Craters of the Moon is an otherworldly experience.  For a geologist, walking up a 2,000 year-old volcanic cinder cone with not one plant growing on its sides is like climbing a staircase built last week for a non-geologist.  Time measured in human terms is so recent that all of human existence would literally be the thickness of a hair lying on top of a yardstick standing on end, where the yardstick represented the rest of geologic time.  So, a geologic event that occurred within the oral history of a people that still exist in southern Idaho, like the Shoshone-Bannock tribes, is not yesterday, but last second, in geologic time.

Craters is an area of not just cinder cones, but both pahoehoe and a'a volcanic flows, spatter cones, lava tubes, and tree trunk molds.  These features are not large, like the Grand Canyon, or spectacularly colored, like Bryce or Zion, but resemble features on the Big Island of Hawaii than like other national monuments in the U.S.

The first photo shows a small spatter cone, a feature built up by small blobs of red-hot lava being ejected from a nearly stationary flow by escaping gas or steam.  This cone is formed from a’a lava, the jagged, broken type of lava flow that is nearly impossible to walk across.

 

 The second photo shows a sloping pahoehoe flow with no vegetation after 2,000 years.  “Pahoehoe” is a Hawaiian word for a “rope-like” flow that forms a smooth skin as it cools, sort of like the skin that forms on a bowl of chocolate pudding after it has cooled.  The smooth skin only forms on cooling lava that contains a considerable amount of gas or water vapor. 

 

The photo below shows the Indian Tunnel lava tube that formed when molten pahoehoe lava continued to flow under a cooling skin until the source finally stopped producing lava and the cooling tube emptied.  This tube is 30 feet high, 50 feet wide, and 800 feet long, but several portions of the roof have collapsed, as can be seen by the “skylights” and rubble in the photo.  This tube got its name because of evidence that Native Americans used the tube for temporary habitation.

 

As it travels farther from its source and begins cooling, the gases begin to be lost and the lava becomes thicker and more brittle.  Instead of stream of flowing lava, the front of the flow becomes a tumbling mass of jagged pieces of lava containing many tiny voids where gas bubbles get trapped.  This is called “a’a” lava in Hawaiian.  The photo below is of an a’a flow that is just beginning to support plant life after 2,000 years!  The weathering and erosion process for lava flows in land-locked, semi-arid southern Idaho is very different from the same geologic processes on the humid Big Island of Hawaii surrounded by the Pacific Ocean.

 

Next time: Driving down a valley cut into an island arc that smashed into the Naorth American Plate 200 million years ago.