Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

The edge of the older flow is seen as a
sloping apron that forms the outer wall
of the lava gutters. Patches of this apron
also can be seen on the walls of the
amphitheater at a level slightly higher
than the top of the upwelled mound.
The tube makes a sharp right-angle
bend to the left (northwest) 125ft downstream.
The gutters continue on, but the
one on the northeast side is partly overridden
by young spiny pahoehoe where
this flow rounded the outside of the bend.
At small alcoves along the wall, the more
recent spiny pahoehoe cuts across the
alcove to reveal wide areas of the older
pahoehoe beneath.
Throughout this upstream several
hundred feet of passage are patches of tan
silt. The silt has slowly filtered down
cracks in the cave's roof and formed four
irregular layered deposits. Amorphous
silica has cemented the upper surfaces of
the silt as well as lining the small drip
pits-called conulites-in these silt
patches.
Lavacicles occur on the roof of Fern
Cave except where roof collapse has
removed them. A few faint high-lava
marks are present on the walls; however,
peeling edges of lava plaster are much
more common than high-lava marks as
records of recurrent fluctuation of lava
level within the tube prior to development
of the two final flows . The walls
have a flowing drapery of generally
unbroken dripstone plastered over most
areas. Nearly 175 ft farther down the
tube is an area of small lava stalagmites
built up of lava droplets from the ceiling.
Downstream 140 ft farther is a lone
rafted block nearly buried at the edge of
the down-peeled east balcony.
Approximately 525 ft downstream
from the right-angle bend, a huge pile of
large collapse blocks fell from the roof,
and for 150 ft they restrict access through
the tube. The easiest and safest bypass is
along the east wall.
The upstream edge of the collapse
breccia is a reference point for finding
two interesting minor features. On the
west wall, 20-30 ft upstream from the
edge of the collapse pile, and just where
the wall turns right (north) downstream,
a patch of hollow dripstone tubelets
emerges from a crack in the wall 5 ft
above the floor. Near a point 100 ft
upstream from the big collapse pile in the
middle and eastern section of the tube are
lava stalagmites built up 4-8 in. above
the spiny pahoehoe floor. They are few
in number but increase upstream over a
distance of 100 ft.
Downstream from the big collapse
pile the next feature to note is the mound
of blocks, pumice, and soil beneath the
hole in the roof. The abundant ferns,
lichens, mosses, and other plants release
water vapor and oxygen to the air. The
feel and smell is somewhat like that in a
greenhouse. Within the area where a
circle of light from the entrance illuminates
the cave are the most abundant
records of early human habitation. The
finest display of pictographs within the
monument is on the walls of the cave
(fig. 60) upstream from the entrance.
Some are faint and possibly quite old;
others appear very clear and fresh. Archaeologists
have enhanced large areas
with a white overlay. Grinding holes for
preparing food pit the surface of several
roof blocks strewn around the edge of the
fern-covered mound.
Downstream, the most notable geologic
feature is the absence of the sloping
apron on the outside wall of the gutters
and its replacement by a lava bench.
Some wider parts of this lava bench
record an earlier history. Here peelings
of thin lava plaster contributed more to
the volume of the bench than solidification
at the high-lava line of the nextto-last
flow that occupied the cave. Some
areas of the early lava floor as much as
110 ft long and 20 ft wide were not
covered by the late lobe of spiny pahoehoe.
The ceiling height lowers to 6 ft or
less in the area within 250 ft from the
downstream end of the tube, and it is
much easier to examine the distribution
of lavacicles in detail on this lowered
ceiling. Near the west wall of the tube,
120-200 ft upstream from the tube's end
(where the tube makes a slight bend to
the left-northwest-as you look downstream),
the lavacicles are oriented along
parallel ribs in the ceiling. The lavacicles
on these ribs are wide triangular blades
resembling large shark teeth more than
icicles. Near the center of the tube are
thin, spindly lavacicles; some are curved
at their tips as if buffeted by gusts of hot
gases while they were forming.
The downstream end of the tube
shows very clearly how the last two lava
flows gradually filled in this large tube
until the spiny lobe met the ceiling. Two
small extensions remained open with a
6-in. clearance on either side of the
central area where the lava first touched
the roof, but each of these is closed tight
in another 5-10 ft. It is unusual that lava
did not pile up in a block jam behind this
constriction. The lava did swell up,
raising its central part, but we observed
no marked fracturing of a congealed
surface as would be created by lava
pushing up from beneath.
Crystal Cave
Crystal Cave consists of collapse
remnants of three major levels and of a
few smaller lava tubes superposed upon
one another (map 18, pl. 6). The cave
extends beneath the floor of the large
collapse trench that marks the course
along which molten lava was delivered
from Mammoth Crater through lava
tubes that extend far beyond the Cave
Loop Road area. The upstream termination
of the mapped part of Crystal Cave
is below a point 50 ft upstream from the
lower end of the deep collapse trench that
lies between Natural Bridge and Ovis
Bridge. Ovis Bridge and Ovis Cave are
uncollapsed sections of a large lava tube
whose course lies almost directly above
the lava tubes that compose Crystal
Cave. To prevent a clutter of heavy lines
on the map of Crystal Cave, only a small
part of the overlying collapse trench near
the cave entrance is shown, but details of
the geometry of Crystal Cave in relation
to the collapse trench, Ovis Cave, and
Ovis Bridge can be seen by visually
superimposing the Crystal Cave map
(map 18, pl. 6) upon the Ovis Cave and
Paradise Alleys map (map 4, pl. 2).
The only entrance to Crystal Cave is
covered by a locked grating. Once across
the ice slope at the foot of the entrance
ladder, it is fairly easy to visit three
levels, which have a combined length of
2,890 ft (see map 18, pl. 6). From the
Crystal Cave 85