into grain-flow deposits. However, this study of the Capelas tuff cone shows that drier Surtseyan periods may also be represented by an increased amount of thin surge deposits that occasionally display dry features. (c) 2006 Elsevier B.V. All rights reserved.

  • 31.
    Troll, Valentin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Deegan, Frances
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Jolis, Ester Muños
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Harris, C.
    Dept. of Geological Science, University of Cape Town, Rondebosch 7701, South Africa.
    Chadwick, J.P.
    Dept of Petrology (FALW), De Boelelaan 1085, Amsterdam, The Netherlands.
    Gertisser, R.
    School of Physical and Geographical Sciences, Keele University, Keele, ST5 5BG, UK.
    Scharzkopf, L.M.
    GeoDocCon, Unterpferdt 8, 95176 Konradsreuth, Germany.
    Borisova, A.Y.
    Observatoire Midi-Pyrénées, Université Toulouse, 14 Avenue E. Belin, 31400 Toulouse, France.
    Bindeman, I.N.
    Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States.
    Sumarti, S.
    Volcano Investigation and Technology Development Institution, Yogyakarta, Indonesia.
    Preece, K.
    School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.
    Magmatic differentiation processes at Merapi Volcano: inclusion petrology and oxygen isotopes2013In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 261, no SI, p. 38-49Article in journal (Refereed)
    Abstract [en]

    Indonesian volcano Merapi is one of the most hazardous volcanoes on the planet and is characterised by periods of active dome growth and intermittent explosive events. Merapi currently degasses continuously through high temperature fumaroles and erupts basaltic-andesite dome lavas and associated block-and-ash-flows that carry a large range of magmatic, coarsely crystalline plutonic, and meta-sedimentary inclusions. These inclusions are useful in order to evaluate magmatic processes that act within Merapi's plumbing system, and to help an assessment of which phenomena could trigger explosive eruptions. With the aid of petrological, textural, and oxygen isotope analysis we record a range of processes during crustal magma storage and transport, including mafic recharge, magma mixing, crystal fractionation, and country rock assimilation. Notably, abundant calc-silicate inclusions (true xenoliths) and elevated δ18O values in feldspar phenocrysts from 1994, 1998, 2006, and 2010 Merapi lavas suggest addition of limestone and calc-silicate materials to the Merapi magmas. Together with high δ13C values in fumarole gas, crustal additions to mantle and slab-derived magma and volatile sources are likely a steady state process at Merapi. This late crustal input could well represent an eruption trigger due to sudden over-pressurisation of the shallowest parts of the magma storage system independently of magmatic recharge and crystal fractionation. Limited seismic precursors may be associated with this type of eruption trigger, offering a potential explanation for the sometimes erratic behaviour of Merapi during volcanic crises.

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