CONTRASTING BASALTIC ERUPTION STYLES OBSERVED AT RED CONES: TWO NEIGHBORING CINDER CONES IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

This study investigates the role that magma composition plays in the temporal sequence of changing eruption styles (explosive versus effusive) at Red Cones volcanoes, a pair of basaltic cinder cones located approximately 10 km SW of Mammoth Mountain, in the eastern Sierra Nevada of California. The Red Cones eruptions (5,000 yrs BP) began with an effusive eruption from the southern cinder cone vent of sheet lava flows ranging in thickness from 0.1 to 0.3 m followed by violent Strombolian eruptions from both the northern and southern cinder cones. The finale of the eruption from southern cinder included effusive production of thin sheet flows 0.1 to 2 m in thickness with pahoehoe surface textures, well preserved flow levees, drained lava lakes, and lava tubes followed by the emplacement of a 6-m-thick aa lava flow. In contrast, the final eruption from the northern cinder cone was characterized by low energy, spatter-fed eruptions and poorly developed spatter-agglutinate flows. Cinder and lavas range in crystal content from approximately 18 vol.% in the earliest erupted lava flow to less than 6 vol.%, in the final products and contain variable amounts of plagioclase, olivine, cpx, opx, and traces of magnetite and ilmenite. Plagioclase phenocrysts from the southern cinder cone display a variety of textures, including oscillatory zoning, coarsely-sieved cores, or dusty rims while those from the northern cinder cone contain only oscillatory zoned and coarsely-sieved plagioclase. Olivine compositions from the earliest emplaced lava flows contain 2 populations of olivine based on major (Fo77- Fo78, and Fo82-Fo84) and minor elements (e.g., Ni), whereas all subsequently erupted material contain olivine phenocrysts that range continuously from Fo79 to Fo88. Our working hypothesis for the eruption of Red Cones suggests a shallow magma chamber existing beneath the present day southern cone prior to eruption is intruded by a rising basaltic dike, which triggers the effusive and subsequently explosive eruptions from the southern vent. The rising dike intercepts the ground surface at the present day northern vent, resulting in explosive eruption and formation of the north cone. Eruption styles from both cinder cones become effusive over time due to decreasing magma supply, ultimately ending the eruption.