Fissure Swarm Research Articles

Tectonics of the thingvellir fissure swarm, sw iceland

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.

Mechanical aspects of postglacial volcanism and tectonics of the Reykjanes Peninsula, southwest Iceland

Four major NE trending postglacial volcanic and tectonic fissure swarms (volcanic systems) occur on the Reykjanes Peninsula, and the westernmost three are the main subject of this paper. Two main types of basaltic volcanoes are associated with these systems: shields of picrite and olivine tholeiite and tholeiite fissures. The average volume of 26 shields is 1.11 km3, and the total production is 29 km3, whereas the corresponding figures for lavas from 101 volcanic fissures are 0.11 km3 and 11 km3. The tectonic fractures are either tension fractures or normal faults of widths up to 20 m, throws up to 10 m, and lengths up to several kilometers. The volcanism and tectonics can be explained by magmatic pressure changes in ellipsoidal magma reservoirs located beneath the fissure swarms. A magmatic pressure increase of the order of 10 MPa is found to be sufficient for an excess uplift of the order of several meters, which is all that is needed to account for the fractures and measured dilation in the fissure swarms. It is concluded that most shield volcanoes, in particular the picrite shields and the large olivine tholeiite shields, formed during the early postglacial period and that their formation was facilitated by the stress field generated as a result of rapid uplift and bending of the crust above the reservoirs. Since that time the reservoirs have become independent systems, the volcanism has been confined to fissures, and the production rate has decreased significantly. During typical fissure eruptions (0.015 km3), only the uppermost several hundred meters of the source reservoir, depending on its magma content, supply magma to the eruption.

Surface and Bedrock Topography of Ice Caps in Iceland, Mapped by Radio Echo-Sounding

Since 1977, large areas on western Vatnajökull have been surveyed by ground-based, radio echo-sounding and the whole ice cap, HofsjökuIl, was surveyed in 1983. Detailed maps of the glacier-surface elevation and the sub-ice bedrock have been compiled. The instrumentation includes a 2–5 MHz, mono-pulse echo-sounder, for continuous profiling, a satellite geoceiver and Loran-C equipment, for navigation, and a precision pressure altimeter. The maps of western Vatnajökull cover about 1500 km2 and are compiled from 1500 km-long sounding lines, which yielded about 50 000 data points for ice thickness and 20 000 points for ice-surface elevation. The maps of HofsjökuIl cover 923 km2, the sounding lines were 1350 km long; 42 000 points were used for determining ice thickness and 30 000 for surface elevation. The maps obtained from these data are the first ones of the ice caps with surface elevation of known accuracy. The bedrock map of western Vatnajökull shows details of volcanic ridges and subglacial valleys, running north-east to south-west, as well as the central, volcanic complexes, Hamarinn, Bárdarbunga, and Grimsvtön and the related fissure swarms. The map of Hofsjökull reveals a large volcanic complex, with a 650 m deep caldera. The landforms in southern Hofsjökull are predominantly aligned from north to south, but those in the northern ice cap run north by 25° east.

Surface and Bedrock Topography of Ice Caps in Iceland, Mapped by Radio Echo-Sounding

Since 1977, large areas on western Vatnajökull have been surveyed by ground-based, radio echo-sounding and the whole ice cap, HofsjökuIl, was surveyed in 1983. Detailed maps of the glacier-surface elevation and the sub-ice bedrock have been compiled. The instrumentation includes a 2–5 MHz, mono-pulse echo-sounder, for continuous profiling, a satellite geoceiver and Loran-C equipment, for navigation, and a precision pressure altimeter. The maps of western Vatnajökull cover about 1500 km2 and are compiled from 1500 km-long sounding lines, which yielded about 50 000 data points for ice thickness and 20 000 points for ice-surface elevation. The maps of HofsjökuIl cover 923 km2, the sounding lines were 1350 km long; 42 000 points were used for determining ice thickness and 30 000 for surface elevation. The maps obtained from these data are the first ones of the ice caps with surface elevation of known accuracy. The bedrock map of western Vatnajökull shows details of volcanic ridges and subglacial valleys, running north-east to south-west, as well as the central, volcanic complexes, Hamarinn, Bárdarbunga, and Grimsvtön and the related fissure swarms. The map of Hofsjökull reveals a large volcanic complex, with a 650 m deep caldera. The landforms in southern Hofsjökull are predominantly aligned from north to south, but those in the northern ice cap run north by 25° east.

