Resting Spring Pass Welded Tuff

Resting Spring Pass Welded Tuff


Approximately 3.7 miles outside of Shoshone, CA, along Highway 178 a cut was placed in a hillside during the construction of the highway. Road workers had to be surprised to find that they had uncovered a black strip of rock during the blasting process. But is it rock? Or possibly coal? Actually it is neither, the strip of black is welded tuff (ie: volcanic glass).

Welded Tuff is formed when a large amount of hot, gas-rich magma is blown from a volcano.  The foamy material consists of glassy particles called pumice. Pumice ranges in size from dust to large blocks, and it can reach temperatures over 1,300 degrees Fahrenheit. The foamy material flows across the landscape beneath a grey cloud of ash. The top and bottom layers of ash cool rather quickly, but the center remained hot; this allowed it to weld together, creating the amazing black vein of glass that we see today.

Geologist have dated the tuff to 9.5 million years old, using radiometric dating.

A small dirt pull-off is located directly across from the welded tuff, allowing for easy access to the site.


Resting Spring Pass Welded Tuff - Close up of the volcanic glass

Resting Spring Pass Welded Tuff – Close up of the volcanic glass


About the author

Jim Mattern

Jim is a scapegoat for the NPS, an author, adventurer, photographer, radio personality, guide, and location scout. His interests lie in Native American and cultural sites, ghost towns, mines, and natural wonders in the American Deserts.