Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

etween the benches. The levees are well
developed here, arching up over the
channel in places and forming at one
point a complete tube-in-tube that persists
for 100ft. Where a medial crack is
present, the tube-in-tube changes in form
and opens into a channel that is confined
by 6- to 12-in.-high near-vertical levees.
Because the confining levees were weak
in places, the spiny pahoehoe broke
through in several spots and filled sections
of the bordering gutter.
The height of the pahoehoe benches
decreases from 4 ft beneath the natural
bridge to 1 ft above the floor 400 ft
upstream from the natural bridge. The
benches are continuous, very flat on top,
and at the same level on both sides of the
tube.
The course of the main large tube
along this 400-ft section is reminiscent of
river meanders. The tube swings
smoothly through several gentle curves
and is preserved without collapse. Highlava
marks are continuous on the walls of
the tube 1 ft above the top of the benches.
Well-developed dripstone and lavacicles
cover the walls and ceiling above these
high-lava marks.
Silver Cave subdivides around a
large pillar 320 ft upstream from the
natural bridge. On the upstream side of
the pillar, a tube at a slightly higher level
than the main tube departs to the north,
turns east, and reunites with the main
tube 80ft downstream. Because the floor
of this side tube is 3-4 ft higher than the
floor of the main tube, the lava drained
back into the main tube from both ends
of the side tube as the final lava flow
lowered and left small frozen lava cascades
at each junction.
The front of a later lava lobe came to
rest on the floor of the main tube just 20
ft upstream from the pillar. This new
flow covered the benches, gutters,
levees, and channel and prevents further
study of them upstream. The flow front
is a tongue of lava 3 ft high; from it, a
pahoehoe toe protrudes into one of the
gutters. The viscous character of this
flow is shown by its slab-pahoehoe surface
upstream, which consists of a 30-
ft-long crust of broken, twisted, and
imbricated slabs of pahoehoe. This flow
continues to form the floor of the main
tube upstream to Silver Cave's connection
with Post Office Cave. From this
junction the flow can be traced upstream
another 60ft until one crawls into a small
chamber 20 ft across blocked upstream
by a lava cascade. A lower lava cascade
just downstream from the chamber
shows steeply plunging ridges of a
smooth yellowish-brown lava. These
parallel ridges evidently resulted when a
smooth-surfaced pool of unusually colored
lava cascaded from the lip of the
upstream chamber. Colorful splashes of
yellow drips tone line the roof on the tube
in the same area.
The branch to the north is a small part
of Post Office Cave. It contains parts of
three levels, each of which is terminated
by lava fill upstream. Successive downstream
breakdown lips drop the two
upper short levels in steps to the lowest
level of the three. The floor of this lowest
level falls away into the Silver Connector.
This connector is the product of a
complicated series of breakdowns between
at least five superposed tubes,
accompanied and interrupted by recurrent
refilling and draining of lava. Its
development evidently spanned several
periods of eruptive activity followed by
enlargement during post-lava collapse.
Vertical layers of lava plaster adhere to
the broken separations between levels in
some parts of the connector. (See "Silver
Connector Level " section and map 15,
pl. 5).
Post Office Cave
Figure 50. Balcony forms natural bridge in Silver Cave (see fig. 4 and map 14, pl.
5). Lower tube was created when interior of late lava flow drained away.
Post Office Cave (map 15, pl. 5) is
geologically the most complex lava-cave
system in Lava Beds National Monument
and is one of the most difficult to
explore. The entrance and exit to Post
Office Cave used when the cave was
mapped in 1975 were closed by a slide
and a rockfall in 1977-78, but by 1988
they had been reopened. The cave is
entered at the downstream end. The cave
system extends upstream for a linear
distance of 2,080 ft, but many different
levels of the cave system are superimposed
one above another. Counting the
footage of each accessible level, the Post
Office Cave has more than 5, 170 ft of
passageway (see longitudinal section the
map). It is much easier to find one's way
from the upstream end of the Cocoa
Pipeline (the deepest lava tube of Post
Office Cave) to the surface via the exit
than it is to climb down into the deep
70 Selected Caves and lava-Tube Systems, lava Beds National Monument, California