Bryndís Brandsdóttir

Färoe‐Iceland Ridge Experiment 2. Crustal structure of the Krafla central volcano

The seismic velocity structure of the Krafla central volcano is characterized by large variations in compressional velocity. A 40 km wide high-velocity dome extends from the lower crust (11–14 km depth) beneath the volcano narrowing upward. A magma chamber sits at its top near 3 km depth. It is defined by both 0.2–0.3 s compressional wave delays and shear wave shadowing to be 2–3 km N-S, 8–10 km E-W, and 0.7–1.8 km thick. The near-surface structure (uppermost 2.5 km) of the Krafla caldera is approximately flat-lying, with only minor lateral heterogeneities. The crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities. Shear waves, reflected from a 19 km deep Moho, are clearly visible for some paths through the crustal volume below the magma chamber, even though the more shallow diving S waves are severely attenuated. The midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near-solidus temperatures. The Krafla central volcano plays a major role in crustal genesis along the plate boundary. The high-velocity dome, in our view, represents crust generated in and around the magma chamber, which has subsequently been advected to greater depths.

Färoe-Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland

Results from the Faroe-Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poissons ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poissons ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

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.

Crustal structure of the northern Reykjanes Ridge and Reykjanes Peninsula, southwest Iceland

Results from the Reykjanes-Iceland Seismic Experiment (RISE) show that the thickness of zero-age crust decreases from 21 km in southwest Iceland to 11 km at 62°40′N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of ∼130°C, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off-axis at 63°N, from 12.7 km thick at 0 Ma to 9.8 km at 5 Ma, most likely due to a ∼40°C change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9–11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seismogram modeling of the wide-angle reflections from this body suggests that it corresponds to a zone of high-velocity (≥7.5 km s−1), high-magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis than beneath older crust, consistent with lithospheric cooling with age.

Center of the Iceland hotspot experiences volcanic unrest

A volcanic eruption beneath the Vatnajokull ice cap in central Iceland (Figure 1) began on September 30,1996, along a 7-km-long fissure between the volcanoes Bardarbunga and Grimsvotn. The eruption continued for 13 days and produced ˜0.5 km3 of basaltic andesite. Meltwater from the eruption site flowed into the caldera lake of the Grimsvotn volcano, where it accumulated beneath a floating ice shelf. The lakes ice dam was lifted off the glacier bed on November 4, and in the next two days more than 3 km3 of water drained out beneath the glacier and flushed down to the south coasts alluvial plain, causing extensive flooding and damage to transportation and communication systems.

Asymmetric plume‐ridge interaction around Iceland: The Kolbeinsey Ridge Iceland Seismic Experiment

We present the results of a seismic refraction experiment that constrains crustal structure and thickness along 225 km of the Kolbeinsey Ridge and Tjornes Fracture Zone and thus quantifies the influence of the Iceland hot spot on melt flux at the spreading center north of Iceland. North of the Iceland shelf, crustal thickness is relatively constant over 75 km, 9.4 ± 0.2 km. Along the southern portion of the Kolbeinsey Ridge, on the Iceland shelf, crustal thickness increases from 9.5 ± 0.1 km to 12.1 ± 0.4 km over 90 km. Gravity inversion indicates a residual crustal gravity anomaly that decreases by about 30–40 mGal toward Iceland. We infer that the variations in crustal thickness and gravity are accompanied by mantle temperature changes of 40° to 50°C. At similar distances from the Iceland hot spot, crustal thickness along the Kolbeinsey Ridge is 2–2.5 km less than at the Reykjanes Ridge, consistent with the asymmetry in plume-ridge interaction that has been inferred from the axial depth and geochemistry of these ridges. Average lower crustal velocities are also higher along the Kolbeinsey Ridge, consistent with a lower degree of active upwelling than along the Reykjanes Ridge. Topography and crustal thickness patterns at the spreading centers around Iceland are consistent with isostatic support for normal crustal and mantle densities. However, we infer that the lower crust beneath central Iceland is considerably denser than that beneath the adjacent ridges. Crustal thickness and geochemical patterns suggest that deep melting is spatially limited and asymmetric about Iceland while shallow melting is enhanced over a broad region. This asymmetry may be due to a mantle plume that is tilted from south to north in the upper mantle and preferentially melts deeper enriched material beneath the Reykjanes Ridge.

