Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

collapse of the upper tube. Molten lava
must have filled the tube after collapse
because in places vertical lava plaster
coats the surface of the broken shelves
and remnants of the floor of these upper
tubes. Above a few such floors are
openings 2-3ft high, sufficiently large to
enter and explore, but because of their
low ceilings and inaccessibility, these
small flat openings were not mapped.
This composite tube extends upstream
for 110 ft and then abruptly
doubles in width. Remnants of the upper
tube are still present in this section but
are less abundant than in the narrow part
of the upper level. The wider section is
blocked 90 ft farther upstream by collapse
rubble encased in ice. A fine
display of icicles 5-15 ft long forms a
drapery along the south wall adjacent to
the collapse. A narrow ice cascade decorated
with ice columns occupies a bottomless
roof tube down the center of the
ceiling at a point 80 ft downstream.
Middle and Lower Levels
The middle and lower levels are
described as a composite unit because
one is superposed exactly above the other
and the two have merged completely,
due to collapse, for over 250 ft downstream
from the Red Ice Room. Moreover,
the two levels are connected by five
additional collapse openings farther
downstream. On the composite map
view (map 18, pl. 6), the lower level is
offset to avoid the lines of the other
levels. The middle level is shown with a
dashed line on this composite view.
Match lines (see map views of each level
on map 18, pl. 6) denote the five breakdowns
connecting the two levels. One of
these match lines marks the connection
and provides the only easily traversable
route between the middle and lower
levels. The other four openings can be
negotiated only with ladders or ropes.
It might appear that the middle and
lower levels were originally one lava
tube, which later became segmented into
two tubes by balcony building. This
hypothesis would imply half-filling of
the original tube with molten lava, which
ponded long enough at the height of the
middle level's present floor to allow
solidification of a thick crust over the top
of the lava pool. Then, perhaps by
collapse of an obstruction downstream,
the still molten lava beneath the crust
would have drained away and left the
crust as a septum dividing a former large
tube into upper and lower parts. Where
the crust was thin, parts of it might have
tumbled into and been carried away by
the lava as it withdrew. Only the margins
were left hanging as balconies along the
wall.
This hypothesis does not stand up,
however, even though there is much
evidence of local balcony building at
various times during the formation of the
middle and lower levels. The detailed
history of lava occupancy and other
events, which shaped these two levels, is
far too complex to unravel completely
from the limited exposures provided by
Crystal Cave-only a small sample of a
lava-tube system many miles long. The
collapse trench and other associated features
show that the tube system extended
far upstream and downstream from the
places where its continuations are
blocked by collapse debris or by ice in
Crystal Cave. Nevertheless, even a leisurely
2-hour traverse through the middle
and upper levels of Crystal Cave
discloses a long and complicated history
of recurrent filling and draining of molten
lava, not a single episode of lava
ponding, balcony building, and drainage.
Significant parts of this record are
summarized below.
1. If we could remove the septum between
the middle and lower levelswhich
is exactly what happened in
the upstream 250 ft of the lower
level-we would see that the cave's
cross section does not have the oval
shape indicative of hydraulic equilibrium
from a single lava tube encased
in a basalt flow. Instead, this
cave is elongated vertically; ceiling
heights are 35-45 ft and widths are
less than 20 ft except where enlarged
by collapse of the walls. This shape
implies the merging by collapse of at
least two superposed tubes.
2. In the upstream 250 ft of the lower
level, more than one basalt flow can
be seen in collapsed parts of the
walls. The downstream half of the
lower and middle levels cuts into red
tuffs and breccias, which underlie the
basalt flows constituting the wall
rock of the upstream part of the cave.
Good contact relations between a
basalt flow and the red breccia immediately
beneath it can be seen in
the middle level just below the wall
of the access breakdown between the
upper and middle levels, and on the
north wall of the middle level 120 ft
farther upstream. In the downstream
part of the lower level, red breccia
and tuff is present nearly everywhere
slides or cracks in the wall lining of
the tube expose the wall rock. Continuation
of the lower level downstream
is blocked at the Red Ice
Room by red breccia rock slides
cemented with ice. Other cracks and
slides in the coatings of lava plaster
and drips tone that line the lower level
show that this part of the lava tube is
completely surrounded by red breccia.
For 120ft upstream, most slides
and cracks in the walls also reveal red
pyroclastic material as the dominant
type of wall rock. (For hypothesis on
the origin of the red color in these
breccias and the probable mechanics
of emplacement of lava tubes within
red pyroclastics, see "Red Tuff and
Volcanic Breccia" section and map
12, pl. 4.)
3. The septum between the middle and
the lower levels is more variable in
thickness than would be expected
from simple balcony building. This
is best seen by comparing the thickness
in the walls of the six breakdown
holes through this septum. Also, note
the measured thicknesses shown in
the longitudinal section and compare
with ceiling heights shown on map
18 (pl. 6); as shown on the longitudinal
section, remnants of a shallowceilinged
tuhe are exposed within
those breakdowns where the septum
is thickest.
4. The walls of the breakdowns also
record episodes of lava refilling after
the collapse occurred. In several
places remnants of lava plaster or
dripstone coat the walls of a breakdown.
The first three breakdowns
Crystal Cave 87