Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

cessive accretionary linings of lava plaster.
In places these have peeled from the
roof and walls in such quantity that the
floor is covered with rubble.
Much additional evidence of repeated
draining and refilling of this compound
tube can be gained from the study
of four breakdowns in the floor of the
tube and from investigation of the interesting
passages, rooms, and subbalcony
passages into which these breakdowns
lead. These breakdowns are described in
order as one proceeds upstream through
the Upper Cataract Tube.
Red Plaster Room
Upstream 45 ft from the crawlway
entrance, the floor of the Upper Cataract
Tube drops away in a semicircular hole
onto a rubble-covered slope 8 ft below.
This slope tilts downstream (northeast),
and one can follow it by squeezing under
a low-ceilinged arch beneath the lip of
the breakdown scarp. Beyond the arch is
an irregularly shaped room 40ft long, as
much as 18ft wide, and 6-10ft high. We
named it the Red Plaster Room for a thin
coating of reddish lava plaster- one of
many lava coatings-that was deposited
upon the walls and roof of this room.
This extremely complicated room is
evidently a short remnant of a lava tube
that once extended much farther downstream.
Direct evidence of its former
extension as a tube is present in the high
shelf just under the roof at the downstream
end of the room. One can extend
a stadia rod more than 20 ft beyond the
thin crawlspace on the top of this shelf.
On the east wall of the Red Plaster
Room, 4 ft above the floor, are the
remains of a lava bench marking the
highstand of a pool of lava that occupied
this room long enough for a thick lava
crust to form inward from the wall. This
bench juts out from the wall as a shelf
2-4ft wide. Several blocks as much as
2 ft long have tumbled from the roof and
walls and landed on this shelf. Some of
these blocks evidently dropped while the
crust of the pool was hot and plastic, for
parts of the bench sag beneath them. The
continuation of the congealing crust toward
the center of the room was broken
off and carried downstream by a resurgence
of new lava in addition to lava still
draining beneath the crust. Soon after
this episode, molten lava again filled the
Red Plaster Room to well above the level
of the bench. This lava completely plastered
over and welded together the remains
of the bench and the fallen blocks
resting upon it (fig. 52). The molten lava
again withdrew and left a 2- to 6-
in.-thick coating of lava plaster. The
outline of the fallen blocks, the brokenoff
bench, and many other features on
the walls of the room can be identified
despite this coating.
On the west wall in irregular alcoves,
projections, and reentrants (made where
fallen blocks were carried away by the
flowing lava), layer upon layer of lava
plaster was accreted at irregular openings
in the walls and roof. Evidently the
Red Plaster Room was repeatedly filled
by surges of lava that drained out of the
chamber almost as soon as it was filled.
Some clues to the possible cause of
these repeated fillings and drainings can
be gained from the geometry of the Red
Plaster Room in relation to the nearby
passages and openings. As noted, a
uniform slope connects the upstream
floor of the Red Plaster Room with the
floor of the Upper Cataract Tube above.
The incline drops 20 ft over a horizontal
distance of 30-35 ft. The slope is so
thoroughly masked under collapse rubble
that the exact nature of its surface
cannot be determined definitely, but it
seems reasonable to assume that this
slope marks a lava cascade, which
dropped molten lava 20ft into an underlying
tube. The Red Plaster Room, at the
base of this cascade, is a remnant of this
tube. Irregular surges and changes in
level would be expected at the base of a
cascade as lava forced its way downstream
through a tube cluttered and
constricted with roof collapses.
Second Breakdown
Leave the Red Plaster Room, proceed
90 ft up the Upper Cataract Tube,
and note a small oval hole in the floor of
the tube. One can squeeze through this
hole and into a well-preserved broad (8
ft) but flat-topped and 3-ft-high tube that
extends back downstream for 95 ft to
where it is terminated by collapse rubble.
Mapping shows that this shallow tube
lies directly beneath the flat floor of the
Upper Cataract Tube and that the walls of
Figure 52. Lava entirely filled this part (Red Plaster Room) of Post Office Cave (see
fig. 4 and map 15, pl. 5) and then drained away leaving a thin coating of lava. Lumpy
shapes in lower right are fallen blocks coated with a late plaster of lava. Scale bar
is 1 ft long.
74 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California