"Lava halfpipe"

On this picture you can see how usually a lava tube is formed:
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. Tubes form by the crusting over of lava channels. 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 conduits beneath the ground. However in this case the flow was not overcrusted therefore the remaining lava flow showed only a "lava halfpipe". The ceiling of the tube is missing. In the background of the picture you can see the smoking "South East Crater" of Mount Etna.

Canon PowerShot G3
Aufnahmedatum/-zeit: 10.12.2006 14:23
Tv (Verschlusszeit): 1/1000
Av (Blendenzahl): 4.0