Lava fields of Iceland's Reykjanes Peninsula 

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 Reykjanes volcanic system is the westernmost of a series of four closely spaced en echelon fissure systems that extend diagonally across the Reykjanes Peninsula. Most of the volcanic system is covered by Holocene lavas and eruptions have occurred in historical time during the 13th century at several locations on the NE-SW-trending fissure system.

Reykjanes Peninsula is the onshore continuation of the Mid-Atlantic Ridge. The rift valley that lies at the crest of the ridge extends in a northeast direction along the peninsula, from Reykjanes, in the southwest, to Vogar and the Keflavík (Keflavik) airport road, in the northeast. Here the rift ends. Another rift starts about 10 km to the east and is parallel to the first. This one passes by Reykjavik on its eastern edge. This stepping pattern, called en echelon by geologists, repeats twice more. The fourth rift is quite long and eventually passes through Þingvellir (Thingvellir) to the northeast. The en echelon rifts are caused by the plate boundary, which runs along the peninsula, being not quite perpendicular to the spreading direction in this area. In terms of energy, it is easier for the crust to break in tension, rather than shear, and it does so whenever possible. With continued spreading, the offset ends of the rifts will eventually become connected by transform faults. A transform fault may be forming further to the east near Selfoss, where an east-west belt of earthquakes is causing some concern.

Brennisteinsfjöll volcanic eruption

Brennisteinsfjöll (Brennisteinsfjoll - "Brimstone Mountains") is a minor ridge with small shield volcanoes and crater rows in the Reykjanes peninsula. It is located about 20 km south of Iceland’s capital Reykjavik, east of Kleifarvatn Lake. An eruption at Brennisteinsfjoll in 1000 AD was dated due to its occurrence at the time of a meeting of Althingi, Icelandic outdoor parliament, at Thingvellir.  The most recent eruption in Brennisteinsfjoll was in the 14th century.

The rocks seen along the Keflavík - Reykjavík road are basalt lava flows, all less than 700,000 years old. These rocks are somewhat similar to those found in Hawaii. Near the village of Vogar lie some of the youngest rocks in the area. Little or no soil or vegetation cover them. Geothermal wells have been drilled into the underlying hot rock and steam is extracted to heat the Icelanders' homes.

Because Iceland is the sub-aerial extension of the Mid-Atlantic Ridge, it is one of the world's most active sites for basaltic fissure eruptions. For this reason, fissure eruptions are also known as Icelandic eruptions.

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