Volcanic geology of Krafla-Námafjall

Karl Grönvold (karlgr@norvol.hi.is)

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.

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.

Historical volcanic activity

Historical accounts mention only one eruption in the Krafla-Námafjall fissure swarm in the years 1724-1729. Contemporary descriptions of this eruption, the "Mývatn fires", exists. On May 17th 1724 a row of explosion craters formed. The eruption was mainly from main crater, Víti, which ejected both silicic pumice and basaltic scoria, but mostly mud and rock debris from the near surface rocks. This eruption lasted no more than a day or two. In January 1725 earthquakes were felt and ground fissures opened along the swarm from near Krafla southwards. Steam and mud craters formed both in the Krafla and Námafjall geothermal fields but no or only minor volcanic activity was noted. Other rifting events occurred in April and Sept. 1725. After that no significant activity is reported until August 1727, when a fissure eruption started at Leirhnjúkur near the centre of the caldera. The next event occurred in April 1728, when the eruptive fissure extended southwards to Námafjall. Further events occurred in December 1728 and finally in June 1728. After that lava continued to flow until September 1729 when the eruption stopped. By then the lava had reached the north shore of lake Mývatn, overflowed two farms and the farmhouses of the parsonage Reykjahlíð, but left the church intact although it was surrounded by the lava.

The lava flows cover an area of about 34 km and the total length of the discontinuous volcanic fissure is 11 km. A minor volcanic eruption was reported on the same fissure in 1746 accompanied by earthquakes and ground movements. Field investigations indicate that it occurred to the north and south of Leirhnjúkur located near the center of the Krafla caldera.

The "Krafla fires"

The events that have taken place in the Krafla caldera and on the Krafla fissure swarm during 1975-84 are, so far, a striking repetition of what happened during the "Mývatn fires" in the 1720's.

The current activity has been termed a rifting episode. It started with increasing seismic activity in the Krafla caldera during the second half 1975, culminating with an earthquake swarm, ground fissuring, and a small lava eruption just north of the hyaloclastite ridge Leirhnjúkur on Dec. 20 1975. Long term gravity changes, tilt variations and geodetic measurements across the axial zones in NE-Iceland are consistent with crustal separation occurring as slow continuous stretching during quiet periods and episodic rifting with up to several m of extension occurring within a period of a few years. During an active episode, magma ascends below the central volcano and is stored in shallow magma chambers at 3 km depth. At Krafla the maximum inflow rate of magma was 5 m/s, a figure based on the rate and area of inflation. During the short deflation/rifting events magma was intruded from the shallow magma chambers into the fissure swarm along laterally injecting dykes. As a result the tensional stress is gradually released and the accompanying magmatism changes from primarily dyke injection in the early stage of the rifting episode to lava outflow at a later stage. These events were accompanied by volcanic tremor, earthquakes swarms, vertical ground movements and widening of one part or other of the fissure swarm and increased geothermal activity. Some 17 volcano-tectonic events of the kind described, have occurred since December 1975, the last in September 1984. Since 1980 these events were accompanied by extrusion of lava.

The longer lasting eruptions started as long fissures but the activity concentrated after the first day on a short segment where sizeable scoria cones grew. After the Sept. 1984 eruption the total area covered by the lavas is 36 km and the volume about 0.25-0.3 km³. The widening of the fissure swarm amounts to a maximum of 9 m at the northern margin of the Krafla caldera and decreasing towards north and south. The subsidence of the central part of the swarm amounts to 2-6 m but on both sides the land has contracted and risen somewhat.

Following the eruption of September 1984 the Krafla magma chamber inflated again eventually fully recovering previous land elevation. During 1989-92 land subsided at 5 cm/yr but during 1992-95 by 2.4 cm/yr.


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Krafla Volcanic System

Overview of the Krafla volcanic system showing the Krafla caldera (red outline) and the Krafla fissure swarm (shaded).

Main fissures and geothermal areas (red dots) in the Krafla-Námafjall area (at the Krafla center).

Lava fields from the 1724-1729 Mývatn fires, and the 1975-1984 Krafla fires.

Lava, craters and eruptive fissures from the Krafla fires 1975-1984.

Eruptive sites and lava fields from individual events during the Krafla fires.