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".
Types of lava surface. Fluid forms grade to stiffer forms on the
right
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.
Last modified on 1 Sept 1998
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