Strike‐slip faulting during the 2014 Bárðarbunga‐Holuhraun dike intrusion, central Iceland

Over a 13?day period magma propagated laterally from the subglacial Barðarbunga volcano in the northern rift zone, Iceland. It created >?30,000 earthquakes at 5–7?km depth along a 48?km path before erupting on 29 August 2014. The seismicity, which tracked the dike propagation, advanced in short bursts at 0.3–4.7?km/h separated by pauses of up to 81?h. During each surge forward, seismicity behind the dike tip dropped. Moment tensor solutions from the leading edge show exclusively left-lateral strike-slip faulting subparallel to the advancing dike tip, releasing accumulated strain deficit in the brittle layer of the rift zone. Behind the leading edge, both left- and right-lateral strike-slip earthquakes are observed. The lack of non-double-couple earthquakes implies that the dike opening was aseismic.

Volcanic Tremor and Low-Frequency Earthquakes in Iceland

Most earthquakes in Iceland are of the usual high-frequency type, reflecting brittle failure of the crust. Earthquakes lacking energy in the higher frequencies (low-frequency earthquakes) also occur, particularly in volcanic regions. Low-frequency earthquakes in the volcanic systems of Iceland span a broad spectrum with respect to amplitude/duration ratios. They often accompany magmatic intrusions or extrusions, and in the cases of Krafla and Hekla, they have been found to correlate in time with the opening of surface fissures. In other cases, such as at SW-Mýrdalsjokull and Torfajokull, low-frequency earthquakes are associated with central volcanoes that have not erupted for centuries. Volcanic tremor has been recorded during all recent eruptions in Iceland. In Krafla, at least two types of tremor have been recorded, one associated with intrusions, the other with eruptions. The two types have distinctly different characteristics, and are produced by different physical processes.

Fracture systems of the Northern Volcanic Rift Zone, Iceland: an onshore part of the Mid-Atlantic plate boundary

Abstract Few divergent plate boundaries are subaerial. Active rifts in Iceland provide valuable surface information on divergent spreading processes, rifting and faulting. The 200 km long and 50 km wide Northern Volcanic Rift Zone (NVZ) is composed of 7 volcanic systems, each consisting of a central volcano with a transecting fissure swarm. Fractures and postglacial eruptive fissures in the NVZ were analysed using aerial photographs and satellite images to study their characteristics and behaviour. While non-eruptive fractures characterize the distal (c. 40–100 km) parts of the fissure swarms, eruptive fissures are most common at distances less than c. 20–30 km from the central volcano. Fractures within the fissure swarms are generally subparallel, with a N–NNE strike. Irregular orientations are associated with calderas within the central volcanoes Askja and Krafla, and at the junction of the NVZ and the Tjörnes Fracture Zone, where high fracture densities also occur. WNW-orientated fractures at the southern end of the Krafla Fissure Swarm, and the northern end of the Kverkfjöll Fissure Swarm, exhibit surface expressions of a transform zone. The fissure swarms within the rift zone are mostly seismically and geodetically inactive, becoming highly active during rifting events that occur at time intervals of tens to a few hundred years.

SEISMIC IMAGES OF CRUST BENEATH ICELAND CONTRIBUTE TO LONG-STANDING DEBATE

Since the 1970s there has been an ongoing debate about whether the crust beneath Iceland is relatively thin ( 20 km) and cooler. New results from the Faeroe-Iceland Ridge Experiment (FIRE) conducted in the summer of 1994 suggest that it is thicker and cooler. This major land-sea study investigated the crust generated where the North Atlantic spreading center intersects the Iceland mantle plume and mapped the transition from thickened oceanic crust to the continental fragment on which the Faeroe Islands sit. Seismic techniques were used to obtain a 600-km profile of the area (Figure 1). Preliminary results suggest that a 20-km-thick crust is being generated beneath the northern Neovolcanic Zone of Iceland, with a high-level crustal magma chamber beneath Krafla. In northeast Iceland, the igneous crust is considerably thicker, reaching 35 km, while along the Faeroe-Iceland Ridge it varies between 25–30 km.