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21-521: Víðgelmir ( Icelandic pronunciation: [ˈviðˌcɛlmɪr̥] ) is a lava tube situated in the Hallmundarhraun lava field (formed around 900 AD) in west Iceland , around 2 km southeast of Fljótstunga farm in Hvítársíða , Borgarfjörður . The roof of the lava tube has collapsed, creating two large openings near its north end which are the only known entrances. Víðgelmir is 1585 m long and

42-468: A greater fall distance tend to form broader ones. Most cave chemistry involves calcium carbonate (CaCO 3 ) containing rocks such as limestone or dolomite , composed of calcite or aragonite minerals. Carbonate minerals are more soluble in the presence of higher carbon dioxide (CO 2 ) and lower temperatures. Calcareous speleothems form via carbonate dissolution reactions whereby rainwater reacts with soil CO 2 to create weakly acidic water via

63-525: A reddish brown color. The presence of manganese oxide can create darker colors such as black or dark brown. Speleothems can also be brown due to the presence of mud and silt . Many factors impact the shape and color of speleothems, including the chemical composition of the rock and water, water seepage rate, water flow direction, cave temperature, cave humidity, air currents, aboveground climate, and aboveground plant cover. Weaker flows and short travel distances form narrower stalagmites, while heavier flow and

84-404: Is a natural conduit formed by flowing lava from a volcanic vent that moves beneath the hardened surface of a lava flow. If lava in the tube empties, it will leave a cave . A lava tube is a type of lava cave formed when a low- viscosity lava flow develops a continuous and hard crust, which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: either by

105-504: Is over 18 kilometers (11 mi) long, due to extensive braided maze areas at the upper zones of the system. A lava tube system in Kiama , Australia , consists of over 20 tubes, many of which are breakouts of a main lava tube. The largest of these lava tubes is 2 meters (6.6 ft) in diameter and has columnar jointing due to the large cooling surface. Other tubes have concentric and radial jointing features. The tubes are infilled due to

126-528: The Viking Age , has been discovered in the cave and is preserved in the National Museum of Iceland . Long stretches of the cave floor are very rough and shouldn't be navigated without a guide. Access and guided tours are provided at nearby Fljótstunga. Lava tubes are formed when a low-viscosity lava flow develops a continuous and hard crust which then thickens and forms a roof above the molten lava stream. When

147-444: The crusting over of lava channels , or from pāhoehoe flows where the lava is moving under the surface. Lava usually leaves the point of eruption in channels. These channels tend to stay very hot as their surroundings cool. This means they slowly develop walls around them as the surrounding lava cools and/or as the channel melts its way deeper. These channels can get deep enough to crust over, forming an insulating tube that keeps

168-468: The eruption subsides, the still-molten lava moving beneath the crust will continue to drain downhill, leaving an open lava tube. Many other lava tubes have been discovered in Hallmundarhraun, most notably Surtshellir and Stefánshellir . 64°45′01″N 20°48′06″W  /  64.750283°N 20.801700°W  / 64.750283; -20.801700 Lava tube A lava tube , or pyroduct ,

189-919: The flanks of Olympus Mons . Partially collapsed lava tubes are visible as chains of pit craters, and broad lava fans formed by lava emerging from intact, subsurface tubes are also common. Evidence of Martian lava tubes has also been observed on the Southeast Tharsis region and Alba Mons. Caves, including lava tubes, are considered candidate biotopes of interest for extraterrestrial life. Speleothem More than 300 variations of cave mineral deposits have been identified. The vast majority of speleothems are calcareous, composed of calcium carbonate (CaCO 3 ) minerals ( calcite or aragonite ). Less commonly, speleothems are made of calcium sulfate ( gypsum or mirabilite ) or opal . Speleothems of pure calcium carbonate or calcium sulfate are translucent and colorless. The presence of iron oxide or copper provides

210-406: The front of one or more separate flows. When the supply of lava stops at the end of an eruption or lava is diverted elsewhere, lava in the tube system drains downslope and leaves partially empty caves . Such drained tubes commonly exhibit step marks on their walls that mark the various depths at which the lava flowed, known as flow ledges or flow lines depending on how prominently they protrude from

