Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

One cannot walk beneath it, as under the
overhanging parts to the south and west,
because an overhang has fallen into a
steep talus of rubble beneath the balcony
edge.
The higher balcony, 32ft above the
ice floor at the foot of the stairs, records
events shortly after the collapse of the
upper tube into the lower tube. It is
developed only in the immediate area
where the collapse occurred; its remnants
consist of a fragile natural bridge that
spreads into a balcony remnant forming
the bridge's abutment for 60 ft along the
south wall. The balcony is missing,
except as a thin bench covered over with
dripstone, in the middle of the semicircular
wall at the east end of the upper
tube. Two alcoves, 8-12 ft long, penetrate
the east end of this semicircular wall
at the level of the high-lava mark crossing
the lava falls that terminated the flow
of lava in the upper level. This high-lava
mark becomes a thin bench as it crosses
the south abutment of the upper balcony.
It gradually widens eastward and is 8 ft
wide where it merges with the floor of the
southeastern alcove. On the opposite
(north) wall the same relations are found .
Another remnant of the balcony forms a
shorter abutment, 30 ft long, which
anchors the natural bridge to the north
wall. This abutment is 4-10 ft lower than
the balcony against the south wall because
the balcony is tilted downstream in
the direction of lava flow . By contrast,
the high-lava mark maintains approximately
the same elevation that it had on
the south wall of the tube. As on the
south wall, it widens into a bench downstream
and merges into the second alcove.
These relations on the north wall,
however, are less conspicuous because
of collapse.
The flow, which left the high-lava
mark across the face of the frozen lava
fall, produced dripstone during its withdrawal
that flowed onto the surface of the
upper balcony. Thus lava ponded at this
level is closely related in time to the
formation of the upper balcony. Draining
of the lava to the level of the lower
balcony left the natural bridge and its
abutments, the benches formed at the
high-lava mark, and the two alcoves at
the east end of the upper level-all
hanging far above the chasm below. It
was from the natural bridge on this
balcony that the two humans met disaster
in attempting to lower their log to the ice
pool beneath.
Red Tuff and Volcanic Breccia
An unusual feature of the lower level
of Skull Cave is that the wall rocks which
enclose the lava tube are of pyroclastic
(fragmental) origin-they are volcanic
cinders and breccia, not the massive
basalt present in the interior of lava
flows. This red tuff and breccia is seen
through numerous cracks in the dripstone
plaster on the walls of the lower tube and
also bursts through the walls in huge
steep slides on the south sides of both the
upstream and downstream caverns.
Their red color easily distinguishes the
slides from the surface and underground
roof collapses, as does the small size of
the fragments oflapilli and frothy scoria,
which contrast strikingly with the huge
blocks of massive basalt characteristic of
the collapse breccia.
Lava tubes typically form in the
interior of thick lava flows . How, then,
did the lava tube of Boneless Cavern
form within loose pyroclastic cinders and
ash beneath the lava flow that contained
the upper-level tube? Surely the lava of
the tube did not melt its way down into
the red scoria, for many contacts between
the lava tube lining and red scoria
can be seen on the edges of cracks in the
walls and at the margins of the red slides
that burst through the walls of both
caverns on the lower level. These contacts
show that the basalt lava plastered
onto the fragments of scoria, penetrating
and molding itself upon this rough surface,
yet heating it only enough to make
a firm bond at the points of contact.
Instead of melting the scoria the invading
lava apparently was chilled against it, as
shown by its finer grain and much higher
content of basaltic glass.
If the lava did not melt its way down
into the ash and cinders, then did it
penetrate and form tubes of sufficient
size to transport large amounts of lava to
lower levels? The answer cannot be
found by examining the passages in Skull
Cave alone, but a plausible explanation
may be constructed by combining observations
in Skull with those from other
nearby caves, and from examining the
surface breakdowns along the paths of
the main feeder tubes as they round the
southeast half of Schonchin Butte. A
look at aerial photographs of these collapse
trenches shows that the lava-tube
system did not curve around Schonchin
Butte, but instead made its way downstream
in a series of jagged offsets of
short tube segments separated by lava
cascades or breakdowns. At each offset
the downstream end of a tube (or breakdown)
drops vertically and is offset to the
right. Or, stated another way, at each
break in direction the tube drops to a
lower level and steps abruptly away from
Schonchin Butte. Moreover, in all of the
tube remnants that can be examined
along this part of the line of collapse
trenches, a tube occupying the interior of
a basalt flow, when followed downstream,
ends abruptly by dropping to a
lower level. In every one of the accessible
two-level caves examined (Castle,
Kirk Whites, Beaconlight, Skull, White
Lace, Frozen River-named in downstream
order), the lava entering the lower
level penetrated pyroclastics.
What is the source of these pyroclastics?
They may have been deposited
during eruptions that built the Schonchin
Butte cinder cone. Basaltic lava is capable
of mechanically displacing lightweight
loose cinders and scoria, as exemplified
by the large number of cinder
cones that are breached and partly carried
away by lava. As the flows of Mammoth
Crater lava piled up higher and higher
over the flanks of Schonchin Butte, they
encroached farther to the north, and if
each major flow formed tubes, successive
tubes rounding the butte at higher
levels would be offset closer to the cinder
cone. Lava in an upper tube might break
through its floor and work its way
through the underlying cinders into an
open tube farther downstream and at the
same time be offset to the right (away
from Schonchin Butte).
Certain additional factors suggest
that a tube-fed tongue of lava might have
little difficulty in penetrating the Schonchin
Butte cinders. Skull, White Lace,
and Frozen River are caves with ice;
62 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California