Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Selected Caves and Lava-Tube Systems In and Near
Lava Beds National Monument, California
By Aaron C. Waters, julie M. Donnelly-Nolan, and Bruce W. Rogers
Introduction
Lava Beds National Monument (fig.
1) lies on the north slope of the huge
Medicine Lake shield (fig. 2), a complex
volcanic edifice of greater volume than
the steep-sided Mount Shasta volcanic
cone, which towers as a snowclad land
mark 40 mi southwest of the monument
(fig. 3).
Much of the north and south flanks of
the Medicine Lake shield were built from
molten lava transmitted through lava
tubes. These tubes formed beneath the
congealing surface of basalt flows in
somewhat the same way that a brook may
continue to flow beneath a cover of its
own winter ice. As molten lava emerges
from a vent and flows downslope, congealing
lava from the top and sides of the
central channel often forms a bridge over
the lava stream. The sticking together of
bits of lava spatter and fragile lava crusts
strengthens the bridge in the manner that
thin crusts of floating ice raft together to
cover a brook during early stages of a
winter freeze. Eruption of basalt lava,
however, is a much more violent and
spasmodic process than the steady gathering
of water that feeds a brook. If liquid
lava stops rising from its source deep
within the earth, the still-molten lava
moving beneath the crusted-over top of a
lava flow will continue to drain downhill
and may ultimately leave an open lavatube
cave-often large enough for people
to walk through. It is rare, however,
to find such a simple scenario recorded
intact among the hundreds of lava-tube
caves in the monument. Even before the
top and walls of a lava flow have time to
cool during a pause in lava supply, a new
and violent eruption of lava may refill the
open tube, overflow its upper end, and
spread a new lava flow beside or on top
of the first flow. Even if the original tube
is large enough to contain the renewed
supply of lava, this tube must deliver the
new lava beyond the end of its original
flow and thus the lava field extends
farther and farther downslope. If the
gradient of flow flattens, the tube may
subdivide into a number of smaller distributaries,
which spread laterally over
the more gently sloping ground.
Within Lava Beds National Monument,
most lava tubes are found within
the basalt of Mammoth Crater (figs. 1
and 4). Complicated and intertwining
lava-tube systems originating from
Mammoth Crater and other vents have
built a broad fan of complexly interfingering
lava flows that form the northeast
perimeter of the Medicine Lake shield.
Most of this lava was delivered through
lava tubes. Some tubes conveyed lava
underground 15- 20 mi from their sources.
Nevertheless, today one cannot
walk for a distance of even 4 mi within
any one lava tube. Large parts of the
roofs of most lava tubes have fallen in,
hiding the floor of the tube under huge
piles of breakdown or angular broken
rock, often stacked so tightly that access
to both upstream and downstream portions
of the tube is closed. In some
places, however, collapse of the tube's
roof has provided a large entrance into
the lava tube through which one can walk
with ease. In some collapse piles where
access appears to be lacking, one can
search the maze of tumbled blocks and
perhaps find a crawlhole into a lava tube.
Openings into caves may be detected by
noticing the runways of small animals or
testing the direction of air flow. On
sparklingly clear, very cold winter days,
openings into underground caverns will
emit a white fog, just as one's exhaled
breath does on such a day.
Holes in the landscape surface
formed by failure of part of a lava tube's
roof are called collapse pits, breakdowns,
or more commonly, collapse
trenches (see maps 2, 5, 10, and 20;
plates 1, 2, 4, and 6). While walking
across the relatively flat surface of the
lava flows, you are seldom aware of their
presence until a large and deep hole
yawns at your feet. Some small breakdowns
are dangerous death traps for
animals. Unwary humans have met a
similar fate (see map 12, pl. 4, and the
"Skull Cave" section).
Once underground within a lava tube
you may find your way impeded or
blocked by a variety of features. Piles of
loose rock that have peeled off the ceiling
and walls of the tube may clutter the floor
of the cave and slow your pace. Where
no fallen blocks are present, the smooth
to ropy (pahoehoe) surface of the lava on
which you walk may change gradually to
a very rough surface composed of bubble-filled
loose blocks of a spiny (aa)
lava. In some cases it may even completely
block the cave entrance. The
words pahoehoe and aa come from the
Hawaiian language. Most lava tubes are
found in pahoehoe lava (e.g. Greeley,
1971a; Harter, 1971), but occasionally
they occur in aa lava (Guest and others,
1980).
Geologists recognize several varieties
of pahoehoe (MacDonald, 1953;
Wentworth and MacDonald, 1953). The
smooth but thin and partly congealed
skin on the surface of the molten lava
may become wrinkled and twisted into
Introduction