Krafla

In North Iceland the neovolcanic zone has a north-south direction. Towards the north it is offset to the west about 100 km by the Tjörnes Fracture Zone to join the Kolbeinsey Ridge. The structure of the neovolcanic zone in North Iceland is dominated by large swarms of faults and fissures which pass through a central volcano forming together a volcanic system. Volcanic fissure eruptions, silicic rocks and high temperature geothermal fields concentrate in the central volcanoes. Two central volcanoes in North Iceland, those of Krafla and Askja, have developed calderas. The volcanic systems are arranged en echelon subparallel to the north-south direction of the main zone. The northernmost parts of the fissure swarms are intersected in the area of Axarfjördur by the Tjörnes Fracture Zone.

The Krafla central volcano forms a low, broad shield some 25 km in diameter, with a caldera in its centre. The caldera measures about 10 km east-west and about 8 km north-south. It formed during the last interglacial period and has since been filled almost to the rim with volcanic material. The collapse of the caldera almost certainly followed the eruption of a sheet of dacitic welded tuff which is exposed around the caldera. Field characteristics indicate that this is an airfall tuff, which was blown mainly towards NE from a source near the centre of the Krafla caldera. A lava shield with a diameter of about 20 km existed prior to the caldera formation. Remnants of this shield structure enclose the caldera on the east and west sides exposing lavas and breccias dipping outward at low angles. Dykes and eruptive fissures that trend parallel to the caldera ring fault occur.

Photo by Halldór Ólafsson

A high temperature geothermal field lies within the caldera. Drilling has revealed temperatures in excess of 340°C at 2 km. Basaltic intrusions together with dolerite and granophyre become increasingly abundant below 1.2 km depth. Corresponding coarse grained rocks are conspicuous among xenoliths of the welded tuff and the ejecta of an explosive eruption in 1724. Another high temperature geothermal field exists at Námafjall to the south. This has been drilled to a depth of 1.8 km revealing temperatures of over 290°C.

Petrological studies in the Námafjall-Krafla area show a range of composition from olivine tholeiite to rhyolite. In Post-glacial times all these rock types have been erupted. The basalts of the fissure eruptions, however, are predominantly quartz normative tholeiites which irrespective of the site of the eruption appear to be of closely similar composition. In Post-glacial times some 18 eruptions have occurred in the Krafla caldera and its nearest surroundings and about 15 in the Námafjall area. Voluminous silicic eruptions have not taken place in Post-glacial time but subglacial silicic eruptions within and around the Krafla caldera have produced large domes or ridges during the last glacial period, among them Hlídarfjall and Hrafntinnuhryggur. Explosion craters that have ejected small quantities of rhyolitic pumice exist in the Krafla caldera. The most recent formed in 1724 at the beginning of the 1724-1729 volcanic episode ("Mývatn fires"). Tephrochronological studies indicate that Post-glacial volcanism in the Krafla and Námafjall areas occurred in two main periods, one in early Post-glacial times, the latter during the last 3000 years. The period of repose may have lasted more then 4000 years. In Krafla the fissure swarm has erupted six times and in Námafjall four times during the latter period of activity. Four of the eruptions are common to both areas. The Krafla fissure swarm is divided into two parallel subswarms in the Krafla and Námafjall area. The rifting activity of the subswarms occurs alternately on the two subswarms for a period of 4000-5000 years. The western subswarm has been volcanically almost quiet during the post-glacial, but tectonically active during the volcanic repose period. Volcanism near Krafla has been concentrated along the central and eastern part of the fissure swarm during this period whereas during the earlier period it was dispersed over the western part as well. Most eruptions have produced less than 0.5 km³ of lava. The basaltic lava from the Threngslaborgir-Lúdentsborgir crater row south of Námafjall 2100 years BP produced the exceptionally great volume of 2-3 km³ with an areal extent of 220 km (Younger Laxárhraun). However this eruption may belong to a separate volcanic system.

The Krafla fissure swarm is 80 km long and 4-10 km wide, with over 1000 tectonic fractures. All fractures in the Holocene lava flows are vertical at the surface, indicating that the surface parts were generated by an absolute tensile stress. The maximum measured throw (accumulated vertical displacement) on a normal fault in the Holocene fissure swarm is 32 m and occurs on a normal fault at the western margin of the Krafla fissure swarm. Throws of 20-30 m are limited to the largest faults in the Holocene swarms, and the mean throw is only a few metres. The shortest fractures detected on aerial photographs at the scale of 1:34,000 are several tens of metres long and are invariably tension fractures. The lengths of the longest fractures are 4-10 km and they are normal faults. The long fractures are normally composed of shorter segments, some of which are composed of still smaller segments down to the scale of columnar joints.

Photo by Halldór Ólafsson