On the mechanism of magma injection and plate divergence during the Krafla Rifting Episode in NE Iceland

Geodetic observations of horizontal and vertical point displacements in a ∼100‐km‐wide network across the Krafla fissure swarm of NE Iceland are analyzed with simple elastic models. The observed displacements accumulated from 1975 to 1980 in several events of magma injection into fissures that were widened. The fissure swarm was extended, and it subsided while the adjacent lithosphere contracted or was compressed accompanied by surface uplift. We applied two models to fit the far‐field deformation: an infinite horizontal pressure line source or a pipe and an infinite, vertically limited, vertical dyke source loaded by normal stress at the walls, both embedded in an elastic half‐space. In the far‐field inversion the two models cannot be distinguished, but geology and the observed axial graben subsidence clearly discriminate against the pipe and for the dyke. The inversion suggests that the widened dykes cut through the whole lithosphere which in the fissure swarm is only 4–6 km thick. During the rifting episode the general magma level in the fissure swarm rose to near the surface by 1980, but the loading increments appear to have decreased whith time from 1975 to 1980.

Dynamics of crustal rifting in NE Iceland

Magnetotelluric measurements have revealed a crustal thickness of 8–10 km in the axial rift zone of NE Iceland and above the proposed hot spot in central east and north Iceland. The crust thickens with age and is 20–30 km thick in the older Tertiary areas to the east and west of the axial rift zone. It also thickens toward north with increasing distance from the hot spot. The crust is underlain by a diapiric updoming asthenosphere over large parts of the country. At the crust asthenosphere interface there is a partially molten basaltic layer a few kilometers thick. This layer partially decouples the crust from the anomalous mantle. Its low viscosity may explain the spatial and temporal instability of the rift axis and the high mobility of the crust during technically active periods. A major volcano‐tectonic rifting episode started in the Krafla area in 1975 involving an 80‐km‐long segment of the plate boundary. Several meters of opening accompanied by subsidence was observed in the center of the axial fissure swarm and contraction and uplift occurred on the sides. The rifting occurred in several short events. Magma flowed horizontally from an upper crustal magma chamber and mixed with magma ascending directly from the semifluid layer at the base of the crust. Crustal accretion was mainly engineered by dike intrusions. Only a minor part of the magma transported reached the surface. During technically quiet epochs, plate‐driving forces, caused by the upwelling asthenosphere, result in long‐term stretching and accumulation of tensile stress in the thinnest part of the crust. The vertical and horizontal displacements during rifting episodes can be explained by elastic rebound, where fissure opening accompanied by magma flow causes the release of tension.

Geodetic measurements of surface deformations during the present rifting episode in NE Iceland

Iceland is part of the accreting plate boundary between the American and European plates. Since 1965 a number of repeated triangulation‐trilateration surveys spanning the neovolcanic zone in NE Iceland were made in order to detect crustal movements. All the measurements for a single observation epoch are separately adjusted by the method of least squares. Statistical tests are carried out to check the observations and adjustment models for outliers and model errors. To detect significant movements in repeatedly measured geodetic networks with multivariate design, a testing rule is described. After unexpected horizontal contractions of the surface of approximately 0.5 m from 1965 to 1971, enormous deformations have been found since the beginning of a major rifting episode in December 1975. During the period from 1971 to 1980 a horizontal expansion of about 7.5 m and vertical displacements of 3 m were observed across the approximately 3‐km‐wide active rifting zone of the Krafla fissure swarm. In adjacent zones, compressional strains of up to 2×10−4 occurred, rapidly decreasing to a value of 1×10−5 at a distance of 40 km from the rifting axis. After 1980 the pattern of ground deformation has altered. The east‐west shifts have decreased and are superposed by a radial component of motion around the Krafla caldera. West of the Krafla fissure swarm a shear zone is found.

