The formation of lava tubes
How do lava tubes come into being?
Iceland´s many lava tubes (of which over 500 are known) have mostly been formed in one of the two ways in which lava tubes typically form.
One is where a river of magma flows on the surface along a pre-existing channel, following the natural contours in the landscape. As the stream gradually cools down from the outside in a crust starts forming on top and at the sides. If the magma is hot enough it may even melt its way down, deepening the channel. If the sub-surface stream of molten lava then continues to flow unobstructed towards an outlet then it will leave behind the overgrown channel as a long, tube like cave.
The other way in which lava tubes typically form is when highly fluid, super-hot magma flows from its source into, or under, a layer of cooling, slowly flowing lava. The hotter lava stream forces its way through the (the then new) ground and bores a tunnel – or a series of tunnels – through it. Similarly to the top-side flowing lava, if the river finds an outlet it will leave in its wake a hollow tunnel shaped tube.
Either way, existing lava tubes can then see multible rivers of magma flow through them, sometimes filling them, at other times remelting them and altering their interiors and pathways. This is why lava tubes can be multi-layered, both vertically and horizontally, and why their interior surface is highly varied in terms of formations and textures, ranging from jagged points and ridges, to silky smooth walls.
The evidence of multiple flows of lava is often evident in the form of layers of flow lines and ledges on the walls formed by successively shallower streams of lava flowing through the tube, sometimes leaving behind hardened lava falls where the magma has flowed down from one layer to the next. At times the river of lava is so hot and fluid that its contents get splashed onto the sides and roof of the tube, creating various types of stalactites, one stranger than the next, as the dripping lava coagulates.
The floors of lava tubes tend to be smooth, as is to be expected given the fact that they are essentially the surface of a river that eventually stopped flowing. However, the floors of lava tubes are often times covered with rocks that have crumbled down from the roof of the cave. This happenes almost exclusively during the cooling phase of the lava that surrounds the tube as it cools from the top down resulting in the still flowing – albeit extremely slowly – lava moving at different speeds. Once the lava has fully hardened and the tube is stable it takes a lot to dislodge any part of the roof. This is not to say that lava tubes are immune to the movements of the crust created by, say, a major earthquake.
However, all existing lava tubes in Iceland that are currently accessible are hundreds, if not thousands, of years old. This means that they have seen their share of massive earthquakes; earthquakes that will, presumably, have already dislodged any precariously placed rock and thereby further contributed to the stabilisation and structural integrity of the tubes.
Nevertheless, nothing is certain when it comes to mother Nature, least of all in Iceland where the forces of nature reign supreme and are continually creating new land and re-shaping (and indeed destroying) older features. That explains the helmets:-)