*3.2. Gravity*

An extensive gravity survey was carried out from 1967 to 1984, covering the Krafla area and its surroundings [25]. Figure 7 shows a de-trended residual Bouguer gravity map based on these data. The map shows a relative gravity high at and within the rims of the visible 110 ka caldera. This gravity high, more or less surrounds a gravity low inside the caldera. Superimposed on this gravity low is a relative gravity high at and east of Leirhnjúkur.

**Figure 7.** A de-trended residual Bouguer gravity map (mGals) of Krafla volcano. The visible 110 ka caldera rim is shown with heavy black hatched lines. An inferred buried inner caldera rim is shown with lighter black hatched lines, an inferred lower density transverse structure is shown with grey lines and a clear density boundary to the north is shown as grey broken line. The location of the lithological section in Figure 8 is shown by straight black line (black stars are wells on the section). Blue and green lines show faults and fissures, and craters and eruptive fissures and explosion craters are shown with yellow lines. Coordinates are UTM, WGS84, zone 28 in km.

**Figure 8.** A lithological cross-section based on drilled wells on a N–S trending profile in Krafla [11]. The location of the profile is shown in Figure 7.

Figure 7 shows other interesting features. The caldera is bisected by two more or less linear gravity lows. One is along the part of the fissure swarm that was active in the Krafla Fires (bounded by green lines in Figure 7). The other is an ESE-WNW trending gravity low from west of Mt. Jörundur in the SE and to the valley Gæsadalur (south of Gæsafjöll) in the NW. Where these anomalies would cut through the caldera rim, the rim is not visible.

If the spreading rate is assumed to be about 1.8–2.0 cm/year [26], the total spreading distance since the formation of the 110 ka caldera is about 2 to 2.2 km. By assuming that 75%–100% of the spreading has taken place in the fissure swarm through the caldera, it should be torn apart by some 1.5–2.2 km. The gaps in the southern and northern caldera rims (as seen on surface) are about 3.5 to 4 km, therefore, parts of the rims are subsided and buried. The same might partly apply to the gaps in the visible eastern and western parts of the caldera rim, but the fact that they are cut through by a low-gravity anomaly suggests that the caldera might be torn apart by an ESE-WNW trending transverse structure with rocks of relatively low density.

The origin of the ESE-WNW gravity low is not clear. It is likely to be due to some transform tectonics where the spreading is gradually migrating westwards, towards the oceanic ridge north of Iceland. Similar structures or trenches are known in transform zones further to the north, where the crustal spreading is migrating westwards. The transverse structure could be of similar origin as Lake Botnsvatn in the Húsavík transform zone, i.e., a pull-apart-basin. The transverse structure in Krafla has almost exactly the same strike as the Husavík–Flatey transform at the southern margin of the TFZ.

There is a clue of the nature of the ESE-WNW low-gravity anomaly from drilling. Figure 8 shows a lithological section from the centre and towards the southern rim of the caldera [11]. The wells on which the section is based, and the location of the section are shown in Figure 7. North of the transverse structure the section shows an about 900–1100 m thick pile of hyaloclastite with interbedded lava flows and dominant intrusions below (wells K-11, K-10 and K-04). In well K-6, within the structure, the intrusions come at about 600–700 m greater depth and with correspondingly thicker extrusive less dense rocks. The higher gravity north of the structure, therefore, reflects intrusions at a shallower depth.

As stated earlier, the gravity is relatively high at and inside the visible caldera rims in the southwest, northwest and east (Figure 7). These high gravity anomalies are bounded by steep gravity gradients towards a gravity low in the centre of the caldera, reflecting less dense rocks. In the eastern part of the caldera, the gradient coincides with arc-shaped eruptive fissures from Hólseldar, about 2 ka old [11] (Figure 7). It might be tempting to argue that the high gravity at and inside the caldera rim is due to dense intrusions, but the steep gradients towards lower gravity show that the density contrasts are at shallow depth. This gravity low can be explained by that there is another caldera with low-density rocks buried inside the visible caldera. The estimated rims of this inferred inner caldera and the bisecting ESE-WNW transverse low-density structure are shown in Figure 7.

Even though the last glacial stage is normally considered to have started at about 110 ka BP, the results of geological studies show that the Krafla area was not glaciated until about 80 ka ago [27]. In the 30 ka between the formation of the outer caldera and until glaciation, it has been mostly filled with subaerial lava flows, up to the lowest parts of its rims in the rift graben. The inner caldera was probably formed shortly after the area was glaciated. It is suggested here that the inner caldera was formed 80 ka ago in sub-glacial eruption(s) forming the rhyolitic mountains Hlíðarfjall, Jörundur and Rani outside the 110 ka caldera (Figures 2 and 7). In [9], several examples of caldera formation are discussed where drained rhyolitic magma is erupted far outside the caldera subsidence. The caldera was later filled with subglacial hyaloclastite of considerably lower density than the subaerial lavas filling the outer caldera, resulting in the gravity low. Any visible signs of the inner caldera are now completely masked by Holocene lavas. The presence of this buried inner caldera and the ESE-WNW transverse structure ge<sup>t</sup> support from a resistivity survey discussed below. Some bounds can be put on the age of the transverse structure. It cuts through the inner caldera, so it is younger than 80 ka. It is, however, not seen cutting through Sandabotnafjall, just south of Mt. Krafla, which is estimated to be 35–40 ka old [7], so the age of the transverse structure is somewhere between 80 and 35–40 ka.

The Bouguer gravity map in Figure 7 shows ye<sup>t</sup> another feature worth mentioning. There is a sharp gravity change at a line in, and parallel to the fissure swarm to the north (grey broken line in Figure 7). The fissure swarm hosts less dense rocks east of this line than to the west. This indicates that after the glaciation, the spreading and subsidence have mainly taken place in the eastern part of the fissure swarm.