Recent volcanic history of the Veidivötn fissure swarm, southern Iceland — an approach to volcanic risk assessment

Abstract The recent volcanic history of the southwestern part of the Veidivotn fissure swarm, southern Iceland, provides a basis for assessment of volcanic risk in an area of large hydropower potential. Local tephrastratigraphy and regional tephrochronology provide relative and absolute dating of individual eruptions as well as information on the volume and distribution of the products formed in each eruption. Three large eruptions took place in this area in ∼ 1480 A.D., ∼ 900 A.D. and ∼ 150 A.D., respectively. Each eruption produced approx. 1 km3 (DRE) of basaltic, and minor amounts of silicic lava and tephra on fissures up to 42 km long. No evidence is found of smaller eruptions during this period. The estimated eruption frequency, one eruption every 600–800 years, implies that this part of the Veidivotn fissure swarm is inactive for long periods between relatively large volcanic events. A change in the mode of eruption from effusive to explosive took place during this period. The hazards posed by this area include far-reaching lava flows, widespread heavy tephra fall with thicknesses in excess of 2 m at distances of 10 km, and damming of a large glacial river with the consequent formation of unstable lakes. A volcano-tectonic model, which explains the observed eruption frequency and provides a basis for a long-term monitoring program, is proposed. Eruptions on the Veidivotn fissure swarm are interpreted as corollaries of rifting episodes initiated in the Bardarbunga central volcano. Volcano-tectonic episodes affect the fissure swarm at an average interval of 100 years. Minor episodes are limited to the central volcano and adjacents parts of the fissure swarm. During the less frequent major episodes, rifting and volcanic activity extends to the extreme southwestern part of the fissure swarm. Seismic monitoring of the Bardarbunga central volcano could provide an early warning of renewed activity on the Veidivotn fissure swarm. A major rifting episode resulting in eruption on its southwestern part can be expected during the next 100 to 300 years.

Magma mixing and hybridization processes at the alkalic, silicic, Torfajökull central volcano triggered by tholeiitic Veidivötn fissuring, south Iceland

