Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

the roof of the East Branch tube prior to
the Boulevard's ponding. During ponding,
molten lava rose to the level of this
chamber's present floor, solidification
formed a crust 1-2 ft thick, and then the
molten lava below drained out, a process
which left this balcony remnant 4-5 ft
above the tube's present floor.
The East Branch tube can be traversed
beyond the balcony in its roof for
only another 100ft downstream. At this
point, the cave ends in a floor jam of
broken and tilted blocks that prevent
further access down the tube. A small
well-like hole in the floor may connect to
a lower tube, but it is too small to enter.
Why is the floor of the East Branch so
extremely rough and wavy-a striking
contrast to that of Boulevard Cave? One
must see and study it throughout its entire
length for a valid answer. This surface
formed from the last surge of lava that
entered the cave after the ponded lava of
the Boulevard stage drained out. It left
the 2-ft-high lava mark in the entrance
chamber. In this part of the East Branch
the lava flowed as a narrow lobe containing
a tube-in-tube only about 3 ft
thick, with an arched roof and steep
sides . Narrow troughs between the lobe
and the former walls of the cave reveal a
pahoehoe floor beneath it. The source
that supplied new lava into this thin lobe
slowed and eventually stopped, but in the
final stages of flow the hot interior of this
small arched-roof tube continued to
drain, possibly through the well-like
feature at the end of the tube. This
drainage left a putty-like skin of halfcongealed
lava on the roof and sides of
the lobe that sagged toward the tube's
interior and was dragged downstream.
The dragging of this lava formed the
wave-like folds , basins, and irregularly
tilted blocks that make the cave so
difficult to traverse. Some of the ridges
rose almost to the ceiling and then broke
open; the opening reveals the hollow
tube-in-tube ofthe flow lobe. Other ridge
parts were pushed down and blocked
parts ofthe outflowing lava. In short, this
buckled and twisted final lobe is another
form of tube-in-tube. Had it acquired a
thicker skin before draining began it
would have remained as a typical small
tube-in-tube, but the skin was so soft and
thin that it collapsed irregularly to form
the wavy and broken surface found
throughout the 140-ft length of the East
Branch.
Balcony Cave
Upstream from the entrance chamber
of Boulevard Cave is a collapse trench,
followed by a second natural bridge, and
then another collapse trench (map 13, pl.
4) containing the entrance to Balcony
Cave. The reason Howard gave it this
name is immediately apparent after
walking a few feet into it. As in Boulevard
Cave, lava ponded in this much
larger lava tube almost to the roof.
Enough time elapsed after the lava pool
was emplaced so that solidification from
the walls formed a crust 2 to 5 ft thick
throughout the entire cave. Later the
molten lava beneath this crust resumed
motion. Because the crust over the deeper
central part of the tube was not strong
enough to bridge the entire 25- to 40-ft
width of this larger tube, its axial section
caved into the molten flood to form a
channel 4-9 ft wide while parts of the
cave system here and upstream drained.
This channel is preserved as a narrow
trough with nearly vertical walls that rise
above the spiny pahoehoe floor at the
bottom of the trench. The trail leads up
the bottom of this narrow trench, passing
under a natural bridge 80 ft upstream
from the entrance. Here, for a distance of
10ft, the balcony remained intact and did
not collapse into the narrow channel.
Upstream from the natural bridge the
central channel continues for another 100
ft to where the channel and balcony
above are both buried beneath a large
collapse that nearly prevents further access
upstream.
The balcony (fig. 49), 11-20 ft above
the trail, is fascinating, but many parts
are difficult to reach without a ladder or
innovative climbing maneuvers. Once
on the balcony, travel is slow because the
balcony floor is so close to the cave
ceiling that one must crawl or walk in a
stooped position. At the upstream end of
the trail, there is an easy ascent to the
balcony where fallen roof blocks form a
natural stairway from which one can
crawl out upon the balcony west of the
central channel. The balcony can also be
reached at a place 60 ft downstream,
where a small roof collapse has made a
hole 5 ft in diameter that is open to the
sky. Fallen blocks have been piled high
enough in a mound beneath this hole to
stand on and reach the west wall of the
trench and pull oneself up onto the
surface of the balcony. This part of the
balcony is particularly interesting because
on the west sides of two pillars
connecting the balcony with the roof are
remnants of a former distributary, which
apparently branched from the main tube
in the large area upstream now covered
by collapse debris. Only a small part of
this distributary tube is visible. On the
west side of the larger (northern) pillar
the balcony surface tilts west into this
small tube. Traced to the north, this tube
diverges from the balcony at its northwest
corner and is almost completely
filled with lava to its roof. Upstream the
continuation of this small tube follows
the west edge of the tilted balcony block,
then dives into the walls of a scarp, and
continues beneath the untilted bJlcony
floor upstream. The course of this tube
upstream beneath the balcony can be
followed (crawlspace only) to the place
where it is demolished by the surface
collapse.
The balcony on the east side of the
collapse trench is a narrow shelf 3- 15ft
wide, hanging 12-20 ft above the trench.
Access without a ladder is difficult. It
can be reached from the balcony on the
west side via the natural bridge across the
trench, but this approach requires crawling
for 25 ft along a narrow ledge formed
by the west balcony around a constriction
in its west wall. An easier approach is to
climb onto the east balcony from the pile
of collapse blocks at the entrance to
Balcony Cave. Here, the ceiling is sufficiently
high for walking upstream on
the balcony for a short distance, but it
lowers to a level where stooping or
crawling is necessary a short distance
upstream.
The most interesting feature along
this east balcony is the mouth of a
tributary tube, which enters the main
tube exactly at the balcony level. Only
the top of the tributary tube is visible, a
fact indicating that lava in the main tube
Boulevard, Balcony, and Sharks Mouth Caves 65