OBSIDIAN  CLIFF

 

The most famous obsidian exposure in North America is Obsidian Cliff on the northern side of Yellowstone.  The cliff itself is rhyolitic obsidian with obvious columnar jointing (referred to by geologists as a vertical colonnade).  Large boulders of obsidian litter the base of the cliff.  Park administrators don't want people walking among the boulders anymore, but they can still be closely examined alongside the road.  The rocks here are part of the Pleistocene-aged Roaring Mountain Member (Plateau Rhyolite).

 

 

 

 

 

The pic above shows the typical character of the boulders at the base of Obsidian Cliff.  The obsidian has nice flow banding, with whitish spotting (“snowflake obsidian”), the result of devitrification.  A little south of here, the obsidian eroding from the cliff above lacks the devitrification spots.  Yellowstone obsidian is of such high quality that early American Indians used it for making various implements.  Archaeological artifacts demonstrably composed of Yellowstone obsidian have been found throughout America.

 


 

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