Abstract Around 19°W64°N, in the Eastern Volcanic Zone of south Iceland, the southwestern extremity of the tholeiitic Veidivotn fissure swarm abuts the northern periphery of the mildly alkaline silicic central volcano Torfajokull. Effusive mixed-magma eruptions in this area have been initiated by crustal rifting associated with lateral injection of tholeiitic magma into the rhyolitic Torfajokull chamber. Mixed rocks, in which cm- to dm-sized mafic inclusions constitute ∼ 10% of the rhyolite lava are distinguished from hybrid rocks which are thorough mixtures having an homogeneous fabric. Mapping of the Domadalshraun lava (0.05–0.1 km3) reveals the early eruption of hybrid spatter and lava followed by less thoroughly blended mixed lava. Geochemical study discloses the participation of three end-members. Plagioclase-phyric tholeiite containing ∼ 10–20% pl + cpx + o1 precipitated at ∼ 1140–1160°C is typical of the Veidivotn component. The Torfajokull rhyolite contains ∼ 10% crystals, dominantly anorthoclase and oligoclase, and is found to be compositionally zoned. Early-late trends include SiO2: 71.6−70.1%, A.I.: 1.07−0.86, Sr: 61–86 ppm and result from feldspar fractionation. The third end-member is a transitional alkali basaltic andesite ( ≡ hawaiite ) belonging to the mafic magma suite associated with Torfajokull. This intermediate magma resided at depth within the stratified Torfajokull chamber and contains xenocrystic feldspars which have settled out of the differentiating rhyolite. The calculated positions in tholeiite-rhyolite-basaltic andesite composition space of eight hybrid rocks show that hybridization was not a random process. No rhyolite/transitional alkali hybrids are present. The only two tholeiite/transitional alkali hybrids are both 0.45/0.55 blends of these two end-members. No rhyolite-bearing hybrid contains more than 20% transitional alkali magma. It is proposed that rhyolite/tholeiite hybrids are most likely to be generated where a shallow tholeiitic fissure has been laterally intruded into and above the roof of the rhyolitic chamber. Near the top of the fissure, tholeiite containing excess water may have been sufficiently vesicular for its bulk density to equal that of the rhyolitic magma. Calculations identify this critical depth as 0.75–1 km-coincident with calculated quenching pressures of small vesicular tholeiitic clots in the mixed lavas (assuming 1 wt.% water). Dense tholeiite at deeper levels in the dyke collapses into the chamber and is replaced by buoyantly rising rhyolite. Hybridization ensues where the rhyolite and vesicular tholeiite come together in the fissure. These and other relative-density controlled processes account for the selective nature of the hybridization process and the order in which hybrid and mixed magmas were erupted. The late 15th century Laugahraun and Sudurnamshraun flows at Landmannalaugar reveal participation of only tholeiitic and rhyolitic end-members. Hybrids are poorly represented and it is hypothesized that this is due to the deep ( ≳ 1 km ) intrusion of undersaturated tholeiite, which leads to the generation of mixed rather than hybrid magmas. These and other tholeiite/rhyolite mixed-magma eruptions in the area were triggered by lateral flow of tholeiite from the Veidivotn system, initiated by the overfilling of the Veidivotn magma chamber, 40 km to the NE, in the style recognized at Krafla and elsewhere in Iceland. Future concern over any renewed activity in Veidivotn should not, therefore, prohibit consideration of rhyolitic or mixed-magma eruptions being induced in the Torfajokull region.

Widening of the Krafla fissure swarm during the 1975–1981 volcano-tectonic episode

Frequent distance measurements across the Krafla fissure swarm, North Iceland, recorded the extension accompanying the sequence of rifting events which started in December, 1975, and lasted for 6 years. An 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events. Maximum widening of about 8 m occurred 10 to 12 km north of Leirhnjukur. which is located above the center of the Krafla magma reservoir. From that location, the amount of widening decreased north-wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting, off the north coast, about 70 km north of Leirhnjukur. The amount of widening also decreased southwards and approached zero at 15 to 20 km south of Leirhnjukur. The ground deformation associated with these rifting events can be summarised as: A narrow strip, 1 to 2 km wide, along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm. This fractured strip has subsided 2 to 3 m relative to its flanks. The flanks of the fractured zone have been uplifted relative to regions farther away. The uplift is not well constrained, but tilt observations at several locations indicate about 1 m uplift. The flanks of the rift zone have contracted, perpendicular to the fissure swarm. The maximum contractional strain exceeds 300 mm per km. The amount of areal expansion (windening of the fissure swarm times the length of the fractured zone) associated with these rifting events is estimated to be about 0.30 km2. For individual events, the area of expansion has been roungly proportional to the volume of subsidence above the Krafla magma reservoir. If the width of a new dike is equal widening of surface fissures, the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2.4 to 2.8 km. This ratio is significantly less for events of large lava production, but even during these events, the majority of magma leaving the Krafla reservoir was apparently emplaced in subsurface fissures.

Stress estimates from the length/width ratios of fractures

The hypothesis is advanced that, provided Young's modulus and Poisson's ratio of the rock are known, the length/width ratios of tension fractures can be used to estimate the tensile stress (assumed constant along the length of each fracture) at the time of fracture formation. The hypothesis is tested on a fissure swarm in a 10,000 year-old basaltic lava in Iceland. The length/width ratios of the fissures give the average tensile stress as of the order of a few MPa.

