Lava tube
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Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like channel.
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Formation
Tubes form in one of two ways: by the crusting over of lava channels and from Pahoehoe 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 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.
Further away from the eruption point, lava can flow in an unchanneled, fanlike manner as it leaves its source, which is usually another lava tube leading back to the eruption point. Called Pahoehoe 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, 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 Pahoehoe flow cools. This forms an underground channel that becomes a lava tube.
Characteristics
A broad lava-flow field often consists of a main lava tube and a series of smaller tubes that supply lava to 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 cave-like conduits beneath the ground.
Such drained tubes commonly exhibit step marks on their walls that mark the various depths at which the lava flowed. Also, lava tubes generally have flat floors and roofs. Lava stalactites that hang from the roof are rare in lava tubes.
Lava tubes can be up to 4-5 m wide, though are often narrower, and run anywhere from 1-5 m below the surface. Lava tubes can also be extremely long; one tube from the Mauna Loa (Hawaii, USA) 1859 flow enters the ocean about 50 km (over thirty miles) from its eruption point, and the Cueva del Viento on Teide (Tenerife, Spain) is 15 km long.
See also
External links
- The Virtual Lava Tube (http://www.goodearthgraphics.com/virtual_tube/virtube.html)
- More lava tube images (http://images.google.com/images?q=lava%20tubes)
- USGS Photo Glossary Entry for Lava Tube (http://volcanoes.usgs.gov/Products/Pglossary/LavaTube.html)