Agust Gudmundsson

Infrastructure and mechanics of volcanic systems in Iceland

The divergent plate boundary in Iceland consists of more than two dozen systems where most of the volcano-tectonic activity takes place. At the surface the volcanic systems are characterised by 5–20-km-wide and 40–100 km-long swarms of tension fractures (~102 m long), normal faults (~103 m long) and volcanic fissures. The Holocene fissure swarms are confined to less than 10,000-year-old basaltic lava flows, mostly pahoehoe, occurring near the centre of the active rift zone. In any particular swarm, the number of tension fractures exceeds that of normal faults. All tension fractures and normal faults are vertical at the surface, indicating that the surface parts were generated by an absolute tension. In addition to a fissure swarm, many volcanic systems have a central volcano some of which have developed collapse calderas. In the late Tertiary and Pleistocene lava pile of Iceland, extinct volcanic systems are represented by local sheet swarms and regional dyke swarms. The sheet swarms are normally circular or slightly elliptical, several kilometres in radius, and are confined to the extinct central volcanoes. Many swarms are associated with large plutons, exposed at 1–2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers, and in short traverses up to 90% of the rock may consist of sheets. The sheets have a very variable strike, dip on average 45–65 °, mostly towards the centre of the associated volcano, and have an average thickness of about 1 m. The regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5–10 km wide. In several-kilometre-long traverses, commonly 1–5% of the rock consist of dykes but occasionally as much as 15–28%. Most regional dykes are subparallel and subvertical. The average dyke thickness in the Pleistocene swarms is less than 2 m but 4–6 m in the Tertiary swarms. While active, the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8–12 km at the boundary between the crust and upper mantle. The reservoirs are partially molten, with totally molten top regions, and of cross-sectional areas similar to those of the volcanic systems that they feed. Some active volcanic systems, especially those that develop central volcanoes, high-temperature areas and calderas, have, in addition to the deep-seated magma reservoirs, shallow crustal magma chambers, located at 1–3 km depth, which, in turn, are fed by the deeper magma reservoirs. It is proposed that the regional dyke swarms are supplied with magma largely from the deep-seated reservoirs, whereas the local sheet swarms are mainly fed from the associated crustal magma chambers. Because the volume of a crustal chamber is less than that of its deeper source reservoir, a single magma flow (dyke intrusion) from the reservoir may trigger tens of magma flows (sheet intrusions) from the chamber, which is one explanation for the enormous number of sheets associated with many central volcanoes. The sheets follow the stress trajectories of the local stress fields around the source chambers, whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes. It is suggested that many dykes develop as self-affine (some as self-similar) structures. When applied to the Krafla volcanic system in northern Iceland, model calculations suggest that a magma flow from the Krafla reservoir, with regional dyke formation, should occur, on average, once every several hundred years, and that tens of sheets might be injected from the shallow Krafla chamber and triggered by a single magma flow from the Krafla reservoir. These results are in broad agreement with the available data.

Emplacement and arrest of sheets and dykes in central volcanoes

Abstract Sheet intrusions are of two main types: local inclined (cone) sheets and regional dykes. In Iceland, the inclined sheets form dense swarms of (mostly) basaltic, 0.5–1 m thick sheets, dipping either at 20–50° or at 75–90° towards the central volcano to which they belong. The regional dykes are (mostly) basaltic, 4–6 m thick, subvertical, subparallel and form swarms, less dense than those of the sheets but tens of kilometres long, in the parts of the volcanic systems that are outside the central volcanoes. In both types of swarms, the intrusion intensity decreases with altitude in the lava pile. Theoretical models generally indicate very high crack-tip stresses for propagating dykes and sheets. Nevertheless, most of these intrusions become arrested at various crustal depths and never reach the surface to supply magma to volcanic eruptions. Two principal mechanisms are proposed to explain arrest of dykes and sheets. One is the generation of stress barriers, that is, layers with local stresses unfavourable for the intrusion propagation. The other is mechanical anisotropy whereby sheet intrusions become arrested at discontinuities. Stress barriers may develop in several ways. First, analytical solutions for a homogeneous and isotropic crust show that the intensity of the tensile stress associated with a pressured magma chamber falls off rapidly with distance from the chamber. Thus, while dyke and sheet injection in the vicinity of a chamber may be favoured, dyke and sheet arrest is encouraged in layers (stress barriers) at a certain distance from the chamber. Second, boundary-element models for magma chambers in a mechanically layered crust indicate abrupt changes in tensile stresses between layers of contrasting Young’s moduli (stiffnesses). Thus, where soft pyroclastic layers alternate with stiff lava flows, as in many volcanoes, sheet and dyke arrest is encouraged. Abrupt changes in stiffness between layers are commonly associated with weak and partly open contacts and other discontinuities. It follows that stress barriers and discontinuities commonly operate together as mechanisms of dyke and sheet arrest in central volcanoes.

