Form and dimensions of dykes in eastern Iceland

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.     

Effects of sedimentary interfaces on fracture pattern,linkage, and cluster formation in peritidal carbonate rocks

The permeability of a reservoir is particularly dependent upon the proportion of its fractures that penetrate or are arrested at interfaces such as contacts and discontinuities. Here we report on fracture penetration and fracture arrest in Lower Cretaceous peritidal deposits exposed in the Pizzicoli Quarry, Gargano Promontory, southern Italy. We measured more than 2000 fractures, in the field and using LIDAR data, of which 564 fractures from the field and 518 from LIDAR studies are the focus of this paper. Fracture arrest/deflection and penetration depend much on the effects of peritidal cycle interfaces such as paleosol horizons, laminated carbonate mudstones, and stylonodular horizons. The laminated mudstones have the greatest effect; 63–99% of the fractures are deflected or arrested at such interfaces, whereas 63–90% are deflected/arrested at paleosols, and 20–35% at stylonodular horizons. In the mudstones, many fractures are arrested at thin, internal laminae, such that few penetrate the entire laminated layer, and fewer still the boundaries between the layers. Paleosol interfaces deflect/arrest more than 60% of all fractures. However, when small-offset fractures above and below paleosols are regarded as penetrating, they are evenly spaced (non-clustered), so that fracture-related fluid transport may occur across the entire paleosol. Stylonodular horizons deflect/arrest and split some fractures, but generally have little effect compared with the other types of interfaces. We present three main mechanisms for fracture deflection and/or arrest: (1) the fracture-induced tensile stress ahead of its tip, referred to as the Cook-Gordon debonding mechanism; (2) rotation of the principal stresses at and across the interface, resulting in the formation of stress barriers; and (3) large elastic mismatch (particularly as regards Young’s moduli) between layers across an interface. All these mechanisms are likely to have operated during fracture propagation and arrest in the carbonate rocks of the Pizzicoli Quarry.     

Loading of a seismic zone to failure deforms nearby volcanoes: a new earthquake precursor

The South Iceland Seismic Zone (SISZ) was loaded to failure in June 2000, resulting in two M6.6 earthquakes. The SISZ is an E–W‐trending zone with an overall sinistral movement. Numerical models indicate that, when the SISZ is loaded to failure, there are stress concentrations at its ends: tensile in the north‐east and south‐west quadrants, and compressive in the north‐west and south‐east quadrants. These model predictions fit well with observations. Geodetic measurements indicate considerable compression, uplift and associated intense seismicity in recent years in the volcanoes of Hengill and Eyjafjallajokull, located in the quadrants of compression, whereas there have been unusually frequent eruptions in the past decades in the Hekla Volcano, located in one of the quadrants of extension. The models predict that following the large June 2000 earthquakes, stress relaxation within the SISZ should lead to stopping of the intense seismicity and deformation in the volcanoes of Hengill and Eyjafjallajokull, again in agreement with observations. However, when similar episodes of deformation and seismicity start again, particularly in the Hengill Volcano, a large earthquake would be expected within several years in the SISZ. The numerical models, and the deformation and seismic data, indicate that monitoring of ‘soft’ inclusions such as volcanoes (many with magma chambers) in the vicinity of a seismic zone may serve as precursors to large earthquakes.     

Progressive cooling of the hyaloclastite ridge at Gjálp, Iceland, 1996–2005

In the subglacial eruption at Gjálp in October 1996 a 6 km long and 500 m high subglacial hyaloclastite ridge was formed while large volumes of ice were melted by extremely fast heat transfer from magma to ice. Repeated surveying of ice surface geometry, measurement of inflow of ice, and a full Stokes 2-D ice flow model have been combined to estimate the heat output from Gjálp for the period 1996–2005. The very high heat output of order 10⁶ MW during the eruption was followed by rapid decline, dropping to  2500 MW by mid 1997. It remained similar until mid 1999 but declined to 700 MW in 1999–2001. Since 2001 heat output has been insignificant, probably of order 10 MW. The total heat carried with the 1.2 × 10¹² kg of basaltic andesite erupted (0.45 km³ DRE) is estimated to have been 1.5 × 10¹⁸ J. About two thirds of the thermal energy released from the 0.7 km³ edifice in Gjálp occurred during the 13-day long eruption, 20% was released from end of eruption until mid 1997, a further 10% in 1997–2001, and from mid 2001 to present, only a small fraction remained. The post-eruption heat output history can be reconciled with the gradual release of 5 × 10¹⁷ J thermal energy remaining in the Gjálp ridge after the eruption, assuming single phase liquid convection in the cooling edifice. The average temperature of the edifice is found to have been approximately 240 °C at the end of the eruption, dropping to  110 °C after 9 months and reaching  40 °C in 2001. Although an initial period of several months of very high permeability is possible, the most probable value of the permeability from 1997 onwards is of order 10^− 12 m². This is consistent with consolidated/palagonitized hyaloclastite but incompatible with unconsolidated tephra. This may indicate that palagonitization had advanced sufficiently in the first 1–2 years to form a consolidated hyaloclastite ridge, resistant to erosion. No ice flow traversing the Gjálp ridge has been observed, suggesting that it has effectively been shielded from glacial erosion in its first 10 years of existence.     

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.