The 1991 eruption of Hekla,Iceland

The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km² and has an estimated volume of 0.15 km³. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km³ of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m³ s^–1. After this peak, the effusion rate declined rapidly to 10–20 m³ s^–1, then more slowly to 1 m³ s^–1, and remained at 1–12 m³ s^–1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.     

Sixty volumes of the Journal du Conseil / ICES Journal of Marine Science

Article 1 of the ICES Convention of 1964 sets out the fundamentalpurposes of the Council and includes as the third and finalpoint: "to publish or otherwise disseminate the results of researchand investigations carried out under its auspices or to encouragethe publication thereof". Publication and dissemination of informationabout the living resources and their environment have been atthe heart of ICES from the first     

Integrity Monitoring of IGEX-98 Data – Part III: Broadcast Navigation Message Validation

At the Delft University of Technology (DUT), data of six stations participating in the first international GLONASS tracking campaign, IGEX-98, were analyzed with integrity monitoring software. The software was developed at the Department of Mathematical Geodesy and Positioning of DUT. The main function of the software is to detect slips and outliers in phase and code observations in real time. In addition, the software also allows the validation of the information contained in the broadcast navigation messages. The results of the IGEX-98 data analyses are presented in a three part series. In this third and final part of the series, GLONASS and GPS navigation message validation results will be discussed in detail. The first part was concerned with the availability of GLONASS observations (Jonkman & de Jong, 2000a), while in the second part, GLONASS slip and outlier statistics were considered (Jonkman & de Jong, 2000b). The discussion of the navigation message validation results concentrates on anomalies detected in the data of an IGEX-98 station in Switzerland. Data collected at this station from September 1998 to December 1999 was analyized. In all, 19 anomalies were detected by the integrity monitoring software, 8 in GLONASS messages and 11 in GPS messages. The cause of the anomalies was established by evaluating the original data files, studying official bulletins of the GLONASS and GPS ground segments, and in some cases tracing the anomaly through the complete IGEX-98 network. ? 2000 John Wiley & Sons, Inc.     

Difficulties in determining electrical anisotropy in subsurface investigations

Surface electrical and electromagnetic methods have a limited resolution capability for determining the conductivity structure of the earth. In one-dimensional modelling a collection of many thin layers is frequently considered as one composite layer, which is then macro-anisotropic. Neither galvanic methods nor inductive methods alone can resolve the anisotropy of the ground, but a joint inversion of galvanic and inductive data may do so. The necessity of including the coefficient of anisotropy in the joint inversion of galvanic and inductive sounding data is demonstrated. An analysis is made of the combined use of geoelectrical and transient soundings to resolve the coefficient of anisotropy of a subsurface layer for varying thickness, resistivity and coefficient of anisotropy. It is found that the coefficient of anisotropy is well resolved only for layers that are many times thicker than the overburden and for coefficients of anisotropy that are not too small. The ability of the joint inversion of geoelectrical and transient sounding data to resolve macro-anisotropic layers is tested using realistic earth models determined from electrical logs.     

Globalisation, State Regulation and Industrial Upgrading of the Oil Seed Industries in Malaysia and Brazil

Palm oil from Malaysia and soybean meal from Brazil are of major importance in the world market. State regulation in both countries has played a crucial role in agro-industrial upgrading, i.e. the establishment, consolidation and diversification of manufacturing capacity in the oil seed chains. In particular, sufficient volumes of raw materials for local processing have been secured through systems of differential export taxes. This core set of regulatory mechanisms has been used in both countries, despite structural differences between the two chains related to the properties of the crops (oil palms and soybeans), the previous organisation of agricultural production, and the size and nature of the local markets. Globalisation in its present neoliberal form, institutionalised within the World Trade Organisation, gradually erodes the opportunity for other developing country states to upgrade agro-industries by adapting similar forms of regulatory mechanisms.     

