Keilir (; 378 m asl) is a Pleistocene subglacial mound or conical tuya G.B.M.Pedersen, P. Grosse: Morphometry of subaerial shield volcanoes and glaciovolcanoes from Reykjanes Peninsula, Iceland: Effects of eruption environment. Journal of Volcanology and Geothermal Research 282, (2014), 115-133. See also for maps therein. on Reykjanes Peninsula in Iceland.Íslandshandbókin. Náttúra, saga og sérkenni. Reykjavík 1989, p. 53 Its basal area is 0.773 km2, summit area 0.004 km2, basal width 0.99 km, summit width 0.07 km, and volume 0.0362 km3.
It is located within the Krýsuvík volcanic systemSee eg. maps herein: Ermias Yohannes Berhane: Geochemical interpretation of thermal water and gas samples from Krýsuvík, Iceland and Alid, Eritrea. The United Nations University. Report No. 18, 2004. and Reykjanesfólkvangur. It is about 17 miles southwest of Iceland's capital, Reykjavík.
In March 2021, there were earthquakes reported near the mountain, followed by a fissure lava eruption further southwest at Fagradalsfjall.
Formation Keilir was formed during a subglacial fissure eruption, which concentrated in the end at one vent.Snæbjörn Guðmundsson: Vegavísir um jarðfræði Íslands. Reykjavík 2015, p. 22-23 Additionally, some small, sub-glacially formed hills to the north are results of this eruption.Ari Trausti Guðmundsson, Pétur Þorsteinsson: Íslensk fjöll. Gönguleiðir á 152 tind. Reykjavík 2004, p. 156-157
Eruptions under the Weichselian glaciers on Reykjanes Peninsula Detailed stratigraphy addresses the different parts of the eruption. The eruption thawed the glacier's ice, forming a subglacial lake within which the volcano continued to develop. The lake's water made contact with the magma within the vent, causing explosive activity. Tephra then settled down into the subglacial lake in layers, building up into a hill and small, elongated mountain covering the vent(s). Like other eruptions of this kind, when continued over a longer period of time, the water eventually does not reach the vent(s) any more, at which point lava begins to flow.
In Keilir's top region, a small cap of lava (0.020 km2) could mean that the volcanic mountain is a tuya, if the lava is from subaerial eruptions that occurred at the end of the eruption series. Alternatively, it could be a volcanic plug if the lava cooled and plugged up the vent.
Ice thickness and a more exact time of eruption are not known other than that it place during the Pleistocene (Weichselian).
Comparisons to an Antarctic subglacial tuff cone Scientists discovered a similar monogenetic subglacial tuff cone within Antarctica's ice in Victoria Land and were able to determine the thickness of ice that had covered the vent during that eruption. However, the Antarctic cone lay under a polar ice sheet, and not a temperate glacier as was the case with Keilir, and is much older—around 640,000 years old—whereas Keilir is up to 100,000 years old.J. L. Smellie, etal.: A tuff cone erupted under frozen-bed ice (northern Victoria Land, Antarctica): linking glaciovolcanic and cosmogenic nuclide data for ice sheet reconstructions. (2017). Retrieved 25 August 2020. There are other key differences: the tuff cone from Antarctica is positioned next to known granitic plutonic complexes and seems to be a parasitic cone of a stratovolcano, whereas Keilir is located next to presently active volcanic systems and looks like it is on top of the shield volcano Þráinskjöldur, which is younger than the sub-glacial formation. Both are located on rift zones, with Keilir on the Reykjanes Rift. They are composed of similar rocks, mafic lapilli tuff (called "móberg" in Icelandic). Helgi Páll Jónsson: Eldfjallagarður og jarðminjar á Reykjanesskaga. MS ritgerð. Leiðbeinendur Ólafur Ingólfsson, prófessor. Jarðvísindadeild Verkfræði- og náttúruvísindasvið Háskóli Íslands. (2011) Retrieved 25 August 2020.
Use as landmark The mountain is identifiable from afar, including from Reykjavík and the sea, so it was used as a landmark by fishermen and seamen for many centuries.
Hiking There is a hiking trail leading up on the mountain from ENE. At the top, a guestbook can be found within a box.
In clear weather, much of the of Reykjanes Peninsula and Faxaflói can be seen from the top.
See also Geology of Reykjanes Peninsula Krýsuvík (volcanic system)
References