Axel Björnsson

Magnetotelluric survey across the active spreading zone in Southwest Iceland

Abstract The results of 10 magnetotelluric soundings, performed along a 110-km-long profile crossing the constructive plate boundary in southwest Iceland, are presented. Apparent resistivities are interpreted by a horizontally stratified earth model to yield a pseudo cross-section along the profile. The crust-mantle interface contains a well conductive layer. The depth to the good conductor increases with age of the crust and the distance from the axial zone. This layer is interpreted as partially molten basalt, at a temperature about 1100°C and a volume fraction of the melt phase in the range 10–20%. The high-conductivity layer probably disappears west of the Borgarnes anticlinal axis, which separates the older (to the west) and younger (to the east) flood basalts in western Iceland, indicating that the temperature below the oldest part of the profile lies below the solidus curve of basalt. Recent seismic crustal investigations in the same area indicate a state of partial melting or a magma chamber, which agrees with the results of the magnetotelluric soundings.

Elastic deformation models of Krafla Volcano, Iceland, for the decade 1975 through 1985

The dynamics of the shallow magma reservoir at Krafla Volcano in NE Iceland have been analyzed with three types of elastic models based on over 70 surveys of tilt and displacement made from 1975 to 1985, a period of continuous volcano-tectonic activity. Modeling results are integrated with geophysical and geological information to estimate the position, geometry, and volume change of magma reservoir domains subjected to periodic inflation and deflation. Dominating influences on magma reservoir dynamics are examined in the context of activity in Kraflas associated rift system and the deformation of its caldera from 1975–1985. Rather than the fluid-filled cavity concept of Mogi (1958) and most recent workers, our models are idealized as strained regions with spherical, double-spherical, or general ellipsoidal symmetry. The models are mathematically generalized from that of Mogi (1958) and are derived by inversion of displacements. Model results from different displacement components are remarkably consistent, although models based on vertical displacements typically have errors-of-fit much closer to expected measurement errors than those based on tilt or horizontal displacements. About one half of the double-sphere and ellipsoid models have significantly better fits than single-sphere models. Double-sphere model results are consistent with a 2 to 3 km center-to-center separation of magma storage zones from at least 2 km to 4 km depth by portions of the fissure system, as implied by S-wave attenuation patterns for 1976–1977. However, all models suggest pressurization zones of more limited extent than possible domains of storage inferred from S-wave attenuation. Ellipsoid models typically implied unrealistically shallow depths of magma storage. Caldera inflation rates decreased after January 1978 when the caldera periodically reinflated to its level prior to the initial December 1975 deflation event. From 1975–1980 the volume and duration of inflation between deflation events was strongly correlated with the volume of the previous deflation. After 1980 there was a significant increase in the duration of inflation periods and a decrease in rates of caldera inflation and fissure system widening. Consistent with these results, the magma reservoir is conceptualized as a hot rock mass containing numerous magma chambers and pressure-sensitive conduits connecting the chambers and deep magma sources. Magma is injected into the fissure system at critical pressures determined by the confining stress and rock mass strength. The duration and volume of inflation required to reach a critical pressure threshold is largely dependent on the volume of magma released in the previous deflation. Reduction of the extension rate across the Krafla fissure system after 1980 suggests that extensional forces were also reduced. A resultant rise of confining pressure on the magma reservoir and a reduced capacity of the fissure system to accommodate dike injection in combination would have increased critical stress levels for reservoir deflation and reduced the pressure gradient driving magma supply from deep sources.

