Steve Sparks

The geology of Nevados de Chillán volcano, Chile

Nevados de Chillan volcano is a large composite stratovolcanic complex in the Southern Volcanic Zone of the Chilean Andes. It is one of the highest-risk volcanoes in Chile due to high levels of historic activity and rapid development of economic activity in the area. High precision 40 Ar/ 39 Ar and 14 C geochronology, geochemistry and petrology have been employed in addition to photogeology and field mapping to elucidate the evolution of this volcano and assess its hazards. Nevados de Chillan has been active since at least 640 ka when a large group of subglacial andesite flows were erupted. Since 100 ka, sequences of andesite and dacite lavas have been erupted into both subaerial and subglacial environments. Ignimbrites were erupted at around 40 ka and may have been associated with caldera collapses. Two separate eruptive centres have evolved since 40 ka: the Cerro Blanco and Las Termas subcomplexes. The two centres are 6 km apart, yet have contemporaneously erupted geochemically distinct magmas. Subglacial lavas have been identified on the high flanks of the volcano and 40 Ar/ 39 Ar dating has confirmed their eruption during recent glaciations (isotope stages 4 and 2). Tephra fall deposits have been dated by 14 C analysis of interstratified organic material and indicate that no proximal tephra fallout deposits older than 9 ka remain. Tephra dispersal indicates that Holocene activity has involved vulcanian to subplinian eruptions. At least, 3 pyroclastic flow eruptions have occurred during the Holocene and lahar deposits are common in the valleys around the volcano. Historically, the Santa Gertrudis vent erupted during 1861-1865 and the dacite lava cone complexes Nuevo and Arrau were constructed during 1906-1943 and 1973-1986, respectively. Historic records indicate that lahars and landslides are major hazards to economic developments on the lower flanks and valleys

Locating a Radioactive Waste Repository in the Ring of Fire

The scientific, technical, and sociopolitical challenges of finding a secure site for a geological repository for radioactive wastes have created a long and stony path for many countries. Japan carried out many years of research and development before taking its first steps in site selection. The Nuclear Waste Management Organization of Japan (NUMO) began looking for a high-level waste repository site (HLW, vitrified residue from reprocessing power reactor fuel) 2 years ago. Over the next 10–20 years, NUMO hopes to find a site to dispose of ∼20,000 tons of HLW in a robustly engineered repository constructed at a depth of several hundred meters.

Expert judgment elicitation

Abstract Two project case histories for geological disposal of nuclear waste are discussed in this and a companion contribution ( Chapter 21 ) with emphasis on the application of formalized treatments of scientific uncertainties in siting considerations. In this chapter, a decision support approach is described, governed by a formalized basis for eliciting and aggregating expert judgments in a rational and auditable way when reasoning under scientific uncertainties. The Classical Model for structured expert judgment elicitation is the theme common to both case histories, serving as a means for determining inputs to a logic tree assessment of the potential evolution of multiple tectonic hazards over extended future periods in the present case history, and providing a way for characterizing potential impacts of climate change on repository performance in the second case history. This chapter first notes the emerging role of structured expert judgment in radioactive waste management and geological disposal facility siting decisions, and describes the properties and attributes of the elicitation method adopted for both case histories. The first, discussed here, is a contribution to a major geological disposal facility siting program in Japan, where expert judgments were elicited in a pioneering approach for parameterizing a logic tree assessment of site-specific impacts due to hazards arising from different long-term tectonic evolution scenarios. Some generic insights on expert elicitation are summarized in the context of facility siting considerations, and suggestions made for further applications, research and methodology developments.

Parametric investigation of a brine lens formation above degassing magma chamber

Formation of porphyry-type ore deposits is associated with degassing of crustal magma chambers. Saline, metalrich magmatic fluid penetrates into a shallow region saturated with cold meteoric water where the metals concentrate in brine lenses. The formation of the lenses and, thus, of the deposits occurs due to phase transitions [1]. The evaporation of H2O results in enrichment of residual fluid in NaCl. At a depth of 1–2 km precipitation of solid halite blocks the pore space and facilitates formation of concentrated brine lenses.