Hydrofracturing stress measurements in the Iceland Research Drilling Project drill hole at Reydarfjordur, Iceland

Two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the Iceland Research Drilling Project deep hole at Reydarfjordur, east Iceland. As indicated by the continuously extracted drill core, the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes. The density of the basalt was used to estimate the vertical stress σV (in megapascals) at depth D (in meters): σV = 0.027D. In the 33‐ to 270‐m depth range the horizontal principal stresses (σHmax and σHmin) as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress: σHmin = 2.1 + 0.016D; σHmax = 3.3 + 0.016D. In the 300‐ to 550‐m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m. Conditions favoring normal faulting (σV σHmax σHmin) were established in the 540‐ to 580‐m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of σHmax and σHmin This would be in accord with the predominant fault type in east Iceland. The relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near‐surface high compressive stresses previously determined by Hast (1973) in east Iceland. The large increase in horizontal stress in the range of 300‐ to 500‐m depth is not understood. A correlation appears to exist between the zone of almost unintruded thick lava flows (200–500 m) and that of high stresses. An intuitive model suggesting ‘locked in’ stresses in this zone is proposed. The hydrofracturing‐determined overall mean direction of σHmax is N50°E, while a more selective sampling of results yields N40°E. The measured stress directions appear to be reasonably compatible with the local structure such as dike direction (NNE) and lava dip (WSW). Significantly, σHmax direction is subparallel to the N20°E trend of the nearest fissure swarms in the axial rift zone. This almost coincidence of directions, like the measured stress magnitudes, is also in agreement with the model of spreading lithospheric plates being pulled apart.

Fracture analysis near the mid-ocean plate boundary, Reykjavik-Hvalfjördur area, Iceland

The geometry and thermal history of fractures have been determined at 59 stations from Reykjavik to Hvalfjördur in southwestern Iceland. The data provide information on crustal stress regimes in the vicinity of mid-ocean ridges. Two major, generalized fracture orientations are present 1. (1) a northeast system, trend 010°–030°, except on Akranes where the orientation is 040°–060° 2. (2) a broad east—west system containing one or more sets with strike between 070°–130°. Thermal history of the host rock and fractures was determined from secondary minerals in vugs and fractures. The thermal history indicates that the northeast fracture set opened while the area was within the relatively hot axial zone of active volcanism and rifting. Some of the east—west trending fractures also opened at this time but many formed later, after the area had begun to cool and drift from the active zone. The northeast fracture set is essentially parallel to the trend of dikes and normal faults in southwestern Iceland. They have been interpreted as extension fractures (resulting in about 0.4% maximum extension) forming generally from the same stress field associated with normal faulting and dike injection in the active zone. Fracturing in an east-west direction (estimated 0.1% maximum extension), mainly near the edge and outside the active zone, indicates a reorientation of this stress field. The dominant mechanism related to the origin of the east—west fractures may be thermoelastic stresses arising from axial and basal accretion and cooling of lithospheric plates. Both fracture systems are inferred to have formed, in the Griffiths idealization, under nearly biaxial effective compressive loading on the order of 200 bar. The discrepancy between this value and the kilobar-order strengths of short-time laboratory tests reflects such factors as high temperature stress corrosion and fatigue. Fracture propagation is assumed to have been stable, but governed primarily by lateral load-diminishing mechanisms rather than by progressive loading. These relaxation mechanisms may have been episodic (northeast-system fissure swarm activity) or steady-state (thermoelastic contraction) in time.

