Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

fall or rock avalanche. At the foot of this
cascade the lobe rolled out over the flat
surface of a pahoehoe pool and then
collided with a wall of collapsed blocks
at the narrow upper entrance of a branch
from the tube around another pillar (map
6, pl. 2). These collapsed blocks may
have fallen from the roof and walls
between the two adjacent pillars at the
same time the floor avalanche advanced.
The avalanche caused unusual
changes when it crossed the pahoehoe.
Evidently the lava pool had a solid crust
above a molten interior because its surface
bowed beneath the weight of the
avalanche debris and collapsed roof
blocks. The displaced lava was squeezed
farther downstream, where its crust lifted
into a pressure ridge. Part of this uplift
also swelled into a rounded blister with
radiating cracks in its top.
Whether the rock avalanche and the
roof collapse at its toe were triggered by
an earthquake, which struck the area
after most, but not all, the lava in the tube
had drained, is a theory that might
explain the time relations within this
small area. However, no concrete evidence
substantiates this theory. The
quake theory does support the observation
that although the lava throughout the
Hercules Leg-Juniper Cave system contains
a large number of rafted roof
block~, almost no large areas exist where
collapse breccia was overridden by lava.
Sf:ntinel Cave
Sentinel Cave consists of the ruins of
four large lava tubes superposed, with
the fourth and lowest level slightly offset
from the upper three. They have tumbled
into a chaos of large angular blocks by
partial collapse of their roof and floors
after volcanism ceased. In places segments
of a tube's floor and walls survive
as benches or balconies perched above
piles of rubble. In other places tube
floors have survived the general collapse,
but are seldom seen because they
are deeply covered by rubble from
above. Many floor segments gradually
gave way under this load and broke
through into the underlying tube. Roof
areas covered by lavacicles or other
primary features are uncommon; indeed,
a light played over the ceiling in many
places reveals only spaces where roof
blocks have tumbled out. Many loosened
blocks remain precariously perched, indicating
that collapse of the roof is still
the north wall of the tube. Above this
constriction the floor jam consists of
smooth, integrated blocks of pahoehoe
broken by deep crevasses. A few blocks
of varying size from the roof were also
incorporated in the jam; three of the
largest rafted blocks are shown on the
map (map 6, pl. 2). The floor jam is not
present in the alcove behind the 40-ft
ridge of collapse breccia, where pull
marks reveal only slow drainage from a
higher level. The collapse breccia on this
floor proves that drainage of the alcove
was complete before the collapse. As
noted earlier, however, the collapse
breccia is older than the pahoehoe of the
floor jam, which was funneled south by
this collapse ridge.
Below this constriction the nature of
the lava floor jam changes. The blocks
are much smaller and have jostled together
into a lobe of loose rubble. Many
are frothy, spiny, highly inflated pahoehoe.
Where this lobe advanced against
the pillar on the south and the tube wall
to the north, floor blocks near the walls
are definitely overridden and shoved
aside. This constricted part of the lobe
has a high gradient; it is essentially a
cascade of jumbled blocks, which may
have moved catastrophically like a rockfigure
31. Dripstone wall in Hercules
Leg Cave (see fig. 14 and map 6, pl. 2).
Pocket knife for scale.
Figure 32. Collapse blocks from tube's roof litter floor of Juniper Cave (see fig. 14
and map 6, pl. 2). National Park Service photograph.
Sentinel Cave 39