**1. Introduction**

The Krafla volcano is the most studied volcano in Iceland. The onset of the Krafla Fires 1975–1984, often referred to as the Krafla Rifting Episode, initiated intensive volcanological research which greatly increased the understanding of volcanism in extensional rift settings. The Krafla volcano has been closely monitored since the Krafla Fires. Geothermal exploration and drilling have also built-up extensive knowledge on the volcano and its geothermal system(s).

The Krafla region has long been known for its geothermal activity. The first geothermal exploration was conducted in 1969 and was continued the following years [1,2]. The first two exploration wells were drilled in 1974. Based on the findings, it was decided to build the first major geothermal power plant in Iceland in Krafla. The construction of a 60 MWe power plant and production drilling started in 1975 and were continued concurrently with the Krafla Fires that started in December 1975. It soon became apparent that the Krafla Fires caused contamination by magmatic gases, CO2 and H2S [3–5]. In the deeper part of the wellfield at that time, to the west and south-west of Mt. Krafla, the volcanic gases increased dramatically in the deeper part of the reservoir and massive precipitation of pyrite and pyrrhotite clogged the wells. The shallower colder part of the reservoir was, however, not affected. Drilling activities were therefore shifted to the southern slopes of Mt. Krafla and Hvíthólar and in 1978, the power plant started the production of 7 MWe. Production drilling continued and, in 1984, the production was up to 30 MWe. In 1990, a new exploration and drilling phase started. The contamination

of magmatic gases was diminishing and, by 1999, the power plant was fully operational and producing 60 MWe.

The complexity of the Krafla volcano and its geothermal activity has puzzled geoscientists for a long time and a convincing conceptual model of the volcano and geothermal activity has, in the opinion of the author, been lacking. In this paper, an attempt is made to shed some light on this complexity. A review of old and new information and data and numerical modelling are used to put forward a new, hopefully sensible, conceptual model for the Krafla volcano and its geothermal systems.

#### **2. Tectonic Setting and Geology**

The Krafla volcano is located within the Northern Volcanic Zone of Iceland (Figure 1). The Reykjanes Ridge (Mid-Atlantic Ridge) comes on shore at the tip of the Reykjanes peninsula, SW Iceland. Crustal spreading veers to the northeast because of an interaction with a mantle plume under central Iceland. The plate boundary coincides with the Reykjanes peninsula with a mixed strike–slip and extensional motion [6] to the Hengill volcano, which is a triple point with the Western Volcanic Zone (WVZ) and the South Iceland Seismic Zone (SISZ). The SISZ is a left lateral strike–slip zone that conveys most of the crustal spreading to the Eastern Volcanic Zone (EVZ) and connects to the Northern Volcanic Zone (NVZ). The spreading then veers north-westward to the Kolbeinsey Ridge, north of Iceland, through a right lateral transform zone, the Tjörnes Fracture Zone (TFZ).

**Figure 1.** A simplified tectonic map of Iceland showing the location of Krafla (black square). Red broken lines represent spreading zones. WVZ is the Western Volcanic Zone, EVZ is the Eastern Volcanic Zone, NVZ is the Northern Volcanic Zone. SISZ is the South Iceland Seismic/Transform Zone and TFZ is the Tjörnes Fracture/Transform Zone. Thin black broken lines show central volcanoes and yellow coloured areas fissure swarms.

Figure 1 shows that the NVZ is arc-shaped towards the transform zones in the north. It hosts five volcanic centres, from south to north, Kverkfjöll, Askja, Fremri Námur, Krafla and Þeistareykir. Each volcanic centre has its own fissure swarm. The fissure swarms overlap and are arranged in a westward stepping en-echelon fashion.

The Krafla volcanic system is believed to have been active for about 200,000 years [7]. The volcanic system consists of a central volcano, approximately 20 km in diameter, bisected by an about 90 km long NNE-SSW trending fissure swarm (Figure 2). The fissure swarm takes up and accommodates most of the crustal spreading in the part of the northern volcanic zone around it. The central volcano is, generally speaking, characterised by gently sloping topographic high with a caldera in the middle. The caldera is about 8 to 10 km in diameter (W–E elongated) and is partly filled with volcanic products. It developed from an explosive eruption producing dacitic welded tu ff about 110 ka ago [7]. The volcano remains active with recurring volcanic episodes. Krafla has a bimodal volcanic character: for long periods, it produces mainly basaltic fissure eruptions and dike injections into the fissure swarm, but intermittently, it erupts semi-silicic to silicic magma or tephra. Three such eruptions have been identified and dated [7,8]. The oldest is the phreatic eruption 110 ka ago forming the visible caldera, then subglacial lava eruptions about 80 ka old forming Hlíðarfjall SW of the caldera, Jörundur ESE of the caldera and Rani NW of the caldera and a 24 ka subglacial fissure eruption forming Hrafntinnuhryggur, an obsidian ridge south east of mount Mt. Krafla (for locations, see Figures 2 and 3). Since the occurrence of the 80 ka silicic formations outside the caldera, volcanism has mainly been centred in the eastern part of the fissure swarm. A simplified geological map of Krafla is shown in Figure 3.

**Figure 2.** An overview of the Krafla area and the central part of the fissure swarm. The figure shows the visible 110 ka caldera (black), faults and fissures close to Krafla (blue), craters and eruptive fissures (orange) and geothermal manifestations (red). The figure also shows the subsidiary geothermal area in Námafjall, south of Krafla. Coordinates are UTM, WGS84, zone 28 in km.

**Figure 3.** A simplified geological map of Krafla. The map also shows inferred structural features discussed in this paper. Note the abundant explosion craters in the western slopes and south of Mt. Krafla. In the upper right inlet, black dots are volcanos and green strips represent fissure swarms. Coordinates are UTM WGS84 in km. (Modified from [9]).

