Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

parts of the cave using the upstream
opening as an entrance.
General Pattern of Post Office Cave
The Post Office Cave system consists
of the collapse remnants of from four to
seven lava tubes superposed almost directly
above one another (fig. 51). Collapse
through their floors and ceilings
now connect and provide access to these
vertically stacked tubes. Most breakdowns
between levels occurred after
volcanism ceased, but fine examples of
collapses that occurred while molten lava
still occupied one or more of the tubes are
also present. In places spectacular lava
falls and lava cascades, frozen upon the
edges of the breakdowns, are preserved
undamaged. Post-lava collapses, however,
allow examination of the rocks that
enclose a lava tube. The last lava to fill
a tube seals its walls and ceiling with
dripstone and lavacicles and its floor
with pahoehoe; thus, the record of earlier
events is hidden beneath a coating of lava
plaster. The broken walls, floors, and
ceilings of the Post Office Cave conduits
reveal extremely complex stages of recurrent
filling, collapse, and draining of
lava tubes.
The superposition of the Post Office
lava tubes in a nearly vertical stack
appears to have been controlled by preexisting
topography. The flow that contains
the deepest tube evidently followed
a valley or canyon. Flow-unit contacts
and platy jointing in these lower lavas dip
toward the axis of the system. As later
flows were added to the pile, the lava
tubes-which formed in the thickest and
most rapidly flowing part of a lava
flow- were superposed by confinement
of the flows between the valley walls.
Eventually, however, some new surface
flows spilled over the valley walls and
sent distributary lobes away from the
axis of the former valley. One of the
more complex examples is Silver Cave
(map 14, pl. 5), which splits off as a
distributary from one of the highest lava
tubes of the Post Office axis. The Silver
Cave lava tube, however, diverged from
the Post Office axis for only a short
distance before flowing roughly parallel
with the axis again.
Before entering Post Office Cave it is
worthwhile to make a brief study of the
profile and map to acquire a general
impression of the cave's complicated
overlapping passageways. A glance at
the profile shows that the central part of
the cave consists of two lava tubes. An
upper larger one called the Silver Connector
level is superposed above a smaller
tube called the Cocoa Pipeline. Closer
examination of the map and the cross
section (map 15, pl. 5), and especially a
visit to the two cave levels, shows at once
that the Cocoa Pipeline is no smaller in
width than the upper level, however, its
ceiling is lower. The Cocoa Pipeline is a
large, fairly intact lava tube-15-30 ft
wide and 8-12 ft high-with a uniform
oval cross section like that of a subway
tunnel. It displays the typical features
that characterize intact lava tubes. On the
other hand, the Silver Connector level is
more varied in width, although on average
it is no wider than the Cocoa Pipeline.
In striking contrast to the Cocoa
Pipeline its cross section is vertically
elongated instead of horizontally oval.
Its ceiling height varies between 30 and
45 ft, and in parts of its course that height
would be doubled if we could remove the
piles of collapse debris which litter its
floor. The Silver Connector level is the
hole left after piecemeal collapse of at
least five lava tubes originally stacked
one above another. This wholesale collapse
of the separations between the
Figure 51. Multiple stacked tubes characterize Post Office Cave (see fig. 4). The
Silver Connector (see map 15, pl. 5) internal breakdown exposes at least seven
superposed levels. The deepest known point in Post Office Cave is 136ft below
surface. Person in lower tube for scale.
Post Office Cave 71