Fate of Point‐Source Pesticide Spill in Clayey Till after 15 Years

This study investigated the development of pesticide pollution two, three, and 17 years after spills of the herbicides dichlorprop, mecoprop (MCPP), MCPA, 2,4‐D (phenoxy acids), simazine, and terbutylazine (triazines) in a former orchard machinery service yard. The spills had occurred over several decades on a 23‐m thick, mainly anaerobic fractured clayey till aquitard. Angled monitoring wells were installed in the aquitard 3 years after the spills ceased in 1989. In 1993, monitoring revealed that high groundwater concentrations of dichlorprop (677 µg/L) and MCPP (139 µg/L) were accumulated as a zone of maximum pollution in anaerobic and largely immobile pore water at 5 to 6 m depth in the aquitard profile. In contrast, 2,4‐D was determined in only one water sample, and MCPA and simazine and terbutylazine were determined only in low concentrations (below10 µg/L), although these pesticides had been handled at the site in greater amounts than dichlorprop and MCPP according to detailed historic information obtained for the site. Repeated monitoring in the same wells after a further 14 years in 2007 revealed that no identifiable degradation of MCPP had occurred, while dichlorprop had degraded by 75% to 80% (estimated half‐life of approximately 5 years). Furthermore, degradation products related to the phenoxy acids had accumulated, especially 4‐CPP with a maximum concentration of 218 µg/L. In the same zone, MCPA and simazine had almost disappeared. As the pollution was mainly accumulated in largely immobile pore water of the aquitard clayey matrix, and the groundwater recharge was low (30 to 60 mm/year), only minor vertical displacement of the zone with maximum pollution zone had occurred during the 15 years of monitoring. However, concentrations of dichlorprop (0.01 to 0.02 µg/L), MCPP (0.1 to 0.2 µg/L), and 4‐CPP (0.6 to 0.7 µg/L) had spread along textural heterogeneities in the aquitard into the underlying sandy aquifer at 23‐m depth.     

Debris entrainment by basal freeze‐on and thrusting during the 1995–1998 surge of Kuannersuit Glacier on Disko Island,west Greenland

Kuannersuit Glacier, a valley glacier on Disko Island in west Greenland, experienced a major surge from 1995 to 1998 where the glacier advanced 10·5 km and produced a ～65 m thick stacked sequence of debris‐rich basal ice and meteoric glacier ice. The aim of this study is to describe the tectonic evolution of large englacial thrusts and the processes of basal ice formation using a multiproxy approach including structural glaciology, stable isotope composition (δ¹⁸O and δD), sedimentology and ground‐penetrating radar. We argue that the major debris layers that can be traced in the terminal zone represent englacial thrusts that were formed early during the surge. Thrust overthrow was at least 200–300 m and this lead to a 30 m thick repetition of basal ice at the ice margin. It is assumed that the englacial thrusting was initiated at the transition between warm ice from the interior and the cold snout. The basal debris‐rich ice was mainly formed after the thrusting phase. Two sub‐facies of stratified basal ice have been identified; a lower massive ice facies (S_M) composed of frozen diamict enriched with heavy stable isotopes overlain by laminated ice facies (S_L) consisting of millimetre thick lamina of alternating debris‐poor and debris‐rich ice. We interpret the stratified basal ice as a continuum formed mainly by freeze‐on processes and localized regelation. First laminated basal ice is formed and as meltwater is depleted more sediment is entrained and finally the glacier freezes to the base and massive diamict is frozen‐on. The increased ability to entrain sediments may partly be associated with higher basal freezing rates enhanced by loss of frictional heat from cessation of fast flow and conductive cooling through a thin heavily crevassed ice during the final phase of the glacier surge. The dispersed basal ice facies (D) was mainly formed by secondary processes where fine‐grained sediment is mobilized in the vein system of ice. Our results have important implications for understanding the significance of basal ice formation and englacial thrusting beneath fast‐flowing glaciers and it provides new information about the development of landforms during a glacier surge. Copyright © 2010 John Wiley & Sons, Ltd.