Middle of July: The eruption progressing into the fifth month

The eruption in Geldingadalir adjacent to Mt Fagradalsfjall develops as time flies. Currently, periods with a lively eruption, with some magma fountain activity, have alternated with periods where no action is visible in the crater.

Low-frequency seismic chart by the MET Office. Graph: Icelandic MET Office

This is evident from low-frequency seismic monitoring by the Met. Office (the blue line in the graph). The lava discharge, however, has lessened somewhat. The average figure is 6-7 cubic metres per second. The lava does flow in rivulets on the surface of the new lava mass but to a large extent also in a plumbing system below the surface. If the eruption continues for months or even years, a lava shield might slowly be built out of countless lava layers. The effusive eruption has entered successive phases as follows:

1. A short lava eruption commences from a new volcano-tectonic fissure with one main crater.

2. Up to eight craters are active on the fissure, following an opening farther to the northeast.

3. One of the new craters develops into the high, main vent – high magma fountains and increased lava discharge.

4. Periodic visible activity follows with lessening fountain activity – somewhat decreased but stable lava discharge.

The 5. phase would include a bubbling, rather quiet lava pond in a top crater of a growing lava shield and lava flowing in

a complex system of runnels and lava tubes.

-Ari Trausti Guðmundsson

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