Frank V. Perry

Scoria cone construction mechanisms, Lathrop Wells volcano, southern Nevada, USA

Scoria cones are commonly assumed to have been constructed by the accumulation of ballistically ejected clasts from discrete, relatively coarse-grained Strombolian bursts and subsequent avalanching such that the cone slopes are at or near the angle of repose for loose scoria. The cone at the hawaiitic Lathrop Wells volcano, southern Nevada, contains deposits that are consistent with these processes during early cone-building phases; these early deposits are composed mainly of coarse lapilli and fluidal bombs and are partially welded, indicating relatively little cooling during flight. However, the bulk of the cone is composed of relatively fine-grained (ash and lapilli) planar beds with no welding, even within a few tens of meters of the vent. This facies is consistent with deposition by direct fallout from sustained eruption columns of relatively well-fragmented material, primarily mantling cone slopes and with a lesser degree of avalanching than is commonly assumed. A laterally extensive fallout deposit (as much as 20 km from the vent) is inferred to have formed contemporaneously with these later cone deposits. This additional mechanism for construction of scoria cones may also be important at other locations, particularly where the magmas are relatively high in volatile content and where conditions promote the formation of abundant microlites in the rising mafic magma.

Decreasing magmatic footprints of individual volcanoes in a waning basaltic field

The distribution and characteristics of individual basaltic volcanoes in the waning Southwestern Nevada Volcanic Field provide insight into the changing physical nature of magmatism and the controls on volcano location. During Pliocene-Pleistocene times the volumes of individual volcanoes have decreased by more than one order of magnitude, as have fissure lengths and inferred lava effusion rates. Eruptions evolved from Hawaiian-style eruptions with extensive lavas to eruptions characterized by small pulses of lava and Strombolian to violent Strombolian mechanisms. These trends indicate progressively decreasing partial melting and length scales, or magmatic footprints, of mantle source zones for individual volcanoes. The location of each volcano is determined by the location of its magmatic footprint at depth, and only by shallow structural and topographic features that are within that footprint. The locations of future volcanoes in a waning system are less likely to be determined by large-scale topography or structures than were older, larger volume volcanoes.

Aspects of potential magmatic disruption of a high-level radioactive waste repository in southern Nevada

Volcanic hazard studies, combining standard techniques of hazard appraisal and risk assessment (probability Xs consequences), are being undertaken with respect to storage of high-level, radioactive waste in southern Nevada. Consequence studies, the emphasis of this work, are evaluated by tracing the steps of ascent of basaltic magma including intersection and disruption of a repository followed by surface eruption. outhern Nevada is cut obliquely by a N-NE trending belt of basaltic volcanism that contains a number of major volcanic fields separated by areas with scattered basalt centers. The basalts of the belt range in age from 8 m.y. to recent. They are primarily of hawaiite composition, and a number of the fields exhibit the straddle-type associations of the alkalic subsuite. The basalts are characterized by high

Volcanism Studies: Final Report for the Yucca Mountain Project

^{87}Sr/^{86}Sr

Hazards and scenarios examined for the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

isotopic ratios (~0.707) and high Sr contents and are evolved from parental compositions by crystal fractionation. Theoretical considerations suggest basalt magma ascends rapidly from mantle depth (10s of cm/sec in the bubble-free regime) but may be trapped temporarily and fractionate at the mantle/crust interface. Basalt centers are fed from narrow linear dikes. Local sheet-like intrusions formed at depths of 200 to 300 m probably due to a combination of extensional faulting during emplacement and trapping within low-density tuff country rock, aided in part by a low magma-volatile content. Basalt centers comprise scoria cones of moderate size and associated lava flows formed during Strombolian eruptions. Local hydromagmatic eruptions occurred at three centers but are unlikely in future eruptions due to the considerable depth to the ground water table and the low flux of moisture in the unsaturated zone. Incorporation of radioactive waste in basalt magma is controlled by the dimensions of basalt dikes at repository depths and the depth of magma fragmentation. Dispersal pathways of waste should follow the pyroclastic component of a Strombolian eruption. The maximum volume of waste deposited with basaltic tephra can be traced approximately by assuming waste material is dispersed in the same patterns as country rock lithic fragments. Based on a basalt magma cycle that is similar to typical Strombolian centers,

Status of volcanism studies for the Yucca Mountain Site Characterization Project

180 m^{3}