Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

stream, the ceiling heights drop, the
walls narrow, the tube takes on an
A-frame cross section (fig. 54) like that
in the Veritable Venturi beneath, and the
walls have spiny pahoehoe benches 1 ft
high that curb a shallow central channel.
Upstream from its intersection with
the Lower Entrance Tube, the tube is
larger and farther upstream the floor is
entirely covered with collapse debris.
Ceiling heights increase markedly 50 ft
upstream from the breakdown, where the
remains of a filled lava tube can be seen
in the roof. A steep-sided collapse pile
covers the floor a little farther upstream,
and the tube widens to 50 ft in the area
of this collapse. The floor of the tube
then descends to a low spot on the
upstream side of this collapse pile, and
from there a soil-covered slope leads
upstream to the vertical chimney, which
is the upstream exit from Post Office
Cave (fig. 55).
lava Transport
Using average underground flow
rates of 0.6-3.6 mi per hour, as determined
by Peterson and Swan son ( 197 4)
from observations through skylights on
active lava tubes in Hawaii , the volume
of lava transported in a full tube of
known cross-sectional area can be calculated.
Assuming these Hawaiian flow
rates and the same gradient, a tube the
size of the Cocoa Pipeline could transport
14-86 million cubic feet of lava per
day, the lower number being comparable
to typical Hawaiian eruption rates (Peterson
and Swanson, 1974). At least five
large lava-tube systems operated during
emplacement of the basalt of Mammoth
Crater, although they were probably not
all active at the same time. The five are,
from west to east, (1) Upper Ice Cave,
(2) north of Bearpaw Butte, (3) Bearpaw-Skull,
(4) Heppe-Cave Loop Road­
Post Office-Craig, and (5) Hidden Valley.
Volume is estimated to be nearly 1.2
cubic miles covering approximately 100
square miles and spreading west and east
beyond the boundaries of the monument
(fig. 1). The basalt of Mammoth Crater
also possesses a single remanent paleomagnetic
direction, and this data suggests
a very short duration , perhaps as
little as a decade, for emplacement of the
entire unit (Donnelly-Nolan and Champion,
1987). If 1. 2 cubic miles of basalt
were emplaced in 10 years, the calculated
flow rate would be 48 million cubic
feet per day; this figure is slightly more
than three times higher than the typical
Hawaiian flow rates cited by Peterson
and Swanson (1974) but within the range
of Hawaiian eruption rates. If the basalt
of Mammoth Crater erupted at approximately
the same rate as in a typical
Hawaiian eruption, 30 years may have
been required for the eruption. Note that
Post Office tubes transported a large
volume of lava (at least some of which
probably traveled through Craig Cave
tube) 5 to 10 mi to the east and northeast
from the east monument boundary nearly
to California Highway 139. However,
some of this large expanse of basalt of
Mammoth Crater may have been transported
by the inferred Hidden Valley
tube system, which is buried by the
basalt of Valentine Cave.
Intermittent changes of hydraulic
pressure and flow rate occur within such
a tube system. Also, roof collapses that
partially or completely plug flowing
tubes affect the hydraulic pressure within
tube systems. Pooling of lava behind an
annealed flow front or a lava collapse,
along with short-term fluctuations in
eruptive rates, probably represented the
only conditions under which any of the
Post Office tubes were completely filled
with lava for an extended period of time,
with the possible exception of the Cocoa
Pipeline. Tube-in-tubes, smooth pahoehoe
benches, and the many thin accretionary
linings in the tubes of Post Office
Cave support this reasoning, for they
record repeated periods of pooling, withdrawal,
and refilling with lava.
Craig Cave and Craig Temple
An oval collapse trench 125 ft long
and 60 ft wide provides entrances for
both Craig Temple and Craig Cave.
Downstream from this breakdown most
of Craig Cave is nearly drained of lava;
the last trickle of pahoehoe lava to
occupy the tube filled only the rounded
central part of the tube's floor and seldom
reached the steep walls except on
the outside of bends. At its downstream
end the lava tube is plugged by a roof
collapse. In contrast, Craig Temple, the
part of the tube upstream from the breakdown,
is two-thirds filled by two tongues
Figure 55. Explorer emerges from narrow exit of Post Office Cave (see fig. 4 and
map 15, pl. 5). Information about cave access, along with safety and exploration
guidelines are obtained at Visitor Center in Lava Beds National Monument.
80 Selected Caves and Lava-Tube Systems, lava Beds National Monument, California