The Rauðhólar () are remnants of a cluster of

rootless cone A rootless cone, also formerly called a pseudocrater, is a volcanic landform which resembles a true volcanic crater, but differs in that it is not an actual vent from which lava has erupted. They are characterised by the absence of any magma condu ...

s in Elliðaárhraun lava fields on the south-eastern outskirts of

Reykjavík Reykjavík is the Capital city, capital and largest city in Iceland. It is located in southwestern Iceland on the southern shore of Faxaflói, the Faxaflói Bay. With a latitude of 64°08′ N, the city is List of northernmost items, the worl ...

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

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”. 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 e.g.

Rauðhólar (Vesturdalur) Rauðhólar (Vesturdalur) is a small chain of volcanoes within the Askja or the Fremrinámur volcanic systems in the north of Iceland. They are located at the lower course of the river Jökulsá á Fjöllum, around 25 km from Öxarfjörð ...

Geography

The cone group is situated not far from Reykjavík's district of Norðlingaholt and between the Hringvegur and the lake

Elliðavatn Elliðavatn (, "Elliði's lake") is a lake in Iceland. It is situated in the area of Reykjavík. Not far from it, there is the well known natural park Heiðmörk with its hiking and biking trails, small forests and lava formations. The eastern ...

. 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 and a length of about . Scientists to the beginning of the 20th century counted 150 cones.“''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 flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s from a

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...

up on Hellisheiði. This shield volcano,

Leitin Leitin () is a small shield volcano in the southwest of Iceland. Name Leitin ("the hills") is sometimes also called Leiti ("hills") or Leitahraun after its eruption products (“the lava of Leiti”). Geography The lava shield is located ...

, is part of the

Brennisteinsfjöll Brennisteinsfjöll (, "Sulfur mountains"
Brennisteinsfjöll. Detailed description. In: Catalogue of Icelandic Volcano ...

volcanic system. A branch of the Leitahraun lava flow, 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 Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

shows a

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

bed (the lake bottom before-eruption), a high lava flow and a high

scoria Scoria or cinder is a pyroclastic, highly vesicular, dark-colored volcanic rock formed by ejection from a volcano as a molten blob and cooled in the air to form discrete grains called clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackso ...

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 tube A lava tube, more rarely called a pyroduct, is a 'roofed conduit through which molten lava travels away from its vent'. If lava in the tube drains out, it will leave an empty cave. Lava tubes are common in low-viscosity volcanic systems. La ...

s 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 unde ...

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 (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

reservoir in the

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

, 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 which are not believed to be random. Most of the craters are in lines or groups, and are found at a short distance of the next crater, and may overlap. This may be due to “the underlying lava pathway geometry and/or by the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

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 drainage basin sustainability. A practitioner of hydrology is called a hydro ...

”, i.e. the form of the lava pathway it the water content of the sediments below the lava when it erupted.

Other rootless cone groups in Iceland

Rootless cone A rootless cone, also formerly called a pseudocrater, is a volcanic landform which resembles a true volcanic crater, but differs in that it is not an actual vent from which lava has erupted. They are characterised by the absence of any magma condu ...

groups are common in Iceland. Other well known rootless cone groups in Iceland are 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 Vatnajök ...

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 pa ...

, the Álftaversgígar on

Mýrdalssandur Mýrdalssandur () is an outwash plain on the south coast of Iceland. Location and description The outwash plain is located between the rivers of Kúðafljót in the east and Múlakvísl in the west. Both rivers carry water from the Mýrdalsjö ...

and the Skútustaðagígar at lake

Mývatn Mývatn () is a shallow lake situated in an area of active volcanism in the north of Iceland, near Krafla volcano. It has a high amount of biological activity. The lake and the surrounding wetlands provides a habitat for a number of waterbirds, ...

in North Iceland. The phenomenon is rarely found elsewhere on Earth, than Iceland. Scientists have identified potentially similar geological structures on

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, and if confirmed they could be a sign of the existence of water at the time of eruptions on that 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 (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

and used for projects such as

Reykjavík Airport Reykjavík Airport ( Icelandic: ''Reykjavíkurflugvöllur'') is the main domestic airport serving Reykjavík, the capital of Iceland. The airport is located about from the city centre. It is the domestic hub of Icelandair flights and has two r ...

and road building.

Nature protection

Today, Rauðhólar are protected as part of Reykjavík's nature reserve of

Heiðmörk Heiðmörk () was proclaimed a municipal conservation area of Reykjavík in 1950. It is located southeast of Elliðavatn, Iceland, and is about from the city of Reykjavík. Its name is derived from its namesake in Norway, Hedmark (; both derive ...

. 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