Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

of Mammoth lava that flowed around it,
united on its north side, and impinged
against the southeast part of Bearpaw
Butte. The collapse trench upstream
from Bearpaw Cave began within this
Mammoth lava. No feeder tube from
Bearpaw Butte intersects this tube or its
breakdowns; instead, the tube cut into
and displaced the loose ash forming the
east slope of Bearpaw Butte.
A small cone on the north flank of
Bearpaw Butte did erupt after Bearpaw
and Merrill Ice Caves were formed.
From this vent a small lava tube, built
entirely upon the surface, coursed down
the north slope of the satellite cone and
stopped at the edge of a large collapse pit
in Mammoth lava that flowed around the
base of Bearpaw Butte.
Shoreline of the Lava Lake
To support the interpretation that
both the Bearpaw-Skull and Cave Loop
Road lava-tube systems originated from
a former lava lake fed from Mammoth
Crater additional details are needed to
document the position of the shoreline of
this lake during its most recent stand.
Evidence of the shoreline can be observed
on the sides of all of the buttes that
helped to contain the lake on its north and
east sides (fig. 68). Red Butte is closest
to the outlet that drained the lake. Remnants
of the black Mammoth lava, joined
to the reddish pyroclastics of Red Butte,
are well exposed where the paved road
rounds the northwest side of the butte.
Here the elevation of the lava shoreline
is 5, 160± 10 ft.
The next butte to the north is Hippo.
On its southwest flank, parts of a bench
of basalt marking the highest shoreline
adhere to the side of the butte at 5, 180 ft
elevation.
Sighting with a hand level from this
point to Modoc Crater and to Bearpaw
Butte, roughly a mile to the west, one can
pick out reference points at the same
elevation. These points, and others identified
on the ground as the highstand of
Mammoth lava against both Modoc Crater
and the southeast side of Bearpaw
Butte, all fall between the 5, 160 and
5,200 ft contours on the U.S. Geological
Survey Medicine Lake 15-minute quadrangle
map. Considering the rough margins
typical of lava lakes, this is good
agreement.
On the southwest side of the lava
lake, no older buttes display a definite
shoreline. However, the fissured and
schollendomed area thins and eventually
disappears against the higher topography
of the preexisting Heppe-Modoc ridge
formed by the Heppe-Modoc lava tube
(fig. 68) below the 5,200-ft contour (see
U.S. Geological Survey Medicine Lake
15-minute quadrangle topographic
map).
A lava lake should not be thought of
as similar to a lake of water in a temperate
climate. An arctic lake (or sea)
with its crust of ice is comparable, but
notable differences remain. Even during
the course of a major eruption, a lava
lake would be partly covered with congealed
blocks as seen during the slow
transit of an almost complete cover of
floating blocks across the Mauna Ulu
lava lake in Hawaii during major eruptive
activity. Such blocks of congealed
crust will tend to jam together and block
any surface outlet, a process which causes
constant shift in the position of the
outlets and corresponding fluctuations in
the elevation of the lake level. Evidence
that tangled surface lobes of block-rich
lava escaped from many points on the
perimeter of the lava lake north of Mammoth
Crater can be seen where irregular
skeins and lobes of lava poured out and
solidified with steep slopes facing to the
east between Red Butte and Hippo Butte,
to the north between Hippo Butte and
Bearpaw Butte, and to the northwest in
the area west of Modoc Crater and
southwest of Bearpaw Butte. Underground
outlets (lava tubes) have a better
chance of being preserved when they tap
the molten lake beneath its solidified
crust. With waning volcanism, however,
they become blocked with lava jams and
drain out as the lake level drops and its
crust collapses into an expanse of schollendomes.
Upper Ice Cave
Upper Ice Cave is a small lava tube
filled with ice that extends beneath the
edge of the younger Callahan lava flow
just south of Eagle Nest Butte (see map
19, pl. 6). Upstream from the cave, a
mile-long chain of shallow breakdowns,
small lava gutters, and short lava tubes
can be traced toward the two pit craters
north of Mammoth Crater. The place
where they join the lava tubes from
Mammoth Crater is lost beneath a blanket
of younger pyroclastic debris and
lava lobes from the crater. It seems likely
that the Upper Ice Cave line of gutters
and shallow breakdowns is a minor
distributary developed in the last stages
of building the Heppe-Modoc lava-tube
system, which extended from Mammoth
Crater through the gap between Eagle
Nest and Bearpaw Buttes and then on to
Canby Bay (fig. 4). Two similar but
smaller lava gutter systems drained off
this ridge to the west, and each of them
originated in small sag basins that cap
this ridge north of the Upper Ice Cave
lava gutters.
Callahan Flow
The Callahan flow composed mostly
of aa and block lava (fig. 69) is the
youngest geologic feature in this area.
Donnelly-Nolan and Champion (1987)
report a radiocarbon age of 1110±60 yr
B. P. Much of this large lava flow poured
from a boca (the Spanish word for
mouth) on the side of Cinder Butte, a
steep-sided cinder cone with a small
undissected crater at its top. Cinder Butte
is located 0.5 mi south of the southwest
comer of Lava Beds National Monument
(fig. 4).
Collapse Trenches
Between Skull Cave and
Three Bridges Area
More than 40 small caves, and a few
that can be traversed for more than 300
ft, lie downstream along the same lavatube
system that contains Skull Cave
(map 20, pl. 6). Collapse has obliterated
most of the primary features of many of
these caves, as well as segmenting the
tubes into short sections with floors so
cluttered with rubble that they are difficult
to traverse.
Collapse Trenches Between Skull Cave and Three Bridges Area 95