The Reykjanes Peninsula (Icelandic: Reykjanesskagi [ˈreiːcaˌnɛːsˌskaijɪ]) in southwest Iceland is the continuation of the mostly submarine Reykjanes Ridge, a part of the Mid-Atlantic Ridge, on land and reaching from Esja in the north and Hengill in the east to Reykjanestá in the west. Suðurnes (transl. Southern Peninsula) is an administrative unit covering part of Reykjanes Peninsula. A great deal of volcanic activity was occurring in the Reykjanes Peninsula in 2020 and into 2024, after nearly 800 years of inactivity. After the eruption of the Fagradalsfjall volcano on 19 March 2021, National Geographic's experts predicted that this "may mark the start of decades of volcanic activity". The eruption was small leading to a prediction that this volcano was unlikely to threaten "any population centers".
Origin The two most important factors for the existence of Iceland, rifting in combination with the Iceland hotspot, were responsible for the existence of Reykjanesskagi. Reykjanes Peninsula originated in a so-called rift jump 6-7 million years ago. At this time, the Snæfellsnes-Skagi rift had drifted so far to the west and away from the presumed steady place hot spot, that activity shifted eastwards in direction of this mantle plume which is now thought to be situated under the big glacier shield of Vatnajökull. Outcropping rocks have ages from 3.2 million years to the present.
Volcanism and glaciations The topography of Reykjanes Peninsula was formed by glaciers and volcanism, basaltic lava fields covering a good part of the peninsula, in between volcanoes of subglacial as well as subaerial origin, namely tuyas, hyaloclastic ridges (tindars), shield volcanoes and crater rows. Some volcanic systems are submarine, so that there is a pronounced continuation between the volcanism of Reykjanes Ridge, a part of the Mid-Atlantic Ridge, and the Reykjanes Peninsula.
Reykjavík region Famous in the area of Reykjavík is the Grágrýti ([ˈkrauːˌkriːtɪ], Gray Lava). These tertiary basalt lava layers cover much of the ground around and under Reykjavík, but their origin is unknown. It is thought that the small hills within the city, some of which were islands during periods of glaciation, could be small shield volcanoes from warm spells of the glaciations. But it is known that during the Plio-Pleistocene (from 3.2 – 1.8 million years BP) two central volcanoes were active in the Reykjavík region, Viðey volcano and Stardals volcano. Both volcanoes partially formed Esja and the smaller mountains near Reykjavík and the hills on the islands and small peninsulas like Viðey and Kjarlarnes. Volcanic and glacial sediments outcrop at some places around Reykjavík, especially in Fossvogur.
Subglacial volcanoes Iceland was heavily ice covered during the glaciations and even completely ice covered during parts of them. As a result, there are hundreds of subglacially formed volcanoes on Iceland. On Reykjanes Peninsula, glaciers were present until around 15,000 -12,000 years ago. Most subglacial edifices are thought to be Weichselian, with a few being older. The subglacial volcanoes can be identified according to type as tuyas (bigger edifices whose upper parts are covered by the products of subaerial eruptions), hyaloclastite ridges (also called tindars) which means elongated subglacially formed volcanic edifices of different sizes, and cone-like subglacial mounds (very rare). There are many hyaloclastite ridges, with most consisting of mixtures of pillow lavas, hyaloclastite and lapilli tuff. There are also elongated pillow structures, called pillow tindars. Examples on Reykjanes peninsula are Sveifluháls, Núpshlíðarháls [ˈnupsˌl̥iːðarˌhauls], Undirhlíðar [ˈʏntɪrˌl̥iːðar̥], Helgafell and Vífilsfell. The tuyas are often sorted according to their form (morphology) into flat-topped tuyas, elongated tuyas, conical tuyas and complex tuyas. The prominent igneous rock is basalt, though there are also some basaltic andesite or andesite volcanoes on Reykjanes, like Húsmúli [ˈhuːsˌmuːlɪ] and Stapafell [ˈstaːpaˌfɛtl̥] within the Hengill volcanic system. Examples for tuyas on Reykjanes Peninsula are Keilir (conical tuya), Geitafell [ˈceiːtaˌfɛtl̥] (Brennisteinsfjöll), Geitahlíð and Þorbjörn are flat-topped tuyas whereas Þorðarfell [ˈθɔrðarˌfɛtl̥] and Syllingarfell [ˈsɪtliŋkarˌfɛtl̥] are complex tuyas.
Postglacial shield volcanoes The Holocene shield volcanoes represent the great bulk of magma production in this part of Iceland and form the base of many other volcanic landforms. Olivine tholeiites constitute about 60% by volume of all post-glacial lava products on the Reykjanes Peninsula." The postglacial shield volcanoes are situated at the periphery of fissure systems. They erupted after the Weichselian glaciation. These shields are mostly circular in form, built up from pāhoehoe lavas and composed of a low-sloping lava cone surrounded by a lava apron; the older shields are made from picrite, the younger, bigger ones from olivine-tholeiite. They were probably formed in long-lived eruptions (years to decades). The best known edifices are Selvogsheiði [ˈsɛlˌvɔksˌheiːðɪ] (height 176 m, basal width 4.8 km, summit width 0.7 km, volume 0.64 km3), Þráinskjöldur [ˈθrauːɪnˌscœltʏr̥] (volume 5.2 km3), Heiðin há (volume 6 km3) and Sandfellshæð [ˈsantˌfɛlsˌhaiːθ] (4.5 km3). Sandfellshæð is a very regularly constructed shield volcano and the largest in the southern part of Reykjanes Peninsula. Another important shield volcano on Reykjanes Peninsula is Leitin, formed around 5,000 years ago.
Reykjanes volcanic belt
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