Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Cave. Collapse rubble half fills Craig
Cave directly beneath the entrance overhang
and slopes steeply downstream to
the cave floor. At the base of this pile the
ceiling height is 20-25 ft. From this
entrance the lava tube extends southeast
on a gentle gradient for 156 ft and then
rounds a broad curve until, 145 ft farther,
the tube turns due south. A blanket of
collapse blocks from the ceiling masks
most of the floor, but three large patches
of pahoehoe are free of rubble. They are
in the central part of the tube, and the
floor rounds upward into the walls on
either side of them.
The left (northeast) wall over this
240-ft stretch of tube has lost most of its
dripstone due to collapse, but on the
opposite (southwest) wall several layers
of dripstone plaster are peeling off near
the base. Some large areas of dripstone
remain attached to the higher parts of the
wall. Very few areas of lavacicles remain
on the high ceiling (from 20 to 25 ft)
because most have peeled loose and
fallen to the floor.
For the next 450ft downstream, the
tube's course is a large smoothly rounded
curve open to the northeast; then it
changes direction to almost due east and
maintains this easterly course for the
remaining 580 ft of the cave.
In the area of the broad curve the
patches of pahoehoe that appear from
beneath the rubble are concentrated
along the south (outside) wall of the
bend, and the tube is deeper here than on
the north wall, as in the deepest part of
a channel (thalweg) of a meandering
river. Moreover, the south wall is relatively
free of collapse rubble, except for
thick slabs of dripstone plaster that have
peeled away from the walls; the ragged
upturned edges of these slabs form thin,
irregular bench-like projections along
the base of the south wall. Patches of
excellent clean pahoehoe in the floor
exhibit ropes that reveal the curving
direction of the last lava flow that swept
around this large meander-like bend.
A similar but relatively straight
stretch continues downstream on an easterly
course. The tube widens to an
average of 60ft over much of this stretch.
Patches of dripstone plaster are peeling
from the base of the walls on either side.
At approximately the middle of this
stretch very large collapse blocks-as
much as 20 ft long-are scattered over a
large collapse pile. These must have
come down in a massive rockfall instead
of by the slow upward unraveling of the
roof when vertical cracks and columnar
joints gradually spread open and divide
the flow units of the roof into small
blocks. Lava stalagmites (small piles of
lava accumulated by drippage from the
ceiling) embellish the pahoehoe floor of
the cave at several places.
At the downstream end of the accessible
part of the tube a huge collapse pile
rises in a steep talus slope 50ft above the
floor. The broad top of this collapse pile
lies beneath a collapse cupola about 20 ft
above the original roof of the lava tube.
The top of this cupola intersects an
interbed between two lava flows that
consists of red tuffaceous material,
chiefly silt and sand, which is 3-4 ft
thick. That abundant tree rootlets extend
downward through cracks into this interbed
shows the surface is not far above.
The broad top of the collapse pile
beneath this cupola reverses slope and
descends to the east on a gentle incline.
Within 20-30 ft, however, the collapse
blocks crowd against the steeper slope of
the roof of the cupola above and seal off
further access. Near the center of the
slope one can work down another 20-30
ft along a small crawl way between fallen
blocks just under the roof of the cupola.
As J.D. Howard wrote after his examination
of the cave 50 years earlier, "the
passageway is caved in at the east, but it
possibly could be opened up."
Entrance Collapse Trench
Figure 57. Entrance to large tube containing Craig Cave (see fig. 4 and map 16, pl.
5) is roofed by numerous thin layers of lava.
In contrast to the huge arched cavern,
which opens Craig Cave to the east, the
entrance into Craig Temple (map 16, pl.
5) at the upstream end of the breakdown
is nearly hidden.
The most interesting features of the
collapse trench between the two entrances
are three small lobes of younger lava
of the basalt of Valentine Cave that
spilled into the trench over its south wall.
Each lobe must have been moving slowly,
almost devoid of energy. A lava
cataract that formed where the eastern
lobe spilled over the near-vertical wall of
the trench is perfectly preserved, still
adhering to the breakdown wall (fig. 56).
Only a small amount of lava made it to
82 Selected Caves and lava-Tube Systems, lava Beds National Monument, California