Gravity and height variations connected with the current rifting episode in Northern Iceland

Following a first gravity survey in 1938, high-precision gravity measurements with LaCoste-Romberg gravimeters have been carried out in northern Iceland, in 1965, 1970/ 1971 and 1975, in order to detect eventual secular gravity variations at that supramarine part of a constructive plate boundary. Since 1975, when an active rifting episode started there, gravity and height variations have been observed in annual intervals. These surveys concentrated on the approx. 100 km long and 5 km wide fissure zone associated with the Krafla central volcano, which is the center of the present activity. The observations, which have been performed at an accuracy level of ± 0.01 mGal, resp. ± 0.03 m, give significant information about the long-term behaviour of the vertical mass shifts, occuring during the rifting process. A steep gravity-variation gradient, observed south of the Krafla caldera between 1965 and 1975, may be interpreted as some kind of precursor of the current process. Since 1975 the rifting event is characterized by shifting activity areas, with gravity decrease and elevation at the flanks, and gravity increase and subsidence at narrow central parts of the Krafla fissure zone, the annual variations reaching ± 0.1 mGal, resp. ∓ 0.5 m and more. Since 1978, the activity has concentrated on the Gjástikki region, north of the Krafla volcano, with decreasing variations in the southern and northern parts of the fissure swarm. The average gravity—height-variation ratio calculated for two profiles south and north of Krafla, over the observation periods between 1975 and 1979, amounts to −0.2 mgal/m, but extreme values are found at recently activated areas.

Rifting of the plate boundary in north Iceland 1975–1978

A rifting episode started in 1975 on the accreting plate boundary in North Iceland after 100 years of quiescence. Horizontal extension of some 3 m has been observed in the Krafla caldera and the associated 80 km long fissure swarm. The rifting occurs periodically in short active pulses at a few months intervals. Between these active pulses, continuous inflation of 7–10 mm/day of the caldera is caused by 5 m3/s inflow of magma into a magma chamber at 3 km depth. The active pulses are caused by a sudden east‐west expansion of the fissure swarm and a contraction of zones outside the fissure swarm. Rapid flow of magma out of the magma chamber and into the fissures toward north or south is indicated. These pulses are accompanied by earthquake swarms and vertical ground movements of up to 2 m and sometimes also volcanic eruptions and formation of new fumaroles. The magma chamber below the Krafla caldera thus acts as a trigger for the plate movement along the constructive plate boundary in North Iceland.

Petrology of the Western Reykjanes Peninsula, Iceland

The active tholeiitic volcanic zone of the Reykjanes Peninsula consists of five volcanic fissure swarms, the two westernmost of which are the subject of this petrological study. The recent (less than 12,000 years) extrusives of the swarms group morphologically and petrographically into small picrite basalt lava shields, large olivine tholeiite lava shields and tholeiite fissure lavas; formed in that chronological succession. The picrite basalts exhibit a primitive mineralogy with chromite, olivine (Fo 89) and plagioclase (An 90) as phenocrysts and may represent a primary liquid from the mantle. Simultaneous crystallization of olivine, plagioclase and augite to form glomerocrysts in the fissure lavas indicate low pressure cotectic crystallization conditions. Twenty-eight new major element chemical analyses of the lavas are presented. They are generally characterized by a low content of alkalies and high CaO. The lavas constitute two main suites, a lava shield suite and a fissure lava suite. There is a positive correlation between the volume of individual lavas and the content of incompatible elements of the lavas within each group. Likewise there is an overall chemical trend through time demonstrated, for example, by a rise in K2O from about 0.02 per cent to 0.24 per cent during the last, approximately, 12,000 years. There is an apparent chemical zoning within each volcanic swarm such that the most evolved and youngest lavas are found in the central axial area of the swarm. This central area is also characterized by graben subsidence, high magnetic anomalies and high temperature thermal areas, all indicative of shallow magma reservoir(s). In spite of indications of fractional crystallization in the evolution of the olivine tholeiites and tholeiites, some other processes must be sought to explain the volume chemistry relations. Cyclic volcanic activity is tentatively suggested to explain the observed regular temporal variations within the swarm, each cycle starting with the formation of picrite basalts.

