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| 2. |
- Arellano, Santiago, 1981, et al.
(författare)
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Degassing patterns of Tungurahua volcano (Ecuador) during the 1999–2006 eruptive period, inferred from remote spectroscopic measurements of SO2 emissions
- 2008
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Ingår i: Journal of Volcanology and Geothermal Research. - 0377-0273. ; 176, s. 151-162
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the results of 7 years (Aug. 1999–Oct. 2006) of SO2 gas measurements during the ongoingeruption of Tungurahua volcano, Ecuador. From 2004 onwards, the operation of scanning spectrometers hasfurnished high temporal resolution measurements of SO2 flux, enabling this dataset to be correlated with otherdatasets, including seismicity. The emission rate of SO2 during this period ranges from less than 100 to35000 tonnes/day (t/d) with a mean daily emission rate of 1458 t/d and a standard deviation of ±2026 t/d.Average daily emissions during inferred explosive phases are about 1.75 times greater than during passivedegassing intervals. The total amount of sulfur emitted since 1999 is estimated as at least 1.91 Mt, mostlyinjected into the troposphere and carried westwards from the volcano. Our observations suggest that the rateof passive degassing at Tungurahua requires SO2 exsolution of an andesitic magma volume that is two orders ofmagnitude larger than expected for the amount of erupted magma. Two possible, and not mutually exclusive,mechanisms are considered here to explain this excess degassing: gas flow through a permeable stagnant magma-filled conduit and gas escape from convective magma overturning in the conduit.We have found thatreal-time gas monitoring contributes significantly to better eruption forecasting at Tungurahua, because it hasprovided improved understanding of underlying physical mechanisms of magma ascent and eruption.
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| 3. |
- Burchardt, Steffi, 1982-
(författare)
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New insights in the mechanics of sill emplacement provided by field observations of the Njardvik Sill, Northeast Iceland
- 2008
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Ingår i: Journal of Volcanology and Geothermal Research. - Amsterdam : Elsevier. - 0377-0273 .- 1872-6097. ; 173:3-4, s. 280-288
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Carn, Simon, et al.
(författare)
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Daily monitoring of Ecuadorian volcanic degassing from space
- 2008
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Ingår i: Journal of Volcanology and Geothermal Research. - 0377-0273. ; 176:1, s. 141-150
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Tidskriftsartikel (refereegranskat)abstract
- We present daily measurements of sulfur dioxide (SO2) emissions from active volcanoes in Ecuador and southern Colombia between September 2004 and September 2006, derived from the Ozone Monitoring Instrument (OMI) on NASA's EOS/Aura satellite. OMI is an ultraviolet/visible spectrometer with an unprecedented combination of spatial and spectral resolution, and global coverage, that permits daily measurements of passive volcanic degassing from space. We use non-interactive processing methods to automatically extract daily SO2 burdens and information on SO2 sources from the OMI datastream. Maps of monthly average SO2 vertical columns retrieved by OMI over Ecuador and S. Colombia are also used to illustrate variations in regional SO2 loading and to pinpoint sources. The dense concentration of active volcanoes in Ecuador provides a stringent test of OMI's ability to distinguish SO2 from multiple emitting sources. Our analysis reveals that Tungurahua, Reventador and Galeras were responsible for the bulk of the SO2 emissions in the region in the timeframe of our study, with no significant SO2 discharge detected from Sangay. At Galeras and Reventador, we conclude that OMI can detect variations in SO2 release related to cycles of conduit sealing and degassing, which are a critical factor in hazard assessment. The OMI SO2 data for Reventador are the most extensive sequence of degassing measurements available for this remote volcano, which dominated regional SO2 production in June–August 2005. At Tungurahua, the OMI measurements span the waning stage of one eruptive cycle and the beginning of another, and we observe increasing SO2 burdens in the months prior to explosive eruptions of the volcano in July and August 2006. Cumulative SO2 loadings measured by OMI yield a total of ~ 1.16 Tg SO2 emitted by volcanoes on mainland Ecuador/S. Colombia between September 2004 and September 2006; as much as 95% of this SO2 may originate from non-eruptive degassing. Approximate apportionment of the total SO2 loading indicates that ~ 40% originated from Tungurahua, with ~ 30% supplied by both Reventador and Galeras. These measurements of volcanic SO2 degassing in Ecuador confirm OMI's potential as an effective, economical and risk-free tool for daily monitoring of SO2 emissions from hazardous volcanoes.
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| 7. |
- Kalscheuer, Thomas, et al.
(författare)
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Electromagnetic evidence for an ancient avalanche caldera rim on the south flank of Mount Merapi, Indonesia
- 2007
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 162:1-2, s. 81-97
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Mattsson, H B, et al.
(författare)
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Crustal xenoliths in the 6220 BP Saefell tuff-cone, south Iceland : Evidence for a deep, diatreme-fonning, Surtseyan eruption
- 2005
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 145:3-4, s. 234-248
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Mattsson, H B, et al.
(författare)
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Eruption reconstruction, formation of flow-lobe tumuli and eruption duration in the 5900 BP Helgafell lava field (Heimaey), south Iceland
- 2005
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 147:1-2, s. 157-172
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Mattsson, H B, et al.
(författare)
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Petrogenesis of alkaline basalts at the tip of a propagating rift: : Evidence from the Heimaey volcanic centre, south Iceland
- 2005
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier B.V.. - 0377-0273 .- 1872-6097. ; 147:3-4, s. 245-267
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Tidskriftsartikel (refereegranskat)abstract
- Heimaey 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 Heimaey 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.
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| 11. |
- Merle, Renaud E., 1976-, et al.
(författare)
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Segregation vesicles, cylinders, and sheets in vapor-differentiated pillow lavas : examples from Tore-Madeira Rise and Chile Triple Junction
- 2005
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Ingår i: Journal of Volcanology and Geothermal Research. - 0377-0273 .- 1872-6097. ; 141:1, s. 109-122
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Tidskriftsartikel (refereegranskat)abstract
- We conducted a detailed field and laboratory study of internal segregation structures of two hand-size pillow lavas samples. They were dredged, respectively, on the Josephine seamount, Tore-Madeira Rise (TMR), and on a small quaternary volcanic edifice located on the continental edge of the trench close to the Chile Triple Junction (CTJ). Both pillows display a combination of four types of segregation structures (spherical vesicles, pipe vesicles, vesicle cylinders, and vesicle sheets) observed so far only within subaerial basalt flows typically 2–10 m thick. In particular, the samples offer a remarkable exposure of the transition between pipe vesicles and cylinders. We show that the vesicle sheets are not generated by the same mechanism in both occurrences; they do not seem to be connected to cylinders in the CTJ pillow as they are in the TMR pillow. The two pillows are geochemically distinct, the TMR being alkaline and the CTJ calc–alkaline. Two types of internal differentiation are proposed. The first one implies the extraction of the residual liquid from the host lava and transport towards the segregation structures, whereas the other one results from in situ crystallization within one given structure. In the latter case, glass composition is highly dependant on the nature of the neighbouring crystallizing minerals. The degree of crystallization required to produce a crystal framework strong enough for generating the segregation structures seems to be lower in pillows (ca. 25% crystallization) than in vapor-differentiated basaltic lava flows (35% crystallization).
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