The Rauðhólar () are remnants of a cluster of rootless cones in Elliðaárhraun lava fields on the south-eastern outskirts of

Reykjavík Reykjavík ( ; ) is the capital and largest city of Iceland. It is located in southwestern Iceland, on the southern shore of Faxaflói bay. Its latitude is 64°08' N, making it the world's northernmost capital of a sovereign state. With a po ...

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...

next to the South Iceland part of

Hringvegur Route 1 or the Ring Road ( or ) is a national road in Iceland that circles the entire country. As a major trunk route, it is considered to be the most important piece of transport infrastructure in Iceland as it connects the majority of towns ...

, the Suðurlandsvegur.

Name

The name Rauðhólar means “Red Mounds”.Thor Thordarson, Armann Hoskuldsson: Iceland. Classic geology of Europe 3. Harpenden 2002, p. 55 It refers to the reddish color of their rocks due to iron oxidations. Some other volcanic cone groups in Iceland also bear the name of Rauðhólar (see eg. Rauðhólar (Vesturdalur)).

Geography

The cone group is situated not far from Reykjavík’s district of Norðlingaholt and between the Hringvegur and the lake Elliðavatn. Sometimes the rootless cone group is also called Rauðhólar við/near Elliðavatn.Þorleifur Einarsson: Geology of Iceland. Rocks and landscape. Reykjavík 1991, p. 78 The cones are placed directly over the connected lava flow, in this case the Elliðaárhraun, which has a width of about 2 km and a length of about 27 km. Scientists to the beginning of the 20th century counted 150 cones.Thor Thordarson, Armann Hoskuldsson: Iceland. Classic geology of Europe 3. Harpenden 2002, p. 56 “The largest cone of the group had a width of 212 m and rose 22 m above the surrounding lava surface.”.

Geology

These rootless cones, also called ''pseudocraters'', are part of the Leitahraun lava, lava flows from a shield volcano up on Hellisheiði. This shield volcano, Leitin, is part of the Brennisteinsfjöll volcanic system. The resp. lava flow, a branch of the Leitahraun, is called Elliðaárhraun and was emitted by the shield volcano about 5000 years ago. The Rauðhólar built up on a location where lava flows entered and covered a small lake just north of today’s Elliðavatn. The stratigraphy shows a mudstone bed (the lake bottom before-eruption), a 7 m high lava flow and a 5 m high

scoria Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) '' ...

platform.

Formation of the rootless cones

"When lava flows over shallow lakes, marshland or in river channels pseudocraters are formed." The heat of the lava causes expansion and boiling, the water content is changed into steam and this initiates hydromagmatic explosions “This often leads to the formation of very regularly shaped scoria craters“. The cones are fed laterally, from feeders which actually are lava tubes of advancing

pāhoehoe Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

flows. In this way, these craters have no direct connection with a

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

reservoir in the Earth's crust, thence the denomination as rootless cones. The internal stratigraphy of the cones shows the different phases of the eruption: “They are typically stratified (formed by multiple eruptive events), inversely size graded (showing decreasing explosivity with time), and capped with welded spatter (indicating cessation of explosivity due to volatile depletion).” Some cones have more than one crater.

Grouping of the rootless cones

They are arranged in irregular groups. The volcanologist Thor Thordarson and other scientists have researched these arrangements which they do not think to be only random.
B. C. Bruno, S. A. Fagents, T. Thordarson, S. M. Baloga and E. Pilger: Clustering within rootless cone groups on Iceland and Mars: Effect of nonrandom processes. Journal of geophysical research, vol. 109, E07009, doi:10.1029/2004JE002273, 2004 The results show that most of the craters have a tendency to build lines or groups, are to be found at a short distance of the next one, if they not even overlap. This is proposedly due to “the underlying lava pathway geometry and/or by the Stratum, substrate

hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...

”, i.e. the form of the lava pathway resp. the water content of the sediments below the lava.

Other rootless cone groups in Iceland

Rootless cone groups are not seldom to be found in Iceland. Other well known rootless cone groups in Iceland are e.g. the Landbrotshólar in the

Eldgjá Eldgjá (, "fire canyon") is a volcano and a canyon in Iceland. Eldgjá is part of the Katla volcano; it is a segment of a long chain of volcanic craters and fissure vents that extends northeast away from Katla volcano almost to the Vatnajöku ...

hraun lava field near

Kirkjubæjarklaustur Kirkjubæjarklaustur ( Icelandic for "church farm cloister", pronounced ; often referred to locally as just Klaustur) is a village in the south of Iceland on the hringvegur (road no. 1 or Ring Road) between Vík í Mýrdal and Höfn. It is part ...

, the Álftaversgígar on Mýrdalssandur and last but not least the Skútustaðagígar at lake

Mývatn () is a shallow lake situated in an area of active volcanism in the north of Iceland, not far from Krafla volcano. It has a high amount of biological activity. The lake and the surrounding wetlands provides a habitat for a number of waterbirds, e ...

in North Iceland. It seems that the phenomenon is rarely to be found on Earth, esp. existing in Iceland. But since some years, scientists are looking for them at

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

, they could be a sign of the existence of water on the harsh planet.

Quarry

Originally there were over 100 craters, but the gravel from them was taken and used for construction. Most of the material was removed around

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...

and used for projects such as

Reykjavík Airport Reykjavík Airport ( Icelandic: ''Reykjavíkurflugvöllur'') is the main domestic airport serving Reykjavík, the capital of Iceland, located about from the city centre. Having shorter runways than the city's larger international airport Kef ...

and road building.

Nature protection

Today, Rauðhólar are protected as part of Reykjavík's nature reserve of Heiðmörk. The Environment Agency of Iceland (Umhverfisstofnunn)
(in Icelandic)

Sources

* Big Map of Reykjavik. 2007 - 2008 / Iceland Road Atlas.

References

{{DEFAULTSORT:Raudholar Geography of Reykjavík Reykjanes Volcanic Belt Brennisteinsfjöll Volcanic System