Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

collapse (map 9, pl. 3). Two others are
still farther upstream at the head of two
branches in Berthas Cupboard Cave; the
branch on the east forms the now rubblefilled
upper Crawl entrance to Berthas
Cupboard, the one on the west is completely
blocked with collapse debris.
During the waning stages of volcanism,
a surface of lava developed within
Tickner Cave that fluctuated only a foot
or two in height because incoming lava
was essentially in equilibrium with the
amount of lava both detouring around the
paddle-shaped barricade and leaking into
Berthas Cupboard Cave. This equilibrium
allowed a crust of solidified lava to
form on top of the flow, and so a "false
floor" was built completely across the
tube upstream from the area of collapse.
This crust, however, was unstable; slight
fluctuations in the amount of lava heaved
it up or let it sag from lack of support.
Moreover, the crust was thin over the
wider and deeper parts of the lava tube,
where large pools of molten lava retained
elevated temperatures for a longer time.
The crust was stronger and thicker in the
narrow, quicker cooling parts of the
tube. Finally, with cessation of volcanism,
much of the molten lava beneath the
crust drained into Berthas Cupboard
Cave. This drainage resulted in large
sections of the thin crust in the wider
parts of the cave collapsing and being
carried away. However, along the walls,
where the crust was thicker and more
rigid, extensive remnants were left clinging
to the walls as benches (fig. 40). In
narrower parts of the tube the crust
remained essentially intact as an underground
natural bridge, with a tube-intube
forming a culvert beneath it.
The difference in shape of Tickner
Cave compared to most other lava tubes
is immediately apparent upon entering
this cave. The floor, instead of rising
gradually into the dripstone walls, intersects
the walls at an acute angle. At the
junction is a bench with an irregular top
1-3 ft high and seldom more than 3 ft
wide. It is made up of thin slabs of
congealed lava, some of which tilt up
steeply from the junction of floor and
wall and have irregular broken edges.
Other slabs extend out nearly flat and
then sag down from the walls, whereas
still others turn down and curl in toward
the wall. These thin slabs of broken or
curled lava record minor fluctuations in
the height of the molten lava surface
within the tube. Such fluctuations allowed
thin, fragile crusts of rock called
peeled linings to congeal and extend
outward from the walls for a foot or more
before they were either broken and
heaved up by a slight rise in the level, or
were let down and curled under as the
height of the lava flood subsided a few
inches. Low benches of these broken and
Figure 39. Pair of collapse holes along Tickner tube forms this natural bridge near entrance to Tickner Cave (see fig. 4 and
map 9, pl. 3).
48 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California