Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

upstream from the one noted by the
match line (map 18, pl. 6) show large
patches of such plaster within their
openings, and through them lava
overflowed from the lower level and
spread as lobes over the Middle Earth
Passage floor (position and direction
of flow shown on map). Many more
lobes undoubtedly spread from several
breakdowns on this level but are
buried beneath the thick mantle of
collapse rubble covering the floor of
the middle level.
5. Many streams of lava that coursed
through the lower level during late
stages of volcanism were not voluminous
enough to inundate the middle
level. Numerous narrow benches
and stretched or sheared curbs of
formerly hot and sticky lava indicate
the position of high-lava marks. A
particularly informative place to examine
how these congealing features
were sheared by the moving lava is a
200-ft stretch of the southwest wall in
the middle of the lower level (fig. 25
and map 18, pl. 6).
6. One of the final streams of lava, only
about 6 ft deep, built a balcony
completely across the floor of the
lower level for a distance of 100 ft.
When the thin crust forming this
balcony was less than 2 ft thick, the
still-molten lava below it drained and
left a shallow flat-topped tube-intube
only 2-5 ft high. Collapsed
areas in its roof give access to this
tube-in-tube (see longitudinal section
on map 18, pl. 6). Downstream from
the lowest breakdown this crust was
too thin to survive collapse, and
farther downstream continuations of
this crust are preserved only in discontinuous
remnants of a 3-ft bench
along the walls.
Overpass Level
At a point 40 ft downstream from the
Blue Glacier, a narrow ramp-like ledge
starts at the floor on the west wall of the
lower level and climbs at an angle of
20°- 30° up the wall until it enters a hole
in the roof. Here the ledge widens and is
obviously the congealed surface of a lava
cataract, which continues upstream another
35 ft, where it opens into a highceilinged
room in an overlying level. The
ramp along the wall is the collapsed
lower end of this cataract. The level at
the head of the cataract can be traversed
for another 200 ft upstream before it is
blocked. Near its upstream end, a hole 10
ft long and 2-3 ft wide extends across
part of the floor of the level. This hole
drops onto a very steep ice cascade,
which descends precipitously into a
room called "Crystal Grotto" that is
evidently an upstream segment of the
lower level. Because this level lies above
a collapsed part of the lower level, to
which it is connected by a lava-cataract
ramp on one end and an ice cascade on
the other, we named it the "Overpass."
A large roof collapse near the downstream
end of the Overpass reveals segments
of still another lava tube, the
Fantasy Passage, 35 ft above the floor of
the Overpass (see longitudinal section on
map 18, pl. 6).
Original details of the walls, floor,
and roof of the Overpass have been
largely obliterated by collapse or obscured
by rimes of hoarfrost. The floor of
the middle section of this level is considerably
lower than either end because
it tumbled into and filled the lower level
below. An ice pool formed in this low
spot, and undoubtedly the ice here is
interconnected with the ice that fills the
spaces of the collapse breccia in Crystal
Grotto upstream from the Blue Glacier
Room. Upstream from the ice pool,
much of the Overpass level's collapse
debris is also cemented by ice. Drip
water collected and flowed along the
floor adjacent to the east wall, producing
an "ice brook" that steepens into an ice
cascade as it approaches the pool.
Ice Deposits
There are two basic requirements for
permanent ice to develop in caves within
a temperate climatic zone: ( 1) Rainwater
or snowmelt must penetrate down to the
cave through cracks or other openings,
and (2) the air in the cave must remain
relatively stagnant throughout the year.
Strong underground circulation of air
through caves-particularly those that
have a surface opening in the low area of
a basin or valley and a second on a high
ridgecrest-prevents the ice that may
form in winter from surviving the following
summer.
Crystal Cave is not only an excellent
example of conditions that are almost
ideal for the growth and stabilization of
ice in caves, but it also demonstrates the
use of the convection principle in discovering
previously unknown caves.
Aside from small pores and cracks
through its roof rock, Crystal Cave has
only one surface opening: the entrance
collapse, located at the highest point of
this cave's underground passageways. In
the winter months the cold dense air
above ground descends through this hole
and displaces the lighter and warmer air
within the cave up to the surface. On a
cold, sunny, and windless day in winter,
especially when the temperature has
dropped substantially overnight to below
freezing, the cave passages appear to be
"breathing." As the cold surface air
descends into the cave, the lighter,
warmer, and more moist air is pushed
upward onto the surface. Chilled in the
freezing temperature above ground, the
small amount of water vapor dissolved in
the cave air immediately condenses into
tiny water droplets, just as your breath
does in cold air each time you exhale.
Thus a persistent plume of fog rising
from a patch of loose boulders indicates
that a minor excavation of the boulder
pile might reveal an opening into a cave.
In caves like Crystal the cold air that
seeps down to fill all the passageways in
winter does not warm in summer. Because
the summer air outside the cave is
less dense than the heavy cold air inside,
it cannot descend to displace the cold air.
By late summer, however, the temperature
of the air in such a cave may rise as
high as the average annual temperature
of the region. As soon as it warms above
32 °F, melting of the ice begins. However,
because it requires 80 calories to
melt one gram of ice and a further 539
calories to convert a gram of ice water to
water vapor, the temperature of the
almost stagnant air immediately adjacent
to the ice mass soon stabilizes at or near
the freezing point of water (32 °F). Ice
will either melt or build up by filling the
openings until a fairly stable equilibrium
88 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California