Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Another filled lava tube is poorly
exposed in the talus slope that forms the
east wall of Mammoth Crater just north
of the bench of basalt lava flows. This
filled tube plunges east and transmitted
lava eastward from Mammoth Crater
into the Hidden Valley area.
Hidden Valley
The head of Hidden Valley (map 19,
pl. 6) is a cliffed amphitheater with walls
60-100 ft high that lies "hidden" by a
forest of conifers on the southwest and a
dense thicket of mountain mahogany on
the north and northeast. It is always a
surprise to emerge suddenly at the top of
a high cliff after beating your way
through the dense vegetation that borders
this amphitheater on the north side. A
short trail, however, leads to the top of
the westernmost cliff and a longer unpaved
trail skirts the south edge and
descends to the floor of the valley.
The origin of Hidden Valley is enigmatic;
it may have been a lava channel
that enlarged laterally and headward as
the eruption proceeded. Downstream
from Hidden Valley, the basalt that must
have flowed through this channel was
buried by the younger basalt of Valentine
Cave. The size of Hidden Valley suggests
that a major tube system from
Mammoth Crater once occupied or
emerged from this valley and fed lava
flows that extend east of the monument
nearly to California Highway 139.
obsidian. Some andesitic volcanoes were
built over the fault zones that define the
caldera rim. Numerous studies of various
aspects of Medicine Lake volcano include
Mertzman ( 1977), Grove and others
(1982), and Donnelly-Nolan (1988).
A geologic field trip guide to Medicine
Lake volcano and Lava Beds National
Monument is also available (Donnelly­
Nolan, 1987).
The Mammoth basaltic shield differs
greatly in form from the many nearby
steep-sided cinder and spatter cones because
its lava flowed downslope in
smooth lava flows and in tubes. Only
rarely did the Mammoth vent expel large
quantities of ash and bombs. Had the
Mammoth shield been built on flat
ground, its flows would slope away on
all sides from the central crater. Because
Mammoth Crater formed on the already
sloping flank of a much larger shield,
however, most of its lava traveled downslope
to the north and east. The lava was
diverted at many points by older cinder
and spatter cones and lava flows such as
Schonchin Butte and the Schonchin
Butte flow. Locally the lava ponded in
swales between obstructions such as the
cones and north-trending fault blocks.
Downslope from Mammoth Crater a
cluster of six cinder and spatter cones in
various stages of erosional truncation
combined to dam a large lava lake (fig.
68). The lake, fed from Mammoth Crater,
lay ponded during the active stages
of volcanism but evidently fluctuated
erratically in level as different surface
outlets along its periphery formed and
closed as the lava congealed. Short-lived
overspills occurred along the periphery
of the lake while occasionally a more
permanent outlet would open underground
as the increasing hydraulic pressure
of the ponded lava overcame a
blockage in a lava tube. In the last stages
of this lake's existence, and after a crust
at least 5 to 10 ft thick had congealed
over its surface, the molten interior of the
lake drained toward an opening approximately
50 ft below the lake's former
level. This opening occurred at a point
0.3 mi downstream (east) of the Heppe
Caves, where the feature labeled "backflow
outlet" joins the line of breakdowns
(fig. 68). As the level of the lake
dropped, its solid surface crust broke into
blocks, which moved toward the newly
opened drain and produced a tangle of
fissures and schollendomes. Heavily fis
Former Lava Lake North of
Mammoth Crater
When volcanism began at Mammoth
Crater, the topography was probably
more diverse than it is today. Clusters of
steep-sided conical buttes, northwardsloping
lava flows, and north-trending
fault blocks dominated the landscape.
Mammoth Crater is located 8 mi northeast
of Medicine Lake, which lies in the
caldera of the Medicine Lake shield
volcano (Anderson, 1941). The flanks of
this huge shield are embellished by more
than 200 satellite volcanoes, including
many steep-sided cinder and agglutinate
cones such as Schonchin Butte (fig. 67),
rounded lava domes, and stubby flows of
Figure 67. View of Schonchin Butte from top of Hippo Butte. Schonchin Butte is
the principal landmark within the Lava Beds National Monument. A trail to a fire
lookout at the top provides excellent views.
Heppe Caves and the Mammoth Crater-Hidden Valley Area 93