Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

flow, then the lava may pool behind the
obstruction, flow over the tumbled
blocks, and cascade off the downstream
side. Alternately the molten lava may
penetrate between the fallen blocks and
buoy them up enough that with the
additional hydraulic energy of lava
ponding behind the obstruction, the flow
is able to entrain and bulldoze enough of
the obstruction for the lava river to
restore its former gradient. Much of this
buoyed material is deposited downstream
in alcoves, where the tube widens,
or on the inside of curves, where the
stream velocity slackens . Examples of
these features are well preserved in the
central part of Valentine Cave (map 8,
pl. 3), in parts of Tickner and Berthas
Cupboard Caves (map 9, pl. 3), and in
many other lava-tube caves.
When a roof collapse is so large that
it effectively plugs a tube filled with
flowing lava, the molten lava in the tube
downstream from the obstruction flows
on, leaving an open lava tube; however,
minor leaks through or around the plug
may continue to feed a small flow into
the eviscerated tube below. Upstream
from the plug the molten lava backs up
and fills the tube to its roof. This process
gradually increases the hydraulic pressure
on all parts of the tube until a weak
spot is opened, generally in the cave's
roof. The lava then pours out of this hole
and forms a new surface lava flow,
which spreads downstream from the
point of egress. As this flow advances
downslope one or more lava tubes may
develop within it. With further spreading
and subdividing, one lobe may find a
breakdown leading to an open tube below.
Thus a part or all of the flow may
be diverted, tumbling as a lava fall
through this breakdown-perhaps into
the same tube that was plugged by a
breakdown upstream.
Studies of the many lava-tube caves
in the monument also provide alternate
interpretations of what has happened in
places under essentially these same conditions.
If the obstruction cannot be
bulldozed away by the lava, the pressure
of backed-up lava may also be relieved
by the formation of a bypass around the
obstruction. Such a bypass is very possible
if the flowing lava remained hot
beneath its already firmly congealed
crust. The hotter liquid magma within
the tube simply pushes the cooler, plastic
material aside, and a bypass is formed
around one side of the obstruction. In
some tubes two bypasses may form, one
on each side. Such a double bypass is
present near the downstream end of
Tickner Cave (map 9, pl. 3).
Relief of the pressure in a backed-up
lava tube can also come from collapse of
the floor of the filled lava tube downward
into an underlying lava tube. In each of
the three major lava-tube systems in the
monument there are numerous examples
where this has happened. If the lava in
both tubes then drains out, the connector,
as the underground collapse conduit is
called, remains open and can provide
access to a cave passage that might never
have been discovered otherwise. Connectors,
once formed, tend to persist.
The Silver Connector, shown on map
and section of Post Office Cave (map 15,
pl. 5) , passes through lava tubes at five
different levels, but it is entirely
underground-not a surface collapse.
Flowage within connectors was not always
down. Some of them transmitted
lava from a lower ponded level to an
upper open level, but the evidence for
this is not likely to be discovered unless
the plugged lower level also obtained
release at some lower point to allow both
it and the connector to drain. Otherwise
the connector remains filled with congealed
lava and so would remain unidentified
or possibly be mistaken for the
vent of a new volcano.
Indeed, open lava tubes, and open
connectors of any kind between lava
tubes, are unusual features. From the
very nature ofthe way they develop, lava
tubes cannot remain open unless the lava
field forms over a topographic slope that
affords sufficient gradient for lava to
drain out of the tube after eruption
ceases. Tubes cannot develop within
lava that remains ponded until solidification.
Furthermore, flowing lava, like
water, spills into any opening available.
So it is quite normal that a walk downstream
within a lava-tube system will
reveal that each lava tube and its distributaries
are likely ponded to the roof with
the final flow of lava that entered. You
will first notice that lava on the floor of
the cave begins to rise against the walls
of the cave, and it acquires a smooth
ponded surface with few of the usual
pahoehoe ropes. The surface of the pond
appears to rise downstream until it intersects
the roof of the cave; actually, it
remains level, whereas the ceiling and
floor of the tube slope downstream. The
lava that rose in the tube was pooled by
an obstruction to this level, and congealed
because it was unable to drain out.
From studying partly eroded shield
volcanoes, geologists find that lava tubes
containing a filling of congealed lava are
much more abundant than open lava
tubes. Open lava tubes will be more
common among the youngest lava flows
in a volcanic pile, for most older open
lava tubes may have been filled with the
lava from later eruptions. It has been
estimated that only 10 to 20 percent of
the lava tubes of a flow drain and remain
accessible to an explorer. Nevertheless,
because of the complexities of intermittent
pauses and recurring floods of magma,
combined with the interruption of
flow in tubes by roof collapse, it is likely
that a few lava tubes will remain open on
the steeper flanks of a shield volcano,
even if they are buried under hundreds of
feet of new flows.
Some lava tubes receive fillings of
material other than lava. Sand, gravel, or
volcanic ash washed in by surface water
may fill them. A rise in the water table
after volcanism ceases may drown the
underground passages. In the monument,
large tubes that are 100ft or more
below the ground surface may be filled
completely with ice, or else have their
walls decorated by a frieze of large frost
crystals interspersed with draperies of
long icicles. Crystal Cave (map 18, pl. 6)
is an outstanding example.
Intact parts of the ceiling in most
caves show fine displays of lavacicles.
As the name implies, they are like icicles
but were formed as molten lava dripped
from the roof of the cave. Undamaged
parts of most tube walls show linings of
drips tone (figs. 8 and 9). These capture
the flow forms taken by congealing
liquid lava as it splashed against or
dripped off the walls of a tube when the
lava surface quickly lowered in the tube.
6 Selected Caves and lava-Tube Systems, lava Beds National Monument, California