Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

perfection and cleanliness of its lavacicle-encrusted
roof and with the untarnished
quality of the intricate patterns of
pahoehoe ropes on its floor. Very likely
the last stream of molten lava filled the
cave to its roof, then slowly lowered 2-4
ft. Lava may have continued to splash the
roof with molten material as long as flow
continued because in places the roof is a
forest of spiny, black lavacicles, some of
which are several inches long. Evidently
some of them continued to elongate as
successive splashes hit the ceiling. The
floor, too, is composed of similarly clean
jet-black glassy lava twisted into pahoehoe
ropes dotted with short lavacicles
that fell from the roof while still plastic.
Scarcely any sediment or dust litters the
floor except near the collapse breccia at
the upper end of the cave.
While crawling and shining a light
ahead to gauge the distance between
lavacicle-studded roof and rough pahoehoe
floor, notice the resemblance of the
cave's cross section to the slightly
opened mouth of a shark. We have
therefore given it the name "Sharks
Mouth Cave."
Other Caves
A maze of dead-end crawlways surround
the deep collapse pit that serves as
the entrance to Balcony Extension, and
some are shown on map 13, plate 4. A
continuous traverse can be made around
the large pillar just west of the entrance
via a group of crawls. There are many
more tight crawls on the south and
southeast side of the breakdown, one
leading to a very small sky light opening
just southwest from the main road. Undoubtedly
the tube from the Balcony
Extension entrance once extended upstream
to the Upper Cavern and it was
connected downstream with Balcony's
South Branch upstream end, but a wide
and extensive area of collapse rubble
now obscures the exact relations.
The numerous passages southward
from Balcony Extension are in an area
where many blocks in the ceiling remain
loosely suspended, and some loose
blocks on the floor shift under one's
weight.
Southwest of the main road, along
what is the upstream continuation of the
Balcony-Boulevard distributary, is the
large Upper Cavern, 90 ft long and as
much as 40 ft wide. It, too, has undergone
extensive collapse, and most of the
original features of its roof and walls
have fallen to the cave's floor. At its
south (upstream) end there is a natural
bridge and a long collapse trench.
Silver Cave
Silver Cave is a near-surface lava
tube, which is relatively undamaged by
collapse. It contains excellently preserved
examples of features common to
lava tubes, such as lavacicles, dripstone
walls, and pahoehoe floors. More complicated
features include several kinds of
tube-in-tubes; levee-like lobes of lava
spilled from cracks in tube-in-tubes;
peeled accretionary walls of lava plaster;
lava cascades, benches, and gutters; and
pillars, alcoves, and skylights.
The area near the head of Silver Cave
(see lower right corner of map 14, pl. 5)
is unusual. It originates in a lava cascade
that blocks further access upstream but
geometrically lies within the Post Office
system of superposed lava tubes. Downstream
100 ft from this cascade, Silver
Cave divides. The north-trending fork
leads to Post Office Cave and furnishes
the only route by which three short levels
within Post Office Cave can be reached.
This overlap between the two cave systems
is shown on both maps 14 and 15,
plate 5.
The core of Silver Cave is 1 ,400 ft
long. More than 850 ft are added if we
include the overlap with Post Office, the
division around one pillar, and the length
of those tube-in-tubes that can be traversed
by crawling. Thus the total accessible
passage is 2,250 ft.
Silver Cave was named by J.D.
Howard, who explored it in January
1918. Near the entrance, percolating
water has deposited white caliche on the
walls and ceiling of the cave. On a winter
day the caliche is frequently covered
with droplets of water, which give off a
brilliant silvery reflection when a light is
played upon it; Howard named the cave
for this effect.
Features at Entrance
The collapse at the entrance formerly
was a skylight in the roof when lava
flowed in the tube. As the tube filled with
lava, the lava must have repeatedly
surged upward through the skylight during
eruptive peaks because the lip surrounding
the skylight is covered with
driblets of lava that were flowing back
into the tube as they solidified. The shell
of lava that partially overhangs the entrance
was caused by gas pressure that
inflated a lava blister of thin semi plastic
crust over the skylight. Subsequent partial
collapse of more than half of this
blister, plus collapse of many similar
blisters and of other debris surging within
the skylight, built a mound on the floor
of the tube beneath the skylight.
Features Downstream from Entrance
Downstream from the entrance the
floor of the tube is hidden by lava driblets
and collapse debris for 30 ft, at which
point a bench along each wall appears
from beneath the rubble. The projection
of these benches 2-5 ft from each wall
left a narrow central channel between the
benches. The floor of this channel is
masked with loose rubble for another 25
ft downstream, where a lava lobe with a
spiny pahoehoe surface is visible beneath
the collapse rubble. The edges of its
rounded upper surface are lower than the
benches on either wall, and so the lobe is
contained in the channel between them.
The gradient of the tube begins to
steepen downstream where the spiny
pahoehoe in the channel cascaded and
broke into an aa surface. In places the aa
in the channel overflowed and covered
the smooth benches. The gradient is still
steeper 150 ft downstream from the
entrance; then the gradient decreases,
and the northwest wall of the tube is
indented by a gently sloping alcove. At
the center of the inner edge of this
alcove, a large block that fell from the
roof stands out conspicuously. Subsequent
to its fall this block was completely
immersed in lava, which then drained
away and left a blanket of lava plaster
and dripstone over the surface of the
collapse block and the adjoining alcove
(see map 14, pl. 5).
68 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California