Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Lavacicles can weld together into a
dripstone drapery where lava drips slowly
from an overhanging ledge.
Because of similarity with features
found in limestone caves many authors
use the name lava stalactite instead of
lavacicle. The process of formation,
however, is utterly different. Limestone
cave stalactites are formed from material
precipitated as a water solution degases
and evaporates. Icicles and lavacicles are
caused by the freezing of a liquid. Many
stalactites in limestone caves have a
companion stalagmite that grows up to
meet them when water droplets falling
from the tip of the stalactite degas and
evaporate on the cave floor, leaving a
deposit.
On the surface of some large rafted
blocks, however, splatters of lava and
pieces of plastically deformed lavacicles
that tumbled onto the block as it traveled
down the lava tube are likely to be
present. Companion lava stalagmites
(fig. 10) are sparse in lava tubes because
drip from the tip of a lavacicle in most
cases fell into the molten flood below.
Where the floor had already solidified,
stalagmites consisting of droplets of lava
welded together are often present. Occasionally
these display frozen rivulets
of lava, which ran down their sides and
partially smoothed their surfaces.
High-lava marks on the walls of a
tube, like the high-water marks of a river
in flood, record the position of lava at
some former high stage in its flow. If
lava remains constant for considerable
time at one level high within a tube, the
congealing of the lava surface inward
from the walls may build a lava balcony;
if ponding occurs lower on the walls (less
than 3ft), a lava bench may form. Most
maps in this report show where balconies
and benches are present. For excellent
examples, see the maps of Silver (map
14, pl. 5), Tickner (map 9, pl. 3),
Balcony (map 13 , pl. 4), and Valentine
(map 8, pl. 3) Caves.
In places, a flow that was building
paired benches-one from each wallmay
form a crust of congealed lava
extending completely across the tube. If
the still-molten lava flowing beneath this
crust drains out later, a two-storied tube
remains-an upper older story-beneath
which a newer lower tube remains active.
If the magma in the lower story then
drains out, a tube-in-tube is formed.
Another type of tube-in-tube forms when
a small lobe of new lava invades an older
and larger open lava tube and then drains
out soon after a thin exterior crust has
solidified. Even more interesting examples
of stacked tube-in-tubes occur in
places where small tubes, 3 to 7 ft in
diameter, have been occupied by brief
periodic surges of lava-a crust forms
that encircles each new surge of lava, but
if the flow is too small to fill the tube, this
new crust develops some distance from
the roof and upper walls, while firmly
attached to the floor. Thus a few flow
surges of diminishing size will produce
tube-in-tubes stacked within one another
that resemble nested concrete culverts of
varying size. Examples may be seen in
Tickner Cave, and at the downstream
terminations of Arch and Silver Caves.
The formation of thin accretionary
crusts of basalt magma at places where it
comes in contact with air or with cold
rock is responsible for many interesting
minor features, both on the surface and
within lava tubes. At first these crusts are
plastic and mobile, and with added cooling
they may be folded into many small
lobes whose surfaces resemble sections
of coiled ropes congealed into stone.
Such accretionary lava crusts are visible
in many lava tubes. Coatings of lavacicles
on the roof of a cave may have
peeled off and exposed another thin layer
underneath, which also has lavacicles.
Observe the dripstone on the wall of a
cave over an area of several square
meters, and you are almost sure to find
Figure 8. Lava dripstone trails down wall
of Post Office Cave (see fig. 4 and map
15, pl. 5). Reddish color was produced
by oxidation of hot lava surface.
Figure 9. Lava driblet on wall of Fern
Cave (see fig. 4 and map 17, pl. 5). Pencil
for scale.
Figure 10. Lava stalagmite formed by
dripping of still-hot lava from ceiling of
Post Office Cave (see fig. 4 and map 15,
pl. 5) onto still-moving flow. The 2-
ft-high stalagmite was apparently rafted
downstream from the ceiling drip that
formed it. Hammer for scale.
Introduction 7