Three-dimensional measurement of columnar jointing in the Bishop Tuff, Owens River Gorge, California

Undergraduates: Tony Bird, Jeff Glazner, Salix Consulting, Inc. Katherine Kelker, Elizabeth Brogden

Faculty Advisor: Allen Glazner
Department: Biology

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The Bishop Tuff is a Pleistocene ignimbrite whose outflow sheet covers ~1300 km2¿¿in eastern California, extending from Mono Lake on the northern end to Bishop on the southern end. Owens River Gorge (ORG) runs through the tuff, providing well-exposed cross sections that allow the study of columnar jointing. Whereas the phenomenon has been extensively studied in basalts, it is almost undescribed in rhyolite. Our study aimed to gain a greater understanding of rhyolitic columnar jointing and its relationship to fumarolic mounds that surround the gorge, by forming a geologic model based on aerial drone photography, satellite imagery, and scanning electron microscopy in the area south of LADWP Powerhouse No. 1. Columnar jointing in ORG appears to be a modification of the standard three-zone model of basaltic jointing, with a lower colonnade, smaller entablature, and upper colonnade. Above the entablature is a section of thin, shorter joints that commonly radiate from a single point in a manner resembling saguaro cacti. Putnam (1960) proposed that these radiating joints occur below fumarolic mounds and record heat flow away from fumarolic pipes, but in our drone survey we found only a weak correlation. Pumice samples taken from the mounds exhibit secondary mineralization in the form of grape-like clusters of ~200¿¿¿¿m spheres, with a tridymite shell enclosing altered volcanic glass. These spheres likely cemented the fragile volcanic ash to form a more coherent, erosion-resistant rock.