The Krafla volcanic system has also shown bimodal behaviour in tectonics and crustal spreading. About 8 ka ago, the spreading moved from the eastern part of the fissure swarm to the western part and back again about 3 ka ago [7] (Figure 4). The shifting of the spreading to the west did not result in increased volcanic activity in the western part of the caldera, only one eruption has been identified in Hvannstóð, which is about 5 ka old [7] (Figure 4). Resistivity survey shows that a mature high-temperature geothermal system never developed in the western part of the caldera (see Section 3.3 below) and only minor extinct geothermal manifestations south of Hvannstóð (Figure 4). Extrusive volcanism is almost exclusively found in the eastern part, before and after the western part of the fissure swarm was active [7], and a high-temperature geothermal system with extensive surface manifestations was developed in the eastern part (see Figure 2). The shifting back of the spreading from the western to the eastern part 3 ka ago had a profound influence on the geothermal system, as discussed below.

The fact that volcanic activity takes place dominantly in the eastern part of the 110 ka caldera could be explained by considering some details of the fissure swarm. Figure 5 shows that south of the Krafla caldera, the crust east of the fissure swarm is moving in an approximate direction of 22◦ south-east, while to the north it is moving approximately 4◦ south-east. These different spreading directions have been confirmed by GPS measurements [10]. The difference is about 18◦, leading to a N–S opening component in the eastern part of the volcano of about one-third of the spreading motion. This opening component favours the ascent of mantle-derived magma and volcanism manifested by subglacial extrusives: Mt. Krafla, Dalfjall and Sandabotnafjall south of Mt. Krafla, abundant explosion craters in the western slopes of and south-east of Mt. Krafla (Figure 3) and post-glacial eruptions and dike injections centred at Leirhnjúkur.

**Figure 4.** Different parts of the fissure swarm active at different times. Blue lines are faults and fissures, yellow lines show the eastern- and westernmost faults and fissures activated in the Krafla Fires and NW–SE fault south of Mnt. Krafla that moved in the Krafla Fires. The purple line marks the Hveragil gulley. Orange lines mark eruptive craters and fissures. Coordinates are UTM, WGS84, zone 28 in km.

**Figure 5.** Di fferent crustal spreading directions south and north of the Krafla caldera leading to an opening component in the eastern part. The inlet bottom right shows how spreading by, d, leads to opening component, c, in the east. Blue lines represent faults and fissures and purple lines eruptive fissures and craters. Coordinates are UTM WGS84 zone 28 in km.

During the past 3000 years, eruptions in Krafla have taken place every 300–1000 years [7]. Geothermal drilling in the central eastern and southern parts of the visible caldera has shown a pile of alternating extrusive hyaloclastites and lavas, underlain by intrusive rocks with similar bimodal compositional distribution of the volcanic and plutonic rocks in the substrata [11,12]. The depth to the intrusive rocks varies from about 800–1100 m in the central part of the caldera to about 1500–1600 m in the southern part (see discussion below).

The two latest eruptive phases of the Krafla volcano were the Mývatn Fires in 1724 to 1729 and the Krafla Fires in 1975 to 1984. The Mývatn Fires started with a phreatomagmatic eruption in the Viti crater (Figure 3) emitting glassy rhyolitic bombs and minor basaltic, felsite and gabbroic lithics, representing intrusive rocks at depth [13]. Repeated dike injections into the fissure swarm, centred at Leirhnj úkur about 2 km west of Víti, started after the initial explosion. Two main basaltic fissure eruptions took place in 1727 and 1729, mainly within the caldera but with a small eruption west of Námafjall, about 5 km south of the caldera [7].

The Krafla Fires started by a small basaltic fissure eruption within the caldera. Repeated dike injections, occasionally with small eruptions, took place until 1980. From 1981 to 1984, the four main fissure eruptions took place. All the eruptions and most of the dike injections were into the northern part of the caldera and fissure swarm, but a few were towards south, one all the way to Bjarnarflag, west of Námafjall [7,14]. During the Krafla Fires, the active fissure swarm widened by about 9 m close to the caldera and subsided by up to about 1–2 m, while the rift flanks were uplifted [15–17].

During the Krafla Fires, periodic uplift and subsidence took place in the caldera which were closely monitored by levelling and tilt metres [14,18–21]. In quiet periods, the ground was up-lifted by about 5 mm/day with a centre of up-lift shown approximately by the red star in Figure 6. During dike injections (and eruptions), a rapid subsidence took place and then up-lift again at the end of injection. Modelling of the deformation due to a single Mogi source indicated the centre of inflation/deflation at the depth interval of 3.9–7.5 km. Detailed study of ground deformation during and after a Krafla Fires eruption in September 1984 [18] and multiple magma reservoirs were suggested, where deeper reservoirs feed a shallow reservoir at about 2.6 km depth. This is an interesting idea and will be taken up below.

**Figure 6.** The location of S-wave "shadows" observed during the Krafla Fires (green). The red star shows approximate centre of uplift and subsidence [18]. Blue lines are faults and fissures and purple lines are craters and eruptive fissures. Coordinates are UTM, WGS84, zone 28 in km.

A study of seismic wave propagation from earthquakes during the Krafla Fires [22] revealed volumes within the caldera where seismic S-waves were highly attenuated or could not pass through. The estimated areal extent of these S-wave "shadows" is shown in Figure 6. The upper boundaries of the shadows were estimated to be at about 3 km depth and the lower boundaries, though poorly constrained, at about 7 km depth. These volumes have been interpreted to contain magma. This will be further discussed below.
