Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

high, but it immediately widens to 40ft
as it divides around a pillar 60ft long and
20 ft wide. A second pillar of about the
same dimensions is present 120 ft farther
downstream.
The final surge of lava to pass
through the tube was only 2-4 ft deep,
and it left many records of its passage. Its
surface began to crust over and solidify
along the sluggish parts of the lava
stream, such as the upstream and downstream
ends of the pillars and in the
alcoves along the outside walls. On the
sides of the pillars and in other restricted
areas of the tube, however, the sticky
lava was pulled and sheared into ribbonlike
masses, which piled up against one
another in vertical sheets. In some places
these accreted layers were later sheared
off while other layers were added; thus,
complex benches were completely torn
off and carried away by the flood, and
only a high-lava mark was left on the
wall of the tube. In these highly sheared
areas the wall below the high-lava mark
is not covered with normal dripstone;
instead it is smooth and shows only
horizontal striations and grooves, where
the viscous and partly solid lava was
sheared and dragged along the wall.
In alcoves of the wall and on the
upstream and downstream ends of the
pillars these benches were able to grow
out into the moving lava flood. Such
areas are particularly significant because
they record the fluctuations of the lava
surface as well as indicating the velocity
of the surges. No two areas are exactly
alike. Moreover, between each accretionary
layer, the dynamics of viscous
flow are revealed not only by sheared
surfaces but also by smeared-out bubbles,
tensional cracks, and extended drag
folds. A particularly good place to see
some of these features is near the alcove
on the east wall of the tube downstream
from the second pillar (280 ft downstream
from the entrance). Other excellent
places are on the east wall between
the pillars and on their upstream and
downstream ends. These benches are
labeled on map 8, plate 3.
The first 350 ft of Valentine Cave is
a good example of a lava tube that has
almost completely drained. The lava
remaining on the floor is mostly pahoehoe;
in places it is glassy and lustrous
whereas at the cascades it is frothy. Only
a small amount of soil and a few scattered
collapse blocks litter the tube floor.
Rodent pellets are present at several
places and, where rainwater seeping
through the roof has moistened them,
their phosphatic composition causes a
green fluorescent glow when a flashlight
is played on them.
The ceiling of this part of the cave
contains excellent lavacicles, and the
walls above the benches display fine
examples of lava dripstone. Many lavacicles
are short thick blades resembling
shark teeth. The roof of the cave is also
riven with innumerable tight cracks,
mostly tensional, formed as the lava
cooled and solidified. They meet in triple
junctions (three cracks radiating from a
point). In addition there are a few long,
straight cracks parallel to the course of
the tube. Striking examples are present
near the entrance west of the first pillar
extending from it for over 100 ft. Percolating
rainwater seeping through the
roof along the cracks has loosened the
lavacicle plaster from the wall rock
above. A few large blocks of the roof
have tumbled to the floor, and many
others appear ready to fall.
Percolating water also produced another
interesting effect on the roof. Water
wets the surface along both large and
small cracks and drips to the floor from
their edges. Most of the water evaporates
and forms a thin precipitate of white
caliche (calcium carbonate). Because of
the strong color contrast between the
white caliche and the shiny black lavacicles,
the roof of many parts of Valentine
Cave appears to be a mosaic of
irregularly shaped black tiles held together
by a white cement.
Two Central Breakdowns
An abrupt change in the Valentine
Cave occurs 370 ft downstream from its
entrance. The width of the tube, 30-50 ft
in the upper part, closes to 12ft, and the
floor drops over two long cascades and
three small lava falls into a broad compound
lava pool at the bottom of the
slope. The tube is over 75ft wide in this
pooled area.
Just above the head of the cascades
three small tubes split off from the main
tube . One enters the west wall a few feet
upstream from the head of the cascades
and is filled with frothy pahoehoe. The
other two tubes, 4-7 ft wide and 3-5 ft
high, take off from the east wall on a
northeast course but then turn back to the
north. Nearly 90 ft downstream they
merge together around a kidney-shaped
pillar 85 ft long and 20 ft thick. In the
lower 25 ft of their course the lava in
these high-gradient tubes cascaded down
a steep slope, which drained their combined
flow into the southeastern part of
a large compound lava pool. This pool
also received the flow from the main
tube's cascades 40 ft farther west.
The upstream parts of these two tubes
are good examples oflava tubes that have
been completely drained. They have
typically flattened oval cross sections
with a broadly rounded roof and floor
that curves sharply upward at their walls.
These tubes also contain excellent examples
of dripstone walls and lavacicles.
At their lower end, where the steep
cascades begin, their floors drop abruptly
8 ft to a lava pool. The final trickle of
lava into this pool arrived in a narrow
channel cut a foot deep into the floor of
the lava cascades. The roof of the tubes,
however, does not drop with these cascades
but instead arches northward over
them to extend high over the eastern part
of the wide pool area.
On the east side directly downstream
from the easternmost of the two small
tubes is a sloping lava bench located 3-8
ft above pool level. Its upper surface is
covered with pahoehoe whose ropes
clearly show that lava from the eastern
small tube once flowed over this bench.
The latest channel of the eastern tube
turned sharply to the left and cut through
this bench. Its truncated edges are evidence
that the bench originally was a
large pile of tumbled roof blocks, which
were later completely immersed in molten
lava. This lava carried away parts of
the collapse and the bench was left as a
remnant. The channel forms a steep
cascade that joins the cascade from the
other small tube. A similar bench, in
which fallen collapse blocks coated with
lava are clearly visible, is present in the
44 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California