IMO reported that a new eruption on the Reykjanes peninsula began at 2217 on 18 December from fissures that opened in a location close to the older Sundhnúkagígar crater row, about 3 km NE of the town of Grindavík. A magmatic dike began intruding beneath the area in late October based on seismic and deformation data; magma continued to flow into the dike causing ground cracking in areas along its axis. The eruption was immediately preceded by an earthquake swarm that began at 2100 on 18 December and the opening of the fissure was accompanied by significant ground deformation.

Increased seismicity and a burst of incandescence indicating the start of the eruption was seen in webcam images. IMO raised the Aviation Color Code to Red (the highest level on a four-color scale) until the situation was able to be evaluated and in case ash was present in any emissions. After the first fissure opened, it propagated S in a series of four additional segments, slightly offset from each other but aligned with the previous dike intrusion. The total length of the fissure was estimated to be 4 km, with the north end just E of the Stóra-Skógfell cones and the south end E of the Sundhnúk cone. Large lava fountains possibly rose hundreds of meters high along the fissures, feeding lava flows in multiple directions. The rate of lava discharge during the first two hours of the eruption was about 100-200 cubic meters per second. The tallest lava fountains were located on the N end of the fissure. The largest earthquake, a M 4.1, was recorded at 2325. The Aviation Color Code was lowered to Orange. By midnight seismicity had declined and the eruption was less intense.

During a second overflight with the Iceland Coast Guard at around 0400 on 19 December scientists noted that the fissure had stopped extending and that most of the activity was concentrated in the central portion. There was minor activity at the S end, near the Hagafell cone, and most of the lava was advancing E towards Fagradalsfjall. Two branches traveled W, remaining N of Stóra-Skógfell. Gas plumes drifted W and NW. Residents of Grindavík, evacuated in November due to ground cracking and unsafe conditions, had recently been allowed to return and allowed to stay until 2100, but not overnight; they again fully evacuated the town due to the eruption. The recently reopened Blue Lagoon resort closed again. Some area roads were also temporarily closed.

Eruptive activity concentrated at five vents during the early part of 19 December. By 1430 the lava discharge rate was about one-fourth the rate measured at the beginning of the eruption and about one-third of the fissure was active. Lava fountains were lower, rising as high as about 30 m. The eruption intensity continued to decline and by 1830 only three vents were erupting. About 5 cm of deflation was detected in Svartsengi; a total of 35 cm of uplift had been recorded there since the beginning of the dike intrusion in November. The power plant in Svartsengi, W of the fissure, was not threatened by the current lava flows, though construction of an earthen barrier around it and the Blue Lagoon continued.

Geological Summary. The Reykjanes volcanic system at the SW tip of the Reykjanes Peninsula, where the Mid-Atlantic Ridge rises above sea level, comprises a broad area of postglacial basaltic crater rows and small shield volcanoes. The submarine Reykjaneshryggur volcanic system is contiguous with and is considered part of the Reykjanes volcanic system, which is the westernmost of a series of four closely-spaced en-echelon fissure systems that extend diagonally across the Reykjanes Peninsula. Most of the subaerial part of the system (also known as the Reykjanes/Svartsengi volcanic system) is covered by Holocene lavas. Subaerial eruptions have occurred in historical time during the 13th century at several locations on the NE-SW-trending fissure system, and numerous submarine eruptions dating back to the 12th century have been observed during historical time, some of which have formed ephemeral islands. Basaltic rocks of probable Holocene age have been recovered during dredging operations, and tephra deposits from earlier Holocene eruptions are preserved on the nearby Reykjanes Peninsula.

Sources: Icelandic Meteorological Office (IMO), Icelandic National Broadcasting Service (RUV)