volcanic tubes

VOLCANIC TUBES.....

 
Lava tubes are a type of lava caves formed when an active 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: 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, fan like 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