Crustal formation and magma genesis beneath Iceland: Magnetotelluric constraints

Two different models of the Icelandic crust and upper mantle have recently competed. The thin-crust model involves a crust ~10–15 km thick beneath the axial rift zones, thickening to ~25 km beneath older Tertiary areas. At the base of the crust is a thin layer containing 5–10% partial melt at temperatures around 1100 °C and with high electrical conductivity. Below the crust is an anomalous ultramafic mantle or an intermediate layer of mantle and crustal material containing 1–5% melt. According to the thick-crust model, the crust is ~20–30 km thick close to the coast and thickens toward the center of the island to as much as 40 km. A large amount of magnetotelluric data were reevaluated and a map constructed, which shows that the conducting layer is continuous beneath whole of Iceland except along the south coast. Joint interpretation of electrical, seismic, and temperature data provides many more constraints and more reliable results than does the use of one method only. There is a good correlation between the depth to the highly conductive layer, temperature gradient, and maximal focal depths. The uppermost 10–15 km of the crust are mainly formed by dike intrusions, lava eruptions, and continuous subsidence and mixing of magma in the rift zones. The lower crust is created by upflow of magma in layer 4, intrusions and underplating causing thickening of the crust with age. The thin-crust model explains the major features of crustal and mantle structure better than does the thick-crust model.

The impact of the 1996 subglacial volcanic eruption in Vatnajökull on the river Jökulsá á Fjöllum, North Iceland

Abstract A subglacial volcanic eruption took place in October 1996 beneath the Vatnajokull glacier, Iceland. The volcanic fissure erupted for some 14 days and it extended between two known subglacial central volcanoes. Most of the melt water drained to the south into the Grimsvotn caldera from where it escaped a month later during a major jokulhlaup (extreme flood) into the glacial rivers flowing to the south from Vatnajokull. At the start of the eruption, the northernmost part of the volcanic fissure extended across the water divide beneath the glacier and into the river basin of Jokulsa a Fjollum, flowing to the north. A few days later, signs of melt water from the volcanic site were detected in the glacial river Jokulsa a Fjollum. Distinct changes in the chemical composition of the water were observed. Both discharge and turbidity of the river were somewhat higher than normal for the season, but there was no extreme flood (jokulhlaup). Total dissolved solids (TDS) and conductivity of the river water, as well as bicarbonate, were found to be higher than previously observed. Traces of sulphide and mercury were detected, which are never recorded at normal conditions. The stable isotope ratios, δD and δ 18 O , the 14 C apparent age and the δ 13 C value were also found to be anomalous. In a few weeks the chemistry was back to normal. The chemical changes were most likely caused by a direct flow of melt water from the northernmost part of the main volcanic fissure or from small cauldrons created at the rim of the Bardarbunga caldera. The flow of melt water to the north from the volcanic centre ebbed soon after or even just before the volcanic eruption in Vatnajokull ended. This experience shows clearly that simultaneous monitoring of chemical changes and flow rate in glacial rivers can deliver valuable data for following subglacial volcanic activity in space and time and may be used to give warning before a major catastrophic flood. Chemical study of glacial rivers can also be used to find out if a jokulhlaup is connected with a simultaneous volcanic eruption or not.

Directional spectral analysis and filtering of geophysical maps

The detection of linear anomalies in map data is facilitated by studying the two‐dimensional power spectrum, because the directivity of the energy in the map is preserved in the Fourier transform. The lineaments associated with individual peaks in the spectrum are then separated from the map data by directional filtering and studied independently of other map features. Gravity and magnetic maps from an active rift area in southwestern Iceland are analyzed in this manner. The agreement between the filtered maps is good and they fit the observed tectonic features quite well.

A GPS Survey in the North-East Volcanic Zone of Iceland 1987 First Results

Iceland lies on the mid — Atlantic ridge at a latitude where the full spreading rate is 2.2 cm / year, which was estimated from seafloor anomalies south of Iceland (Talwani and Eldholm, 1977). The ridge on land is comprised of transform zones and the so-called neovolcanic zone that is made up of numerous volcanic systems. The first geodetic measurements in the neovolcanic zone were carried out in 1938 by Niemczyk (Moller, 1989) in north-east Iceland. Subsequently a classical geodetic network with an extent of 110 km in the east-west direction has been developed, spanning the main part of the rift zone.