Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Waters agreed to do the cave mapping
and to prepare a report on each major
cave. In 1974 Eric Pittenger and William
Ruddiman III from Oregon State University
assisted in the fieldwork. Jamie
Gardner and David Kimbrough from the
University of California, Santa Cruz,
joined William Ruddiman III and Waters
during the 1975 field season. In 1976
Alison Till, also ofU.C. Santa Cruz, and
Waters returned for one month to complete
some details of the underground
mapping, and to map some of the important
lines of surface breakdowns. The
cave mapping was completed and mylar
copies of all cave maps and descriptive
reports were delivered to the National
Park Service in 1977. Subsequent to this
work, Julie Donnelly-Nolan began work
in the monument in 1979, and that work
resulted in a surface geologic map (Donnelly-Nolan
and Champion, 1987). She
updated the geologic interpretations in
this book and added some figures and
references. Where interpretations in this
bulletin differ from those on the surface
map, the interpretation is that of the first
author in each case. Bruce Rogers redrafted
and added to many of the maps
and their descriptions and contributed
new figures including many new photographs.
Keith Howard reviewed the
manuscript. James 0. Sleznick, Superintendent
of the monument from 1978 to
1986, encouraged publication of the cave
maps and descriptions. Janet Sowers and
other National Park Service personnel
provided useful information.
CAVES EASILY ACCESSIBLE
FROM CAVE LOOP ROAD
lava-Tube Caves of the
Headquarters Area
The Cave Loop Road area, south of
the Headquarters building, is composed
of many small and large lobes of lava,
most of which subdivide downstream
into new lobes. Many of these lobes
contain from one to several small lava
tubes, which also may bifurcate and
spread out like the distributaries on a
delta. Beneath this complex pile of overlapping
lava lobes and lava tubes is a
chain of large feeder tubes, which delivered
molten lava from Mammoth Crater
to this area and also far to the
northeast. Figure 14 shows some of the
complex relations in this set of tubes that
originated at Mammoth Crater. Figure 4
shows that this is only part of a much
larger array of lava tubes in the basalt of
Mammoth Crater (Donnelly-Nolan and
Champion, 1987).
Mushpot Cave
The entrance to Mushpot Cave is a
hole in its roof, located in the parking lot
that serves the Visitor Center and Headquarters
building (map 1, pl. 1). This is
the only cave in the monument that
contains lights and interpretive signs,
which point out geologic features. It is an
excellent place to get acquainted with
lava-tube caves. A staircase leads to the
floor of the cave, and from this point one
can traverse the main branch of Mushpot
lava tube downstream (northeast) for 520
ft. Upstream the tube is blocked 25 ft
south of the foot of the stair by a floor jam
of broken and deformed lava blocks.
Downstream 65 ft from the entrance
the main tube widens into a broad domelike
area and is intersected on its southeast
wall by another wide tube, which
diverted part of the flow in Mush pot tube
to the east. Only about 50 ft of the length
of this east flowing tube is visible because
both upstream and downstream it
is filled to its roof with congealed lava.
Yet another tube, a small tributary,
spilled a thin flow of rough-surface
pahoehoe into the main tube at a point
low on the west wall 25 ft downstream
from the foot of the stairway. This small
tributary is accessible only by crawling
for 180ft, where further access upstream
is blocked by a lava lobe that leaves just
a 6-in. space between floor and roof.
Features Near Entrance
Upon entering Mushpot Cave pause
about halfway down the staircase and
look around. An open lava-tube cave
extends downstream on the right and an
alcove extends upstream on the left.
Notice that the walls of this alcove are
covered with lava dripstone-a thick
plaster of sticky lava that oozed and
dripped down the wall in thin lobes as the
surface of molten lava lowered within the
tube. Similar dripstone, somewhat obscured
by lichens and dirt, extends to the
top of the entrance pit on the east and
south; the presence of this dripstone
indicates that this entrance was a former
skylight-a hole in the roof of the tube
that was open to the sky while molten
lava was in the tube. Most entrances to
lava tubes are not former skylights; they
have been opened by collapse of a part of
the roof long after volcanism ceased.
Actually, some collapse has occurred on
the west side of this skylight. Note that
this overhanging west edge is not covered
with dripstone but instead was pared
back by blocks that tumbled from it and
were carried away in the molten flood.
One large block that fell from this edge
was too heavy for transport; it landed on
the floor of the alcove at a point just
upstream from the foot of the stairs.
Examine its rounded top and sides and
note that it is completely plastered over
with a thin coating of lava which is
smooth on the top and pulled into dripstone
on the sides. After its tumble this
block was completely immersed in the
molten lava flowing down the tube.
Descend to the foot of the stairs and
inspect the upstream end of the cave
before starting on the trail downstream.
Looking upstream from the base of the
stairs, one of the first things you notice
is the feature that gives the cave its
name-a small rounded mound of
smooth lava with a hole in its top (fig.
16). Sticky lava emerged from this hole,
spread radially, and built up a low cone,
in the same way boiling porridge spills
from an overfilled pot. Peer into the hole
at the top of this Mushpot bubble, and
you can see at a depth of 2 ft a miniature
tube inside the thin smooth lobe of
yellowish lava, which floors this part of
the chamber. The opening through which
the lava porridge spilled out is a skylight
on the top of this tiny tube-in-tube. The
tube-in-tube developed as a later flow of
lava flowed along the larger tube. The
Mushpot bubble and the tiny tube-intube
beneath it were produced by leakage
through a lava jam that blocked the main
tube. This trickle of smooth yellow lava
12 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California