Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

into the distributary in the general area
where the Mushpot tube expands to 40 ft
in width. The low roof over the west side
of the Mushpot tube is, therefore, the
continuation of the eastern distributary
roof, upstream from the point where the
two tubes merge. Lava from Mushpot
tube filled the lower distributary tube to
its roof, except for the low-ceiling area
on the west side of the enlarged Mush pot
tube and the 50 ft that the distributary
tube extends to the east. Detailed evidence
of exactly how the two tubes
joined is obliterated by the deep lava fill
in the lower tube, but enough of the roof
remains to establish the overall relation
of one tube to the other.
Western Tributary
The western tributary is a small tube,
less than 6 ft wide and 3 ft high. It is
floored by a small stream of spiny pahoehoe,
in places only a few inches
thick. In the upper part of the tube the
lava flowed along an incline of 11 o for 54
ft. A traverse up this tube is strictly a
crawl; in only one spot, adjacent to the
small pillar halfway along this passage,
is it possible to stand upright. Access to
Lava Brook Cave is blocked upstream by
a passage less than 6 in. high, but this
same point can be approached in the
tube's continuation for 20ft as a distributary
from Lava Brook Cave.
Balconies and Skylights
Downstream along the Mushpot tube
trail, beyond the complex area of tube
junctions, are three features that interrupt
the smooth contours of the cave's arched
roof and walls.
The largest is a balcony located 130
ft downstream from the cave entrance. It
hangs as a graceful semicircle 9-12 ft
above the floor of the tube. The smooth
upper surface of the balcony is plastered
against the upstream half of a dome in the
roof of the tube. It probably formed when
a lava pool filled most of the tube
completely but rose only part way to the
top of the dome. Cooling lava at the top
of the pool evidently attached to the tube
walls and formed a narrow platform.
Before thickening, the lava beneath the
platform drained out of the tube and left
the platform hanging at its present level.
This crescentic balcony overlooks the
underground lecture hall on the floor of
the cave.
A second feature in the cave's roof is
located another 160 ft farther downstream.
It resembles part of a culvert
from which the bottom half has been
removed. This rounded ceiling channel
can be followed for 35 ft along the top of
the larger tube, until it turns into the
south wall and is lost within a few feet
under a plaster of dripstone. It definitely
records the presence of a small open lava
tube, only about 8 ft in diameter, whose
floor collapsed into the large Mushpot
tube below.
The third and highest feature is a
niche in the south wall 17 ft above the
tube floor. This niche is part of a high and
narrow cupola in the tube's roof. It is
located at the sharp bend 150 ft upstream
from where the main tube is blocked by
ponding of lava. The cupola and niche
probably are a roofed-over skylight in the
tube's ceiling. Its original walls are
obscured by a plaster of dripstone. Many
of the thin plates of lava that formed by
the bridging of congealing lava in
skylights-as observed on active lava
tubes in Hawaii (Greeley, 1971b, 1972;
Peterson and Swanson, 1974)-are
clearly visible as horizontal ridges partly
smoothed over by the dripstone plaster.
Repeated freezing-over of the lava surging
within this narrow skylight built a
cover across a hole, which formerly
opened to the surface. When the lava in
Mushpot Cave withdrew to its present
level, only a cupola and niche in the wall
remained to indicate the position of the
roofed-over skylight. Today an electric
light illuminates the niche. Downstream
from this point the trail ends. Though the
ceiling is low, you can still crawl for
another 100 ft by avoiding two areas of
collapse blocks. The tube is closed by a
lava filling to its roof.
Dripstone and Lavacicles
Both Mushpot and Lava Brook
Caves provide such excellent examples
of dripstone and lavacicles (such as those
shown in figs. 8 and 9) that further
descriptions and locations of these features
are useful. Fine examples of lavacicles
cover the large domes in the area
where Mushpot tube widens at its junction
with the eastern distributary. Excellent
places to study them more closely at
eye level can be found along the main
tube roof throughout its length, except in
areas of roof collapse. In these areas,
many stages and changes in the development
of the lavacicle and dripstone
plaster are recorded. Many interesting
patterns result from the response of
molten lavas of varying viscosities to the
pull of gravity. On steep and smooth
walls, pull-out patches in the dripstone
are common (fig. 11). Fine examples of
such sloughing are present on the southeast
Mushpot wall between the large
balcony and the roofed-over skylight.
Good examples of pull-out patches from
a lavacicle-covered roof can be seen on
the high domes in the tube junction area.
Other interesting forms, better seen in
Valentine and Post Office Caves, are
near those high-lava marks where the
moving lava dragged and distorted hot
and sticky dripstone. In places the dripstone
is sheared into narrow benches and
strips or broken into foliated masses.
Caliche, False Gold, and
Phosphorescent Deposits
Changes after solidification also produce
interesting patterns superimposed
upon the lavacicles and dripstone. Water
from rain and melting snow seeps down
into the caves and then may spread out,
wetting the surface of the lavacicles and
dripstone. Some of this water evaporates
and leaves a thin coat of caliche (calcium
carbonate plus minor amounts of other
soluble salts and clay) as a white filigreelike
tracery upon the dark lava surface
beneath (fig. 17). Excellent examples of
this lace-like natural ornamentation can
be seen on both walls of Mushpot Cave
between the large balcony and the
roofed-over skylight. J.D. Howard gave
the name "White Lace" to a cave with
walls of this kind of caliche. If cave
walls are completely covered with white
14 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California