CRATERS OF THE MOON
Southern Idaho’s Snake River
Plain represents the track of the Yellowstone Hotspot. Yellowstone is
presently located in northwestern Wyoming, of course. In the geologic
past, the center of Yellowstone volcanism was in southern Idaho, and before
that - southeastern Oregon.
Eight large, distinctive,
dark-colored lava fields cover portions of the modern Snake River Plain
(Shoshone Lava Field, Craters of the Moon Lava Field, Wapi Lava Field, Kings
Bowl Lava Field, Cerro Grande Lava Field, North Robbers Lava Field, South
Robbers Lava Field, and Hell’s Half Acre Lava Field).
These lavas have erupted during the late Pleistocene
and Holocene, well after the Yellowstone Hotspot “migrated” away to the
northeast. The lavas are so young in general that very little vegetation
has covered them. Portions of the Snake River Plain's basaltic lava
fields have been designated as Craters of the Moon National Monument. The
flows & cones in the Craters of the Moon Lava Field range in age from ~15 ky
to ~2 ky. The landscape is harshly beautiful and provides an excellent
opportunity to closely examine terrestrial basalt volcanism.
NORTH CRATER &
NORTH CRATER FLOW
North Crater is a cinder
cone that erupted basaltic aa lava and pahoehoe lava ~2200 years ago.
Fragments of the wall of this cinder cone collapsed into the flowing lava -
they are readily recognized as large, reddish/grayish blocks surrounded by
low-lying lavas.
Above: North Crater Cinder Cone
(northern slopes), looking S.
Above: North Crater Cinder Cone
(southern slopes), looking NW from Inferno Cone.
Above: Grassy Cone, a ~7.5 ky
now-vegetated cinder cone sitting just to the west of North Crater & the
North Crater Lava Flow. Looking W.
Above: ~1350 year-old tree
sitting atop the ~2.2 ky North Crater Lava Flow. Looking SW.
Above & below: pahoehoe lava showing the
classic ropy surface morphology. North Crater Lava Flow, ~2.2 ky, upper
Holocene.
Above: lava tube rupture.
The basaltic lava flows in the Craters of the Moon area have many lava
tubes. During eruptions, the shallow underground lava tubes would
sometimes completely fill with flowing lava. Or, a lava tube may have
become blocked by partially solidified lava. In such situations, pressure
would cause the top of the lava tube to rupture, and lava would ooze out at the
surface.
Above & below: large cinder cone
fragments (derived from the western wall of North Crater Cinder Cone) caught up
in lava.
Above: more large cinder cone
fragments (derived from western wall of North Crater Cinder Cone) caught up in
lava.
INFERNO CONE
Inferno Cone is a moderately
large cinder cone that completely lacks vegetation on its slopes. The
surface rocks of Inferno Cone are blackish, lightweight, somewhat glassy
scoriaceous basalt (scoria), often with iridescent coloration. Cinder
cones such as this are the result of high-energetic eruptions of basaltic
lava. If lava has a relatively high dissolved volatiles content (H2O,
CO2, SO2, etc.), material gets thrown relatively high
into the air. As the lava cools and solidifies, dissolved gases bubble
out. The gas bubbles get preserved as the lava finishes
solidifying. The resulting lightweight rocks have a frothy texture
(lots of tiny gas bubbles).
Hiking to the top of Inferno
Cone allows a magnificent panoramic view of the Craters of the Moon Lava
Field.
Above: Inferno Cone (northwestern
slopes)
Above: linear chain of small
spatter cones striking NNW-SSE in the Craters of the Moon Lava Field, as seen
from summit of Inferno Cone. Looking WSW.
SPATTER CONES
The Craters of the Moon Lava
Field has excellent examples of both cinder cones and spatter cones.
Spatter cones tend to be smaller structures. Spatter cones are the result
of low-energetic eruptions of basaltic lava. If the lava has relatively
little dissolved volatile content (H2O, CO2, SO2,
etc.), lava doesn’t get propelled very high into the air. A pile of
moderately dense blobs of lava is the end result - a spatter cone.
Above: small unnamed basaltic
spatter cone near Big Craters & Inferno Cone, Craters of the Moon Lava
Field.
Above: JSJ at a small basaltic
spatter cone called Snow Cone, in reference to the presence of unmelted
snow at the bottom of its vent in high summertime. This is located on the
southeastern side of Big Craters.
Above: small lava arch developed
on one side of Snow Cone.
LAVA CASCADES
The Lava Cascades are
somewhat reminiscent of the rapids of a frozen rushing river - a river of
lava. This is a good place to see slabby pahoehoe, an intermediate
lava morphology between aa lava & pahoehoe lava. Slabby pahoehoe
consists of jumbled & broken slabs of ropy pahoehoe lava tops.
Above: slabby pahoehoe lava flow
at Lava Cascades, Craters of the Moon Lava Field. Looking ~WNW.
Above: Lava Cascades, looking NE.
LAVA TUBES
Idaho’s Craters of the Moon
Lava Field has many excellent & accessible lava tubes (see also Lava Beds
in northern California). The Caves Trail at Craters of the Moon provides
easy access to several lava tube caves.
Above: spectacular juxtaposition
of aa lava and pahoehoe lava along Caves Trail. Aa is the rough,
blocky material at upper left. Pahoehoe is the smooth, ropy
material at lower right.
The most spectacular lava
tube cave here is Indian Tunnel. It is quite large, and has
occasional ceiling breaks that allows in considerable amounts of natural light
along most of its accessible length.
Above: Indian Tunnel lava tube
cave, near tourist entrance. Looking ~SSE.
Above: large basalt rubble pile
below ceiling break in Indian Tunnel lava tube cave. Looking ~S.
Some of the lava tube caves
have collapsed or slumped. Collapse involves brittle failure of solidified
basalt lava rocks in the ceiling of a lava tube. Slumping involves
ductile failure of still-soft basalt lava rocks in a lava tube's ceiling.
Above & below: collapsed lava tube &
basalt lava rubble.
Above: broad depression in
foreground here is a slumped lava tube.