Rifting episode in north iceland in 1874–1875 and the eruptions of askja and sveinagja

In 1874 and 1875 the fissure swarm of Askja central volcano was activated during a major rifting episode. This rifting resulted in a fissure eruption of 0.3 km3 basaltic magma in Sveinagja graben, 50 to 70 km north of Askja and subsequent caldera collapse forming the Oskjuvatn caldera within the main Askja caldera. Five weeks after initial collapse, an explosive mixed magma eruption took place in Askja. On the basis of matching chemistry, synchronous activity and parallels with other rifted central volcanoes, the events in Askja and its lissure swarm are attributed to rise of basaltic magma into a high-level reservoir in the central volcano, subsequent rifting of the reservoir and lateral flow magma within the fissure swarm to emerge in the Sveinagja eruption. This lateral draining of the Askja reservoir is the most plausible cause for caldera collpse.

The simultaneous production of basalts, enriched and depleted in large lithophilic trace ions (LIL), within the same fissure swarms in Iceland

The simultaneous production of basalts, enriched and depleted in large lithophilic trace ions (LIL), within the same fissure swarms in Iceland

Seismicity, earthquake swarms, and plate boundaries in the Iceland region

abstract The seismicity of the Iceland region is concentrated along the crest of the Mid-Atlantic Ridge and in two shear zones connecting the two ends of the eastern volcanic zone in Iceland to the ridge crest on both sides of Iceland. Earthquakes of magnitude exceeding 6 occur in these shear zones, but not on the ridge crests. The volcanic zones of Iceland, with the exception of the Reykjanes peninsula, are characterized by low seismicity with no observed earthquakes of magnitude larger than 5. Earthquake swarms are mostly confined to the crest of the Mid-Atlantic Ridge, the Reykjanes peninsula and the north Iceland shear zone, but very few swarms occur in the south Iceland shear zone or in the eastern volcanic zone of Iceland. The magnitude-frequency relation of earthquakes within each earthquake swarm deviates from the linear relation, Log N = a−bM , in that the “ b ” value decreases with decreasing earthquake magnitude. A better expression for the magnitude-frequency relation within each swarm is Log N = 3.5 − A · 1.5 M , where A is a constant related to the size of the swarm, and N is the number of earthquakes of magnitude equal to or exceeding M . The spreading axes follow the crest of the Reykjanes Ridge and the volcanic zones in Iceland. North of Iceland the axis of spreading is not well defined, but it probably has a nearly north-south strike from the Eyjafjordur deep across the Kokbeinsey Island to the Spar Fracture Zone. The spreading of the eastern volcanic zone of Iceland is confined to two or more parallel bands of volcanic activity and fissure swarms, with more stable strips in between. The transfer of spreading from the Reykjanes Ridge to the eastern volcanic zone and from the eastern volcanic zone to the spreading axis north of Iceland takes place in zones of shear deformation, but not along single transform faults. These shear zones are the only areas in the Iceland region where earthquakes of magnitude 7 or larger are known to occur. The postulated central Iceland shear zone (or transform fault) is seismically inactive and, therefore, not similar to the shear zones in south Iceland and off the coast of north Iceland.

A comparative account of the flood basalt volcanism of the Columbia Plateau and Eastern Iceland

An introduction to the flood basalt volcanism of the Columbia Plateau and Eastern Iceland is followed by more detailed comparative notes. These stress that the volcanism in the two areas was of the same general type. In both regions sub-aerial fissure eruptions gave rise to very extensive basalt flows, particularly on the Columbia Plateau, where some individual lavas cover more than 10,000 km2. The feeding fissures were localized in swarms, and this led in each case to the development of thick, low, shield-like accumulations of flows over the source areas. Progressive (isostatic?) subsidence of the central parts of the basalt pile accentuated the natural tendency for the succession to be thickest in the neighborhood of the feeding fissure swarms. Related differentiates were erupted from the central parts of the fissure vent areas, while olivine-rich basalt flows were apparently often erupted from the edges of the main swarm.