Emplacement of dikes, sills and crustal magma chambers at divergent plate boundaries

Abstract A model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1–3 km of the oceanic crust, the number and lengths of dikes in any particular swarm should increase, but that the thickness should decrease, with depth in the crust. These predictions are supported by field observations. It is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that, at a certain level in the crust, subsequent dikes change into sills. Such sills may develop into crustal magma chambers, either when many nearby sills combine into a large one, or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid. For the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m, whereas at the slowest spreading ridges the initial thickness must exceed 170 m. Because thin sills are much more likely to form than thick sills, the probability of sills developing into crustal chambers increases with dike intrusion frequency, hence with spreading rate. Furthermore, the model predicts, first, that with increasing spreading rate the length of the chamber along the ridge axis should increase, and, second, that the most likely site for magma chambers is at depths of 2–4 km. Both these predictions are supported by available data.

Vertical and lateral collapses on Tenerife (Canary Islands) and other volcanic ocean islands

Recent studies demonstrate that Tenerife has undergone large lateral collapses. This has led to the suggestion that the Las Canadas caldera, one of the best exposed calderas in the world, is the result of lateral collapse. We have tested this idea using the available structural, stratigraphic, volcanological, and geochronological data. We conclude that the Las Canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of Tenerife. Our results indicate, however, that vertical collapses may have played a major role in triggering lateral collapses. We propose that the association of vertical and lateral collapse events, such as inferred for Tenerife, can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands.

Formation and geometry of fractures, and related volcanism, of the Krafla fissure swarm, northeast Iceland

During the past 12 yr, a major volcano-tectonic episode occurred in the Krafla fissure swarm at the divergent plate boundary in north-east Iceland. This swarm is an 80-km-long and as much as 10-km-wide zone of tension fractures, normal faults, and volcanic fissures. The average length of 1,083 measured tectonic fractures is about 350 m, the maximum length being 3.5 km, and the average estimated depth is of the order of 102 m. Most fractures strike north to north-northeast, with widths as much as 40 m and throws of as much as 42 m. Pure tension fractures are most common, but as they grow they commonly change into normal faults. Most fractures gradually thin out at their ends, but several exceptionally wide tension fractures end in tectonic caves, several tens of meters long, only a few meters beneath the surface. The total dilation measured in 5 profiles across the Krafla swarm reaches a maximum of at least 80 m and decreases from south to north along the swarm. Some 20 intrusive events and 9 eruptive events occurred during this volcano-tectonic episode. New lavas covered many old fractures, but several new fractures were also formed and many old ones grew. New lava flowed into some of the major fractures in the area, presumably forming pseudodikes. Locally, magma used a part of a pre-existing fracture as a pathway to the surface. Small width: length ratios of the normal faults, as compared with such ratios of the tension fractures, are attributed to the tendency of tension fractures to close as they develop into normal faults. It is concluded that divergent plate movements with dike intrusions, or pressure changes in a deep-seated changes in a deep-seated magma reservoir, are viable models for formation of the fractures.

Tectonics of the thingvellir fissure swarm, sw iceland

Abstract The Thingvellir fissure swarm dissects 9000 year-old pahoehoe lava and contains about 100 fractures of average orientation N29.3E. The average length of fractures is 620 m, the minimum being 57 m and the maximum 7.7 km. The maximum width and throw on a single fracture are 68 m and 40 m, respectively. Most fractures are vertical at the surface and must be the result of an absolute tensile stress. The geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures. It is concluded that most fractures attain depths of the order of several hundred meters or less, but that the largest faults attain depths of many kilometers. Comparison with the Vogar fissure swarm on the Reykjanes Peninsula suggests that the Thingvellir swarm may have the greater rate of dilation; the total maximum postglacial dilation of the Vogar swarm is only 15 m, whereas the corresponding figure for the Thingvellir swarm is about 100 m.

