Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

can be followed down the floor of Mushpot
tube, as a narrow lobe, to its terminus
about 20 ft downstream from the stairway
(west of the trail).
Crouch beside the Mushpot bubble
so you have a clear view upstream
(south) and you will see the lava jam
blocking the tube (fig. 16). Filling a 3-ft
space between floor and roof of the tube
is a jumbled mass of frothy and distorted
lava blocks-the crusted-over surface of
a moving flow that broke and stuck,
creating a constriction comparable to an
ice jam in an Arctic river after the spring
thaw. Note that the roof of the cave
against which the lava has jammed is
covered with lavacicles, some of which
punctured the rising lava-jam blocks.
Notice that these lavacicles embellish the
roof of the cave not only here, but also
above the Mushpot bubble and down the
course of the Mushpot tube. The lava jam
blocked the tube for only about 30ft; the
upstream area beyond this jam is in Lava
Brook Cave (map 1, pl. 1).
High-Lava Marks
Another important feature well displayed
in the area that contains Mushpot
bubble is a high-lava mark, similar to
that of a flood mark left by a river. One
high-lava mark is present 20 in. above
the floor on the east wall of the tube in
the alcove containing the Mushpot bubble.
It marks the maximum depth of lava
before the jam blocked the tube. Note
that the mark is plastered across the
dripstone wall of this alcove. One small
patch of dripstone at the south end
merges with the high-lava mark, and one
small tongue of drips tone that slid off the
side of the fallen block at the stairs is
younger. These small patches of dripstone
may be from lava that splashed up
onto the wall by violent emission of
gasses ("fountaining") of the flow that
produced the high-lava mark. Follow the
high-lava mark downstream-it is not
continuous because it is covered or removed
in places by collapse or by human
activities connected with trail construction.
It slopes downstream, but at a lower
gradient than the surface of the flow that
now forms the floor of the tube. At the
Mushpot the high-lava mark is 20 in.
above the floor. Where the intersecting
distributary tube takes off into the east
wall 80 ft farther downstream, the highlava
mark is 6 ft above the floor and five
Figure 16. Drawing of Mushpot bubble, which was formed by overflow through
tiny hole in small lava tube. The small lava tube was formed by trickle of lava that
leaked through the floor jam of blocks which forms the upstream termination of
Mushpot Cave (see map 1, pl. 1).
other faint high-lava marks visible below
it mark brief halts in the lowering of the
lava flood. Three of the most conspicuous
ones are present on the peninsulalike
hump that flares out from the wall of
the eastern distributary at the junction of
the two tubes. Trace these high-lava
marks back upstream toward the Mushpot
and note that all converge southward.
To the north, the top one rises to the roof
of the tube; the trace of this high-lava
mark indicates that below this point the
Mushpot tube was completely filled with
lava.
Eastern Distributary
The Mushpot tube is abnormally
wide in the area where it is joined by its
western tributary and its eastern lavafilled
distributary (see map 1, pl. 1).
Note also that the roof is abnormally
high, 9- 13 ft, over the eastern half of the
tube in this area but is abnormally low
(3--4 ft) in the western half of this wide
tube, providing only crawlspace. Three
pillars connect roof and floor in the
western part. Directly across Mushpot
tube from this area of pillars, the large
distributary tube takes off into the eastern
wall. It is larger than Mushpot tube but
is filled almost to its roof with lava. Its
roof is only 3 to 4 ft above its floor at the
junction with Mushpot tube, but downstream
35ft east ofMushpot tube the roof
rises to a maximum height of 6 ft in a
partly collapsed dome. The northern half
of this dome was destroyed by a roof
collapse before the final flow of lava
occupied the tube. The swirling action of
spiny pahoehoe against the collapse
blocks left a high-lava mark 1-3ft above
the present tube floor. Collapse blocks
above this high-lava mark are loose and
were never covered by lava. East of the
collapse pile the floor of congealed lava
intersects the top of the tube in a wide
arc, but at the northeast corner the
pahoehoe surface swirls to the right, the
tube steepens, narrows, and is closed
shut in the top of a cascade of spiny
pahoehoe.
From these relations, it seems apparent
that the eastern distributary is a
somewhat older and deeper tube than the
higher Mushpot tube, which collapsed
Lava-Tube Caves of the Headquarters Area 13