THE SMALL PICTURE INSIDE THE BIG: VOLCANIC FEATURES IN TUFF CANYON, BIG BEND NATIONAL PARK, TEXAS

National Parks provide broad overviews of geology, but tourists and students can learn even more from detailed exploration of small areas within the parks. The logic that links interpretation to observation can be most readily explained in a place where the visitor is face to face with the geology. One such site is Tuff Canyon, southern Big Bend National Park, where a creek has cut through 29 Ma rhyolitic pyroclastic rocks and into 34 Ma mafic lava. Tuff Canyon is an underutilized teaching resource containing a wide variety of well displayed volcanic features. The National Park Service has built three viewing platforms on the edge of the canyon, and an easy trail extends from the canyon mouth upstream for 400 m. A one-hour stroll shows hyaloclastites, pillow lavas, and pahoehoe flow tops with stretched vesicles, overlain by nonwelded pyroclastic flow, surge, and debris flow deposits. Among the features seen in the canyon are pumice lapilli, lithic fragments, pyroclastic flow units with gas escape pipes, ballistic blocks with impact sags, crossbeds, sorting variations, imbrication, filled channels, normal faults, case hardening, potholes, rapid stream incision in response to overgrazing, and records of flash floods. In the surrounding area are lava domes, rheomorphic folds, dikes, and air-fall, near-vent spatter, and welded pyroclastic flow deposits. A 40-page guidebook to Tuff Canyon has enticed tourists out of their vehicles and off the pavement. Users respond favorably and want more opportunities for detailed examination of rock exposures. This poses problems for the Park Service, but most can be lessened by careful exposition in guidebooks.