231-457: The largest part of the cave passage is 15.8 m high and 16.5 m wide, making it by far the largest of its kind in Iceland. The cave has a wide entrance but narrows down in some places. An iron gate was installed at the first constriction in 1991 to preserve those of the delicate lava formations or speleothems which hadn't already been destroyed. Evidence of human habitation, probably dating to

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252-418: The latter may grade into a form known as a tubular lava helictite . A runner is a bead of lava that is extruded from a small opening and then runs down a wall. Lava tubes may also contain mineral deposits that most commonly take the form of crusts or small crystals , and less commonly, as stalactites and stalagmites. Some stalagmites may contain a central conduit and are interpreted as hornitos extruded from

273-518: The lava molten and serves as a conduit for the flowing lava. These types of lava tubes tend to be closer to the lava eruption point. Farther away from the eruption point, lava can flow in an unchanneled, fan-like manner as it leaves its source, which is usually another lava tube leading back to the eruption point. Called pāhoehoe flows, these areas of surface-moving lava cool, forming either a smooth or rough, ropy surface. The lava continues to flow this way until it begins to block its source. At this point,

294-565: The low slope angle of emplacement. Lunar lava tubes have been discovered and have been studied as possible human habitats, providing natural shielding from radiation. Several holes on the lunar surface, including one in the Marius Hills region, have been observed with angled satellite imagery to lead into voids wider than the holes themselves. These are considered as possible collapses into lunar lava tubes. Martian lava tubes are associated with innumerable lava flows and lava channels on

315-595: The major types of speleothems. Speleothem transects can provide paleoclimate records similar to those from ice cores or tree rings . Slow geometrical growth and incorporation of radioactive elements enables speleothems to be accurately and precisely dated over much of the late Quaternary by radiocarbon dating and uranium-thorium dating , as long as the cave is a closed system and the speleothem has not undergone recrystallization. Oxygen ( δ O ) and carbon ( δ C ) stable isotopes are used to track variation in rainfall temperature, precipitation, and vegetation changes over

336-433: The past ~500,000 years. The Mg/Ca proxy has likewise been used as a moisture indicator, although its reliability as a palaeohygrometer can be affected by cave ventilation during dry seasons. Variations in precipitation alter the width of speleothem rings: closed rings indicates little rainfall, wider spacing indicates heavier rainfall, and denser rings indicate higher moisture. Drip rate counting and trace element analysis of

357-415: The reaction: As the acidic water travels through the calcium carbonate bedrock from the surface to the cave ceiling, it dissolves the bedrock via the reaction: When the solution reaches a cave, the lower pCO 2 in the cave drives the precipitation of CaCO 3 via the reaction: Over time, the accumulation of these precipitates form dripstones ( stalagmites , stalactites ), and flowstones , two of

378-404: The subsurface lava is still hot enough to break out at a point, and from this point the lava begins as a new "source". Lava flows from the previous source to this breakout point as the surrounding lava of the pāhoehoe flow cools. This forms an underground channel that becomes a lava tube. A broad lava-flow field often consists of a main lava tube and a series of smaller tubes that supply lava to

399-532: The tube floor. Lava tubes can be up to 14–15 metres (46–49 ft) wide, though are often narrower, and run anywhere from 1–15 metres (3 ft 3 in – 49 ft 3 in) below the surface. Lava tubes can also be extremely long; one tube from the Mauna Loa 1859 flow enters the ocean about 50 kilometers (31 mi) from its eruption point, and the Cueva del Viento–Sobrado system on Teide , Tenerife island,

420-438: The walls. Lava tubes generally have pāhoehoe floors, although this may often be covered in breakdown from the ceiling. A variety of speleothems may be found in lava tubes including a variety of stalactite forms generally known as lavacicles , which can be of the splash, "shark tooth", or tubular varieties. Lavacicles are the most common of lava tube speleothems. Drip stalagmites may form under tubular lava stalactites, and

441-506: The water drops record short-term climate variations, such as El Niño–Southern Oscillation (ENSO) climate events. Exceptionally, climate proxy data from the early Permian period have been retrieved from speleothems dated to 289 million years ago sourced from infilled caves exposed by quarrying at the Richards Spur locality in Oklahoma. Speleothems take various forms, depending on whether

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