Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

ft and passes through several levels. Its
base is in a short tube that slopes north for
an additional25 ft and ends in a rounded
pocket 85 ft below ground, but above the
floor of the Cocoa Pipeline. Details of
some lava tubes that intersect the Silver
Connector have not been worked out
because they are inaccessible, and the
edges of all levels that intersect the
connector are very loose. Moreover, the
top three lava tubes can only be approached
by a difficult crawl through a
constriction in Silver Cave.
The Silver Connector level changes
markedly near the Silver Connector and
upstream from it. It spreads out into at
least seven superposed lava tubes, all of
which were truncated by the collapses
that formed the Silver Connector. The
tube that we call Lower Entrance Tube is
readily accessible from the base of the
Silver Connector.
Lower Entrance Tube
To enter the Lower Entrance Tube
from the base of the Silver Connector,
you must pick your way upstream beneath
a natural bridge and then up a steep
rubble slope into the tube. Farther upstream
50ft another breakdown, only 8
ft deep, interrupts the floor of the tube for
another 45 ft. From the upstream end of
this breakdown it is possible to descend
through an opening and work down into
still lower tubes; we continue, however,
in the main part of the Lower Entrance
Tube.
This tube has ceiling heights of
10-12 ft and is characterized by thin
layers of accretionary lining that are
partly peeled away from its walls and
ceiling. Upstream 240ft from the Silver
Connector, these accretionary linings
arch steeply from the tube's walls and in
places are vertical along the axis of the
roof. The roof has a steep gable instead
of a rounded top. The tube continues to
narrow upstream to the Veritable Venturi
and acquires an A-frame shape (fig. 54)
where it makes a smooth 70° tum to the
right (west) . Just beyond this tum, 290ft
upstream from the Silver Connector, a
breakdown closes the continuation of the
tube to the west but permits one to climb
into the upper entrance level and continue
to the cave exit, a tight crawlhole
leading to the surface.
Cocoa Pipeline
The 1 ,040-ft-long Cocoa Pipeline,
approximately 110 ft below the surface,
is the deepest well-preserved lava tube in
the upstream half of Post Office Cave.
This tube is oval in cross section and,
except at one small cascade near its
downstream end, measures 12-14 ft high
and 18-20 ft wide. The pahoehoe floor
has an overall gradient of less than one
degree. The oval cross-section is close to
the ideal hydraulic form to be expected of
a heavy liquid flowing through a viscous
medium. The thick roof that separates
the tube from the overlying Silver Connector
level also indicates that the Cocoa
Pipeline developed in an unusually thick
lava flow, perhaps near the bottom of the
first major flow to occupy the inferred
canyon or valley that determined the site
of the superposed lava tubes comprising
Post Office Cave.
The Cocoa Pipeline is nearly intact,
with very little collapse except at the two
connectors that join it with the overlying
level. There are four possible entrances
through the roof of the Cocoa Pipeline:
the Central and Cocoa Connectors and
two upstream-end crawlways leading
through collapse blocks within an overlying
maze of partly collapsed small
tubes. The evolution of the collapse-free
Cocoa Pipeline contrasts strikingly with
that of the Silver Connector level. The
Pipeline seems to have been little affected
by the complex series of breakdowns,
drainings, and recurrent lava fillings that
characterize the overlying levels. Its
basal position and its thicker roof may
have allowed it to remain insulated and
full of molten lava during fluctuation of
lava in the shallower levels. Some of the
draining and refilling in upper tubes may
have been caused by surging of lava
upward through connectors as hydraulic
pressure changed within the completely
filled Cocoa Pipeline.
The accessible part of the Cocoa
Pipeline lacks tributaries or distributaries
and is so uniform in shape that routine
description of its upstream course is
unnecessary. Only a few of the more
interesting localities are described.
Figure 54. Small triangular-shaped passage was left in Post Office Cave (see fig. 4
and map 15, pl. 5) when late, stiff lava flow failed to fill tube and formed
pronounced lava curbs. Triangular shape may have been caused by inward
slumping of tube walls.
78 Selected Caves and lava-Tube Systems, lava Beds National Monument, California