ERUPTIVE HISTORY OF RATTLESNAKE CRATER, ARIZONA, SAN FRANCISCO VOLCANIC FIELD REU

Rattlesnake Crater is located near the eastern edge of the San Francisco Volcanic Field in northern Arizona. The San Francisco Volcanic Field contains more than 600 volcanoes and is late Cenozoic in age. Rattlesnake Crater is composed of a tuff ring on the north side, a cinder cone on the south side, and two lava flows. Rattlesnake Crater has a complex eruptive history that has not been studied extensively and can provide a better understanding of volcanic processes in the San Francisco Volcanic Field.

Basalt of the Rattlesnake Crater complex is rich in pyroxene xenocrysts and feldspar xenocrysts more rarely throughout the deposits. The basal deposits of the tuff ring are pyroxene- and feldspar-xenocryst welded- and nonwelded-cinder basalt. Phreatomagmatic deposits have a sandstone matrix and consist of alternating poorly and well-sorted layers that suggests a pulsing eruption. These deposits include basal megablocks of pyroxene- and feldspar-xenocryst lava. A feldspar-xenocryst basalt lava flow appears to originate from the eastern base of the tuff ring. The cinder cone is composed of pyroxene-xenocryst agglutinated and welded cinder as well as welded and loose cinder and spatter. The cone includes three valley-shaped breaches, with lava-flow deposits found farther up the western breach, suggesting the presence of a lava lake. This lava lake fed a pyroxene-xenocryst basalt flow that is widespread to the south and east of the cone.

Three eruptive episodes make up the history of Rattlesnake Crater. The basal cinder on the north side of the crater originated from a Strombolian eruption and coincides with the oldest eruption. A phreatomagmatic eruption created the tuff ring as well as the two different facies, which overlie the cinder. The diversity in clast size and poorly sorted beds along with the variation of facies, such as clast size and sorting of the beds, distinguish the poorly sorted layers as surge. The well-sorted layers resemble fallout deposited from an eruption column. Xenocryst composition of the basal megablocks in the phreatomagmatic deposit matches the composition of the basal deposit of cinder. Preliminary seismic data show the phreatomagmatic deposit dipping beneath the cinder cone to the south, suggesting the eruption of the cinder cone is the youngest eruptive episode in Rattlesnake Crater.