Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.     

Expert judgment and climate forecasting: A methodological critique of “climate change to the year 2000”

The National Defense University's study of climate change to the year 2000 was based largely on the judgments of the members of two expert panels. Although the study has been widely distributed and apparently read by policy makers in the U.S. and abroad, the method of eliciting and analyzing expert judgment has not been critically reviewed. This paper uses the literature on judgment and subjective probability to evaluate the expert judgment methods used in the study.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The Delphi technique and judgmental forecasting

The Delphi technique for judgmental forecasting by expert groups is described and the controversy surrounding its use is summarized. The technique clearly does not eliminate all unwanted psychological effects on group judgment. Furthermore, the design of most Delphi studies makes it impossible to separate the signal from the noise in expert judgment. A methodological standard for evaluating judgmental forecasts is proposed.     

Transport and economic development in the less-developed countries: Some reflections on the seaports of Kenya and Tanzania

The significant role of transport in the development of less-developed countries is widely accepted, but the critical importance of seaports is often under-appreciated. Evidence from the Indian Ocean ports of Kenya and Tanzania is used in this paper to illustrate and support the argument that efficient port operation is an essential contributory factor in the development process.     

An experimental investigation of volatile exsolution in evolving magma chambers

Previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas. Turbulent transfer of heat between a layer of dense, hot and volatile-rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer. Small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing, followed by further exsolution of gas and explosive eruption. These processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of CO₂. We have investigated both the initial overturning produced by gas release in the lower layer, and the subsequent evolution of gas due to intimate mixing of the two layers. The latter experiments, in which the reactants remained isolated in the two layers until overturning occurred, demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat. This shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other. The magmas can coexist in the same stratified chamber, until their bulk densities become equal and they mix together. The processes illustrated in these experiments could occur in H₂O-bearing magmas such as in the calcalkaline association and in CO₂-bearing mafic magmas such as in silica undersaturated suites.     

Long-term erosion on granitic roadcuts based on exposed tree roots

Exposed roots were used to estimate soil and bedrock erosion on the cut slopes of a 45-year-old road constructed in granitic soils of the Idaho Batholith. The original roadcut surface was defined by projecting a straight line from the toe of the cut past the end of the exposed root to the intersection of a straight line projected along the surface of the hillslope. A cross-sectioning technique was then used to determine erosion to the present roadcut surface. A total of 41 exposed root sites were used to estimate erosion on a 1350 m-long section of road. Average erosion was 1·0 and 1·1 cm/year for soil and bedrock respectively. Buttressing by tree roots caused lower erosion rates for soil as compared to bedrock. Both soil and bedrock erosion rates showed statistically significant correlations with the gradients of the original cut slope. The bedrock erosion data provide a reasonable estimate of the disintegration rate of exposed granitic bedrock exhibiting the weathering and fracturing properties common to this area. The road is located in a study watershed where long-term sediment yield data are available. Sediment data from adjacent study watersheds with no roads were compared to sediment data from the roaded watershed to estimate the long-term increase in sediment yield caused by the road. The increase amounts to about 2·4 m³/year. This figure, compared to the average annual on-site road erosion, provides an erosion to sediment delivery ratio of less than 10 per cent. Based on study results, road construction and maintenance practices are suggested for helping reduce roadcut erosion.     

Compaction localization in the Earth and the laboratory: state of the research and research directions

Localized compaction in porous rocks is a recently recognized phenomenon that has been shown to reduce permeability dramatically. Consequently, the phenomenon is relevant to a variety of technologies involving fluid injection or withdrawal. This article summarizes current understanding of localized compaction and impediments to further progress. The article is based on discussions at a small workshop on localized compaction sponsored by the Office of Science, U. S. Department of Energy.     

Temporal Zooming

Spatio-temporal knowledge representation often requires changing from one level of detail or granularity to another so users can carry out a desired task. Meteorological occurrences, geological processes, or population movements, for example, can be examined at different granularities. This includes different spatial perspectives as well as temporal views where phenomena may be examined under real time conditions, or over hourly, daily, weekly, or longer periods. Moving back and forth among granularities is a necessary routine for many domain scientists. Changing to a more detailed view uncovers information that otherwise is unknown. Conversely, moving to a simpler view can improve our understanding of phenomena. Although people routinely abstract information from their environment at different granularities and perform mental shifts that increase or decrease detail, formalization of these alterations for use in information systems has proved difficult. Geographic information systems typically treat changes in granularity from the perspective of changes to the geometric properties of objects through graphic zooming. None of the current approaches to zooming offer support for performing this operation over time. This work focuses on the temporal aspects of changing granularity or temporal zooming . The approach is based on a model of change to identifiable objects. In this paper, temporal zooming involves expanding or collapsing the transitions among identity states of the same object as well as revealing or omitting other objects that are linked through transitions. A set of operations to support refining and coarsening the evolution of objects over time is presented. This work offers a new perspective on zooming important for spatio-temporal query languages and for users of large spatio-temporal databases who require the means to shift back and forth among simpler or summarized views of data and more detailed views.