Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Downstream 60 ft from the alcove
the benches on both sides of the tube are
completely inundated by spiny pahoehoe
that welled up and issued from a break in
the surface of the aa lobe of lava between
the benches. Lava beneath the aa surface
apparently ponded and swelled up when
it reached the gentle slope downstream
from the area of steep gradient. The
hydraulic pressure on the molten material
beneath the aa crust became great
enough to rupture the aa surface; this
rupture allowed the molten lava to spill
out and continue downstream as a new
flow unit of spiny pahoehoe that spread
over both the benches and the downstream
surface of the aa. Downstream
another 40ft, this lobe of spiny pahoehoe
in tum changes into a collapsed tubein-tube,
and several short sections of
crude benches built by its collapse are
preserved along the walls.
Collapse blocks litter the floor of the
cave near its downstream end whereas
the walls and ceiling are lined with thin
accretionary layers of lava. The inside of
each successive tube lining has lavacicles
and dripstone on its surface. Some
lavacicles were sheared off and embedded
in the outer surface of the next
successive tube lining that rose and
crowded against them. Forty feet from
the end of the tube, one of these linings
peeled back from the walls while still in
a semiplastic state, but 20ft from the end
this lining is not peeled off; it still sticks
tightly against the walls and ceiling of the
tube. The downstream end of Silver
Cave is closed by lava fill and by many
thin accretionary linings peeled from the
roof and walls.
Features Upstream from Entrance
The floor of Silver Cave is concealed
by collapse debris for 75 ft upstream
from the entrance skylight (map 14, pl.
5). Within this area of collapse blocks,
the tube widens abruptly from 25 to 50 ft.
On the east wall, 80ft upstream from the
entrance, a lava cascade joins the tube.
This cascade apparently was fed into the
main tube from a surface flow that found
access through a breakdown.
A smooth pahoehoe floor emerges
from the collapse rubble just upstream
from this lava cascade. This is the same
pahoehoe floor that forms the smooth
benches downstream from the entrance
to the cave, but here it has no channel.
Two shallow breakdowns (collapse
trenches) in this floor, however-one
120ft and the other 160ft upstream from
the entrance-reveal a shallow tubein-tube
that underlies this floor and extends
far upstream. The channel between
the benches downstream from the entrance
is probably the downstream continuation
of this episode of draining.
At the upstream end of the shallow
breakdowns, a narrow medial crack developed
along the middle of the pahoehoe
floor and continues upstream for at
least another 80ft. During volcanism the
underlying shallow tube-in-tube overfilled,
heaved up, split open the pahoehoe
floor above it, and spilled lava out
through the medial crack. The overflows
formed thin lobes of spiny pahoehoe,
which flowed down the shallow gutters
along the walls of the tube.
Upstream from these overspill lobes
the tube makes a sharp 40° bend to the
southwest. The ceiling and floor almost
close together in the section where the
tube changes direction. This results in a
very tight 12- to 18-in.-high crawlspace
for 15 ft along the tube. This difficult
crawl is enough to discourage all but the
most enthusiastic spelunkers, but it is the
only way to visit the upstream part of
Silver Cave and three short levels of Post
Office Cave.
At the upstream end of this route the
tube widens to 30 ft, and ceiling heights
gradually increase to 3 ft. Lava pooled
here and formed a wide smooth pahoehoe
floor. Upstream 30 ft from the
crawlspace, a breakdown in the smooth
pahoehoe floor opened a 2- to 3-ft-high
tube-in-tube in which one can crawl back
downstream for 125 ft, directly beneath
the upper tube. This tube-in-tube lies
exactly beneath the medial part of the
upper main tube; it is the upstream part
of the tube-in-tube that overflowed
through the medial crack and formed the
spiny pahoehoe lobes that spread into the
gutters along the walls.
Farther upstream from the breakdown
that gave access to this tubein-tube,
a larger shallow breakdown extends
upstream along the middle of the
tube for 80 ft, paralleled by flat benches
that project from the walls on either side.
The breakdown floor defines a channel
between the benches generally 3-4 ft
deep. A small natural bridge (fig. 50)
unites the benches between the two
breakdowns (see map 14, pl. 5). Large
collapse blocks on the pooled surface in
the area around this natural bridge were
later plastered over with lava. Collapse
blocks such as these could easily have
broken through the thin crust in the
middle part of the pooled surface and
probably contributed to the 80-ft-long
breakdown in this area.
The floor of the channel between the
smooth benches on either side of the
80-ft-long breakdown contains a 1- to
2-ft-high tube-in-tube, which extends
upstream far beyond the breakdown.
This small tube-in-tube records a series
of instructive transformations as it is
followed upstream from the natural
bridge for the next 410 ft (see cross
sections on map 14, pl. 5). Its top is
missing 45 ft upstream from the natural
bridge, and the remaining sides of the
tube form narrow levee-like ridges that
contain a shallow channel between them
partly filled with spiny pahoehoe.
Whether the roof of this small tube
collapsed and was carried away by molten
lava, or whether it was never roofed
over by the buildup of the steep levees on
either side is a moot question because
each process probably occurred along
different parts of this 410-ft stretch.
Where steep levees are present, they are
bordered by gutters between them and
the wall of the adjoining lava bench. In
these places the flat-topped benches projecting
from the walls overlook gutters,
levees, and a partly filled central channel
in the breakdown between them.
Upstream from the natural bridges
the pahoehoe benches decrease in width
until they are completely absent 90 ft
above the natural bridge. They are
present 40 ft farther upstream where the
tube turns 40° toward the south. Farther
upstream the top of the small tube between
the benches is open, and the
gutters, steep compound levees, and a
narrow channel half-filled with a small
flow of spiny pahoehoe are all contained
Silver Cave 69