The Reykjanes Peninsula as a geological entity extends eastward beyond the geographical limit to a triple junction south of Mt. Hengill. Thus defined the Reykjanes Peninsula marks a plate boundary of the transtensional type.
The extensional component comprises northeast-southwest trending normal faults and crater rows, which group together to form volcanic systems, tens of kilometres in length. The transform component comprises north-south trending strike slip faults and push-ups, which are a few km long at most. They occur in an east-west trending zone along the peninsula coincident with seismic activity. Periods of rifting of the volcanic systems and periods of seismic activity associated with transform faulting alternate on a time scale of 600-800 years.
An overview of the distribution of volcanic eruptions of individual volcanic systems on the Reykjanes Peninsula with time (Saemundsson and Sigurgeirsson, manuscript accepted for publication). Periods of volcano-tectonic activity of the NE-SW volcanic systems alternate with periods of seismo-tectonic activity along the east-west trending seismic zone. Only the last three volcanic episodes have been sufficiently documented so far to be included. It is important to note that activity in Brennisteinsfjöll volcanic system, which is the most productive system on the peninsula, has preceded activity in the other systems by about 200 years. The numbers refer to the time-interval between eruptive periods or the time elapsed since the latest eruption.
At the present, a seismic period is active. The preceding volcanic period began in the 8th or 9th century, shortly before Iceland was settled, and ended in the mid-13th century. Dating of volcanic episodes is based on ash chronology and C-14 dating but also written records for the last episode. Only the last two are reasonably well known, the third last has been verified as well, but only a few lavas have been properly dated so far. The volcanic systems have not been active simultaneously but rather in a succession separated by long quiet intervals. Seismic events on the peninsula have been reported occasionally since late 18th century. Earthquakes are frequent but generally small and occur in swarms. They rarely exceed magnitude 6. Six volcanic systems occur on the Reykjanes Peninsula. Their centers are defined by a high number of fissure eruption foci. Five of the volcanic systems host geothermal systems in their center areas where the heat source is hot intrusives in their roots. In boreholes, intrusives constitute a large proportion (20-60%) of the rock mass below 1000-1600 m depth. Neither magma chambers nor calderas have been discovered. Only Mt. Hengill has produced silicic rocks (andesite to rhyolite) north of the triple junction. The other volcanic systems have only produced basalt ranging in composition from picrite to quarz tholeiite.
The Brennisteinsfjöll volcanic system has been most active of the volcanic systems and produced the most lava by area and volume during the Holocene. Each of the last three volcanic episodes began at Brennisteinsfjöll a few hundred years before the others became active. If the same pattern continues, Brennisteinsfjöll might be due to erupt in the near future with the other systems following in a succession over a time span of a few hundred years.
Main structures of the Reykjanes Peninsula. NE-SW trending volcanic systems are shown in pink. Geothermal fields are shown in yellow at the centers of volcanic activity. These line up within a seismic zone (red), which marks a trans-tensional plate boundary. The seismic zone is discontinuous in the eastern part of the peninsula (Einarsson, 2008). The inset map shows the main volcano-tectonic zones of Iceland. Arrows show spreading direction (half rate 1 cm/y). Spreading axes are shown black. The two in the south of Iceland complement each other in rate of spreading. This is indicated by showing them pinching out, one towards northeast the other towards southwest. Flank zones where no spreading occurs are shown blue. They host large stratovolcanoes. The presently active Eyjafjallajökull (1666 m) is one of them (arrow). Non-volcanic transform zones are shown red. Earthquakes of M = 6.5-7.1 have occurred there (compiled by Saemundsson).
Kristján Sæmundsson, 2010