## 2 Geological Background  
The study area is located along the 100-km long KFS, the second westernmost rift of the Northern Volcanic Zone (NVZ) (Figure 1a), which is composed of an active central volcano-caldera system, several normal faults and extension fractures, and eruptive fissures that extend mainly northward from the caldera, affecting both Pleistocene and post-Late Glacial Maximum (LGM) deposits [35; 36; 37], as shown in Figure 1b. All structures are mostly oriented N to NNE and have been subject to volcano-tectonic deformation for 100 thousand years [34; 38]. More in detail, the study area, characterised by the presence of the Hituholar late Pleistocene higgc (Figure 1b), is located 8 km north of the Krafla caldera border, along the western side of the northern KFS. The hyaloclastite deposits have been dated to the younger Weichselian [39], which in Iceland has been dated, by Radiocarbon method,more recent than 29.1-45.7 ka BP [40]. The Weichselian High Glacial, or LGM, was then reached sometime between 24.4 and 18.6 ka BP [41]. This corresponds also to the latest glacial cover of the nearby area of Melrakkasletta, which became deglaciated at 12.1 ka BP [42]. Thus, the age of the hyalocalstite deposits can be ascribed to the Weichselian High Glacial, at 29.1-12.1 ka BP. The hyalocalstite ridge, elongated N-S, is affected by several N-S to NNE-SSW-striking normal faults and extension fractures, and is surrounded by post-LGM horizontal lavas. These lavas belong to three basaltic, Holocene main sequences: the oldest two sequences are 11-12 ka old, whereas the uppermost lavas belong to the 1975-1984 AD Krafla eruptive period. The KFS has been affected by the most recent rifting event within the NVZ [43; 44], that began in December 1975 [45; 46] and lasted until 1984, accompanied by about 20 rifting episodes and numerous lava flows. During each rifting episode, magma escaped from the chamber below the Krafla volcano (situated between 3 and 7 km depth [47]) and intruded northward along the fissure swarm in the form of kms-long dykes [48]. Such dykes typically propagated up to the surface in the caldera region and to a depth of about 3 km at the northern end of the KFS [49], accompanied by diffuse seismicity [50]. These repeated events caused extension of the fault and fissure swarm above the dykes, resulting in subsidence of the axial part of the swarm and uplift of the flanks, followed by viscoelastic relaxation that further contributed to the total extension [51]. The total widening across the KFS during this rifting phase ranged from 2 to 6 m, with a local maximum of 9 m [43; 44]. In the successive years, GPS geodetic monitoring has shown a large, systematic, rift-normal expansion that was interpreted as post-rifting relaxation of compressional stress accumulated in the near-boundary region of the plates, as a consequence of the 1975-1984 repeated dyke intrusions [48; 51; 52; 53]. Previous authors indicate a higher extension rate than the computed regional rate, which in north Iceland is 1.8 cm/yr [54], with a maximum rift-normal extension rate of 4.5 cm/yr near the rift, decreasing to 3 cm/yr at large distances. A more recent work [55] indicates that the current horizontal velocity field, holding North America Plate fixed, is of 2.34 \(\pm\) 0.38 cm/yr in direction N106.9\({}^{\circ}\) E \(\pm\) 8.9\({}^{\circ}\) in North Iceland. Further rift expansion was measured through satellite synthetic aperture radar data acquired in the years 1993-2008, which was explained by the combination of plate spreading, viscoelastic relaxation following the 1975-1984 intrusions and inflation/deflation of shallow magma chambers beneath the Krafla central volcano [56]. Another major rifting episode at KFS occurred in 1724-1729, known as 'Myvatn fires episode' [45]. Regarding the spreading direction in the NVZ, [PERSON] et al. [57] suggested a spreading direction of N112\({}^{\circ}\) E, based only on GPS data and for a limited time window of 4 years.