Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

too, is filled with lava and fallen blocks
from roof collapse.
Balcony Extension
The more interesting parts of Balcony
Extension are accessible near the
main road. A large collapse pit lies 20 ft
northeast of the road. At the bottom of
the north side is an arched entrance
through which you can walk into Balcony
Extension.
The last flow to enter the cave emerges
15 ft inside this entrance from beneath
the pile of collapse rubble that covers the
floor on the north and west sides of the
entrance. This basalt flow advanced
downstream along the floor of the cave as
a sticky lobe of spiny pahoehoe only 3-6
ft thick. Its surface was pushed in the
direction of flow into complicated arcuate
ridges. The surface crust of this lobe
is also riven by large longitudinal fissures
along its crest and by curving cross
fractures. Some of these breaks opened
far enough so that you can see the hollow
interior of the flow. This flow lobe, in
other words, was a collapsing tubein-tube,
similar to the one in the East
Branch of Boulevard Cave. The main
difference is that the crust of this lobe
was thicker; therefore, brittle cracking
accompanied the bending as support of
the semiplastic crust was withdrawn by
draining of the tube-in-tube.
As in the East Branch, this sticky
lobe did not spread to the walls throughout
the cave. The old smooth pahoehoe
floor that it covered is generally visible
as a narrow strip between the lobe and the
cave's walls. Sixty feet downstream
from the entrance two branches of the
main tube reunite after having flowed
around a large pillar just west of the
entrance. The floor of the tube in this
area is a pool. The lobe of spiny pahoehoe
evidently spread across this pool
when it had a congealed crust but a still
plastic interior. The crust on the pool
bowed down toward the edge of the new
lobe and cracked in places or heaved up
to accommodate the weight of encroaching
spiny pahoehoe.
This partly deflated tube-in-tube of
spiny pahoehoe can be followed downstream
180 ft toward Balcony Cave from
the entrance. It is lost under extensive
collapse debris 80 ft upstream from the
crawlway exit that leads to the surface
collapse basin. Parts of this large pile of
debris consist of broken sheets of lava
plastered onto the roof of the cave during
early ponding. One such sheet 20ft long,
8 ft wide, and 2 ft thick still hangs
attached to the roof but is pulled away
from it as much as 3ft at one end. Along
the east wall a small tributary tube
floored with pahoehoe enters the main
tube but is filled with lava 25ft upstream.
Access through the collapse block to the
exit hole at its end is found adjacent to
either wall of the tube.
It would be interesting to know
whether the collapsing tube in Balcony
Extension is the same flow as the spiny
pahoehoe lobe that occupies the bottom
of the trench in Balcony Cave, but the
evidence is hidden beneath collapse
breccia that denies access for 160 ft
between the two caves.
Another distributary branch leaves
the Balcony Extension tube to the left
(west) 130 ft downstream from the entrance.
It flows northwest for 45 ft and
turns in a broad arc to the north and
northeast for another 100 ft, where it
expands into a large room over 50 ft
across. This room is shaped like an
inverted saucer because it is filled nearly
to its roof with collapse debris. Anyone
wishing to traverse it will find that the
easiest course is along the wall on its
west and northwest side. The north end
of this tube has been demolished by a
roof collapse. Two crawlholes through
blocks at the north end of this room serve
as alternative exits into a surface collapse
pit, as noted earlier. The route along the
west wall leads directly to one of them.
Except for the large debris-choked room,
this distributary is a normal small lava
tube, which drained and left a floor of
ropy pahoehoe. However, this distributary
affords the only entrance into Sharks
Mouth Cave-one of the most fascinating
caves of this group.
Sharks Mouth Cave
In the big room partly filled with
collapse debris described previously, a
small tube extends from the floor into the
east wall. Although 8 ft wide, this tube
is scarcely noticeable because its roof is
so low that one must lie on the floor to
peer into it (map 13, pl. 4). Shine a light
down it and note that as the tube drops
lower its floor appears from the rubble as
a gently sloping lava cascade, and that
roof and floor are close together until
only 2ft or less of space separates them.
Focus light to illuminate the area beyond
this tight opening and see that within
10-15 ft the floor ends against a vertical
wall. The logical conclusion is that this
is a short dead-end cave filled to the roof
with congealed lava. However, if you
crawl down into the tight section, you
find the tube makes an abrupt right-angle
turn to the north. Beyond the area where
it turns right, the tube is wider, its
gradient steeper, and the ceiling height
doubles to 4 ft. Follow the tube downstream
for 70 ft, past a pile of collapse
rubble on the left, and it widens out into
a room over 80ft wide but only 5 ft high.
Evidently this room is the upper part of
a large tube, supported in its middle by
two pillars, which extends downstream
(to the north) for another 275ft to where
access is blocked completely by lava
filling. The cave is 50-80ft wide where
it divides and reunites at the ends of the
two pillars. Downstream it narrows to 30
ft and then 20 ft, yet the ceiling heights
are only 3-4 ft throughout nearly all of
the cave.
How was a cave of such great width
and low ceiling height formed? It seems
that the original lava tube was large, with
ceiling heights of at least 6 to 10 ft. It
occupied a deeper level than the Balcony
Extension distributary upstream. There
was probably another connection between
these two lava tubes, perhaps a
lava fall now buried under the surface
collapse that demolished parts of both
caves. In the last stages of lava occupancy
this lower tube filled almost to its
roof, and then the lava congealed completely
without further draining of the
molten interior. A tight seal of lavacicles
on the cave's roof has slowed or prevented
ingress of pumice, clay, and
caliche from the surface.
Although this cave is difficult and
even painful to traverse, the lava-cave
enthusiast will be delighted with the
Boulevard, Balcony, and Sharks Mouth Caves 67