Form and dimensions of dykes in eastern Iceland

Abstract A study was made of about 700 dykes in eastern Iceland. The majority of these belong to three swarms. About 73% dip within 10° of the vertical. Most strike between 10° and 40°NE. The strike of the dykes within the southernmost swarm (Alftafjordur) changes along its trace, from almost N at the north end to NNE-SSW southward along the swarm. The average thickness of the dykes is about 4.1 m, and the thickness does not change notably along the Alftafjordur swarm. The thinner dykes tend to have smaller grains than the thicker dykes. Of five dykes followed laterally, the longest is over 22 km. The thickness of individual dykes changes irregularly along their length, and the dyke is often offset where its thickness changes. Many dykes appear to be completely discontinuous, but some parts are connected by veins. Where the dykes end in a vertical section, most of them simply taper away. Only about 10.5% of the dykes occupy faults. The mechanical and thermal effects of the dykes on the country rock are small. Many of the dykes appear to be non-feeders, i.e. dykes that never reached the surface to feed lava flows. Using the length/width ratio, the depth of origin of three dykes has been estimated. The maximum depth of origin of these three dykes is 7.5 km, 9 km and 10 km below the original surface.

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.

Magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics

The volcanic rift zone in Iceland is characterized by elongate volcanic systems, consisting of tension fractures, normal faults and volcanic fissures, where most of the volcanotectonic activity takes place. Most volcanic systems develop central volcanoes, the formation of which is still poorly understood. There is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes. In the central volcanoes, eruptions are frequent (typically one every several hundred years), of small volume (normally less than 0.1 km3) and fed mainly by thin (average thickness of 0.5 m), inclined sheets. Outside central volcanoes the eruptions are rare (typically one every several thousand years) but of large volume (typically more than 1 km3) and fed mainly by thick (average thickness of 4–5 m), subvertical dikes. Using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi-infinite plate, these empirical relations, as well as the formation of specific central volcanoes in the volcanic systems, can be explained. Once a cavity-like magma chamber has formed, its existence in the rift zone concentrates tensile stress. This stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system, which partly explains the formation of central volcanoes and their high eruption frequencies. This concentration also gives rise to a local stress field that encourages injection of small-volume sheets in all directions and at various dips from their source magma chamber. The magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma, through inclined sheets, toward a limited area at the surface where the central volcano gradually forms. The inverse relationship between eruption frequency and eruption volume, when the central volcano is compared with other parts of the volcanic system, is also partly due to the trap-like nature of the magma chamber. The chamber is normally much smaller than its source mantle reservoir, so that a single magma flow (through a dike) from the reservoir (lasting perhaps many years) may trigger tens of magma flows (through sheets) from the chamber, many of which would reach the surface in the central volcano.

Effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies

What controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood. I propose that for volcanoes at divergent plate boundaries, these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them. This stress concentration is mainly a function of the applied tensile stress, associated with spreading, and the aspect (height/width) ratios of the chambers. High spreading rates and/or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies. It is found that for chambers at the same depth in a volcanic zone, the one of the highest aspect ratio will normally intrude magma most frequently. Also, if the chambers are of equal aspect ratios, the one at the greatest depth will intrude magma most frequently. Because the extrusion frequency of a volcano is a fraction of its intrusion frequency, the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano. These results are applied to the divergent plate boundary in Iceland, where many of the volcanoes appear to be fed by “double chambers”, that is, shallow chambers fed by deep-seated chambers. It is found that, except when the aspect ratio of the deep-seated chamber is much less than that of the shallow chamber, the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep-seated chamber. It is concluded that whereas the short-term (i.e., ≤103 yrs) extrusion frequencies of volcanoes at the plate boundary in Iceland may be similar to the dike intrusion frequencies of the source chambers, the long-term (i.e., ≥104 yrs) extrusion frequencies may be about ten times lower than the intrusion frequencies.