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Formations found in Volcanic Caves

The lava caves contain a distinctive suite of lava structures or "decorations", some of which are illustrated in the figure.
Diagram, 9kb GIF
Diagram 3kb GIG The level of lava within the tubes tended to fluctuate during the course of the eruption, and so we find thin linings plastered onto the walls and roofs, and 'tide-marks' are indicated by benches or shelves on the sides of the tubes. Some shelves can reach right across a passage to form a false floor. The floor of the tube is often flat or slightly arched - being the surface of the last flow of lava through it. If a lava flow within a tube forms a solid crust, then drains away from beneath it, we get a tube in tube effect with a thin false-floor bridging the tunnel.
Small round-tipped lava stalactites, (lavacicles, lava drips) form where molten lava has dripped from the roof. Lava Ribs form where lava dribbled down the walls of the cave, or where the whole lining has sagged and wrinkled. If the floor was already solid (unusual) drips of lava from the ceiling can build up lava stalagmites. Stalagmites often have a form in which the original drips can still be seen welded together as a lumpy mass. The wall linings can rupture, peel back and curve over to form draperies and scrolls. Sometimes the lining appears to have burst open under pressure of gas built up behind it. Some linings are smooth, but others have a sharp hackly surface which may be due to the bursting of many small gas bubbles. Lava hands of semi-solid lava can be squeezed out through cracks or holes in the lining. Some forms that have been extruded or dribbled through small orifices evoke scatological terms. Analogies to sheep, dog, cow etc can be seen.

The floor can be smooth or have a 'ropy' surface of pahoehoe lava with wrinkles and other patterns that indicate the flow direction. As lava cools and looses gas it becomes stiffer and breaks up into a knobby or hackly rubble (aa lava), which eats both overalls and their contents. Transitional types which are still solid, but have jagged surfaces, are sometimes called "cauliflower aa".

Figure, 9kb GIF
Types of lava surface. Fluid forms grade to stiffer forms on the right

Diagram 3kb GIF Small lava mounds, or tumuli, may be heaved up by pressure from below - these are analogous to the much larger versions we see on the surface (e.g. in the Harman Valley). Lava 'puddings' or 'boils' can form from pasty lava that oozes up through holes and cracks. In some caves the crusted floor has buckled and broken into a jumble of heaved up plates, or cracked into a mosaic of jostling plates with rounded or upturned edges. Splash concentrics are frozen ripples formed where a piece of roof has fallen into the liquid lava. Lava cascades or falls form where lava flows or drops from a higher level. Hornitos are hollow mounds (miniature vents) built up by spatter blown up out of a skylight. Where a lining has pulled away from the wall we may find tacky forms resembling toffee or sharks teeth.

Material falling from the roof may be rafted some distance downstream and may end up welded into the floor, or piled up in 'log jams'. Piles of breakdown may be flooded by lava and then left plastered with a thin layer of lava when the level drops. Rafted slabs on a flow surface may leave scratches and grooves on the semi-solid wall linings.


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Last modified on 1 Sept 1998
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