The long-awaited eruption has started, and has been confirmed by IMO. Just after 9pm the first light became visible on the cameras. At 21:40 official confirmation came. IMO has decided that the eruption began at 20:45. The location is not entirely clear yet, but the most recent earthquakes would indicate it is pretty much where expected, just south of the peak of Fagradalsfjall. There was no seismic signal, but in hindsight the near-surface M3.1 this afternoon was the straw that broke the camel’s back.
This solves the question whether this event was tectonic or magmatic. This is the first eruption in Reykjanes since about 1200 AD, and is likely the start of a longer period (centuries) of new activity on the peninsula. The current eruption may or may not last long. We do not know very much about how volcanics on the peninsula works. But eruptions here are likely to produce of order 0.1 km3 of lava, enough perhaps to reach the ocean and cause inconvenience, but not do major damage.
The fact that there were no earthquakes announcing the eruption suggests that the magma was already very close to the surface. A filled conduit is silent. The earthquakes come from breaking the conduit open, and tremor from filling it up. There was almost no tremor either (a15-min low frequency burst before the eruption was reported, only noted in hindsight), so magma may already have been ready since a few days. It was just waiting for the weekend.
The red glow suggests fountaining but we are waiting for further reports. We will add news here as it appears. We actually rely mostly on you!
And as a final point, when we announced the 1-year postponement of the eruption last year (April 1), we had no idea how close we were to reality.
A direct view of the rift is from the following webcam. It was at first pointing in the wrong direction but not has a great view of the eruption
Another suitable webcam is https://www.ipcamlive.com/60401b8d3413c
Road cameras are at http://www.road.is/travel-info/web-cams/south-west-iceland
The eruption is confirmed to be in the Geldingaladir. A 500-meter fissure is reported with 4-6 small fountains. In this location the lava will be confined and does not appear to endanger infrastructure. It is a perfect small eruption. The valley is enclosed, and if the the eruption lasts long enough to fill up the valley (less likely) it would flow into another valley and go north away from any people. (It seems more likely that the eruption will stop well before but may erupt in a new location.) For a detailed map, we recommend http://kortasja.lmi.is/mapview/?application=kortasja
Keep posted! And keep posting.