Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

less than half the size of the one that
originally occupied the upper level. It
ranges from 20 to 35 ft wide where its
thickness can be seen in the broken
sections and from 10 to 45 ft high in the
lower (downstream) part, not counting
the areas where balconies create overhangs.
Despite its smaller size, this tube
was large enough to transport the vast
amounts of lava needed for the downstream
continuation of the Mammoth­
Bearpaw lava-tube drainage system.
The lower tube is accessible for only
400ft along its length. Of this, the 100-ft
section upstream from the ice floor at the
foot of the stairs, known as Boneless
Cavern, is almost blocked off by a
50-ft-long collapse through the roof. To
reach the Boneless Cavern section beyond
this roof collapse requires a crawl
through a narrow passage that begins at
the northeast corner of the ice floor and
rounds the north side of the roof collapse.
One enters through a small hole between
and beneath large collapse blocks on the
south, and alongside a lava wall lined
with dripstone on the north. After crawling
15 ft one emerges in a narrow
dripstone-lined slot nearly 3ft wide. The
crawlspace continues upward and westward
as a wider and higher opening,
which skirts a slide of red breccia. The
western extension of the narrow lava
tubelet was broken into and pushed
southward by this slide, as deduced from
small broken remnants of its walls and
one large remnant of its roof exposed just
south of the highest point on the crawl
across the red slide. Upon descending
into the Boneless Cavern one reaches
still another intact fragment of the narrow
tube, which arches over the trail as
an unbroken roof segment 15 ft above the
floor of the cavern. Its eastward continuation
was demolished by the red slide.
The south wall of the eastern continuation
of the Boneless Cavern is buried
beneath collapse blocks. No lava occupied
either the Boneless Cavern, or the
small tube adjacent to its north wall, after
these collapses occurred.
Where the west foot of the crawlway
emerges in Boneless Cavern, it is immediately
apparent that this cavern is the
upstream continuation of the lower tube.
It is more than 100 ft long, blocked by
roof collapses at either end, and broken
into slides of red scoria. The scoria is
composed of loose bombs, blocks, abundant
lapilli, and cinders. These red slides
burst through the tube walls and roof at
many points. One large slide demolished
over half of the south wall, carrying huge
fragments of the dripstone wall and
fragments of one-third of the south lavacicle-lined
roof northward past the
middle of the tube. A similar but smaller
slide occurred on the north wall, at the
upstream end of the cavern. At numerous
other points the relatively intact parts of
the walls are cracked and broken; these
openings allow the red pyroclastics,
which encircle both walls and roof, to
dribble into the cave. It appears that the
thin dripstone walls are barely preventing
the collapse of the entire cavern.
The intact upstream sections of this
room reveal a history of lava occupation
surprisingly different from the downstream
eastern part of the lower level. No
balconies are present, and there are no
indications of higher lava stands on the
walls except for a narrow (1-3 ft) bench
2-3 ft above a lava pool floor, which is
partly covered with ice. At the time the
bench was formed it must have been just
a plastic scum less than 2 in. thick on the
margin of the lava pool, because as the
lava lowered, the thin plastic layer
drooped down from the walls and in
places broke and curled up like a jelly
roll. Above this bench the walls of the
cavern are plastered with well-formed
dripstone, which rounds upward into a
typical lavacicle roof. This evidence
implies that the cavern was brimful of
lava just before it drained.
One small feature in the southwest
corner of this cavern is worth observing.
Set back into the wall at a height of 8 ft
above the floor is a box-like cupola 18ft
long, 7ft wide, and 10ft high. The top
of the alcove is a few feet above the top
of the tube at this point. The ceiling of the
cupola tapers upward into two pipe-like
extensions, each 2 ft long and 1 ft in
diameter. They project upward less than
a foot and end in the loose red tuff and
scoria breccia, which form the wall rock
outside the tube. The entire interior of
this cupola and its two pipes are lined
with dripstone, which deviates radially
downward from the pipes. The pipes
appear to be gas vents, which released
steam and other gases, as well as some
spatter, from the top of the lava-filled
tube into the porous breccia above.
In contrast to Boneless Cavern, the
part of the lower level reached by the
stairs (map 12, pl. 4) records two periods
of highstands of the lava surface, during
which solidification set in from the walls
and top of the lava long enough to
produce extensive balconies when the
lava lowered. A high-lava mark 4-6 ft
higher than the upper balcony records a
brief but higher stand.
The lower balcony lies 5-12 ft above
the mixed ice and rubble floor of the
eastern cavern. It forms extensive overhanging
flanges along the south wall and
is also developed across the west end of
the cavern and along its north side. On
the south wall it is a sloping shelf 6-25
ft wide, with an overhang of 2-12 ft
beyond the wall beneath it. Just upstream
from the foot of the stairs this shelf
swings across the tube in a wide arc.
Here, it resembles the downstream entrance
to a natural bridge, but the "other
side" of the natural bridge is not present.
Instead, that part of the tube beneath the
overhang extends upstream with a
smooth ice floor and gradually lowering
roof to the point where the trail across the
ice ends at a point 90ft upstream from the
base of the stairs. It is at the north side
of this collapse that the crawlspace into
Boneless Cavern is found.
The upper surface of this balcony
also extends upstream from the stairs,
where it becomes the floor of a separate
small lava tube-a middle level 10 ft
above the ice level of Skull Cave. This
low, flat tube becomes inaccessible farther
upstream; its roof slopes down to the
point where it meets the floor 60 ft
upstream from the stairs. Parts of this
small tube, especially its north wall and
roof, appear to be in collapse breccia,
which was penetrated and smoothed over
by lava.
On the north wall of the lower level,
this lower balcony is represented by a
narrow ledge or bench; in the eastern part
of the cave, opposite the big red slide
through the south wall, the lower balcony
is a shelf 2-12ft wide and 40ft long.
Skull Cave 61