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 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.
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.
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.
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.