Rhyolitic pumice in the basaltic pyroclasts from the 1971 eruption of Teneguía volcano,Canary Islands

Fragments of acid pumice have been found among the basic pyroclastics thrown out by the Teneguía volcano during its explosive phases. The presence of these pumice fragments brings up the problem of their origin and their relationships to the mechanism of the eruption.The composition of the analysed samples coincides exactly with the low temperature trough in petrogeny's residua system, albits-orthoclase-silica. The samples can be classified as rhyolites and trachytes.The authors propose that the pumice was produced by the fusion of the acid phase in a subvolcanic complex located beneath the island of La Palma.     

Second order elasticity theory: An improved formulation of the Grüneisen parameter at high pressure

The second order theory of elasticity, in which terms to second order in strain are retained in calculating atomic bond length changes and elastic moduli, is extended to describe thermal vibration of a face-centred cubic crystal. Coupled with equations relating the pressure dependences of elastic constants, this yields a new formulation of the thermal Grüneisen parameter γ in terms of pressure P, incompressibility K and rigidity, μ

$\text{λ}\text{1}\text{2}\text{d}\text{K}\text{d}\text{P}\text{?}\text{1}\text{2}\text{+}\text{1}\text{9}\text{P}\text{K}\text{?}\text{1}\text{3}\text{?}\text{1}\text{9}\text{P}\text{K}\text{f}\text{1?}\text{2}\text{3}\text{P}\text{K}\text{?}\text{2}\text{3}\text{P}\text{K}\text{f}$

where f = 24 (3 K ? 2 P)/(3 K + 115 μ + 90 P). The factor f arises from bond interactions and the case f = 1, representing independent bonds (no interactions), yields the free-volume γ- Since we have shown earlier that the second order elasticity theory provides a convincing explanation of the elasticity of the inner core, we believe that the new formula is appropriate for the inner core. It is, however, inadequate to describe the lower mantle γ, in which atomic bond angle rigidity, not considered here, may be appreciable.     

Tectonic and sedimentary basin evolution of the eastern Bangong–Nujiang zone (Tibet): a Reading cycle

The ophiolite-bearing Bangong-Nujiang zone (BNZ) traversing central Tibet from east to west separates the Qiangtang block in the north from the Lhasa block in the south. Their stratigraphic development indicates that both blocks once formed a continuous continental platform until the Late Triassic. Following Late Paleozoic-Triassic rifting, ocean crust formed between both blocks during the Late Triassic creating the Dongqiao-Naqu basin (DNB) among other basins (Yu et al. 1991). The analysis of the rift flank sequences reveals that rifting was dominated by transtension. The basin was shortened by post-Mid-Cretaceous transpression. Thus, the overall basin evolution represents a Reading cycle despite some active margin processes which gave this cycle a special imprint. Major basin parts were preserved despite transpressional shortening suggesting that the eastern BNZ represents a remnant basin. Our understanding of the DNB solves the prior problem of viewing the BNZ as a Mid-Late Jurassic collisional suture although typical collision-related deformation, thickening, mountain building, as well as related molasse formation are lacking. Our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting.     

Thermal histories of the core and mantle

Recognition that the cooling of the core is accomplished by conduction of heat into a thermal boundary layer (D″) at the base of the mantle, partly decouples calculations of the thermal histories of the core and mantle. Both are controlled by the temperature-dependent rheology of the mantle, but in different ways. Thermal parameters of the Earth are more tightly constrained than hitherto by demanding that they satisfy both core and mantle histories. We require evolution from an early state, in which the temperatures of the top of the core and the base of the mantle were both very close to the mantle solidus, to the present state in which a temperature increment, estimated to be ～ 800 K, has developed across D″. The thermal history is not very dependent upon the assumption of Newtonian or non-Newtonian mantle rheology. The thermal boundary layer at the base of the mantle (i.e., D″) developed within the first few hundred million years and the temperature increment across it is still increasing slowly. In our preferred model the present temperature at the top of the core is 3800 K and the mantle temperature, extrapolated to the core boundary without the thermal boundary layer, is 3000 K. The mantle solidus is 3860 K. These temperatures could be varied within quite wide limits without seriously affecting our conclusions. Core gravitational energy release is found to have been remarkably constant at ～ 3 × 10¹¹ W. nearly 20% of the core heat flux, for the past 3 × 10⁹ y, although the total terrestrial heat flux has decreased by a factor of 2 or 3 in that time. This gravitational energy can power the “chemical” dynamo in spite of a core heat flux that is less than that required by conduction down an adiabatic gradient in the outer core; part of the gravitational energy is used to redistribute the excess heat back into the core, leaving 1.8 × 10¹¹ W to drive the dynamo. At no time was the dynamo thermally driven and the present radioactive heating in the core is negligibly small. The dynamo can persist indefinitely into the future; available power 10¹⁰ y from now is estimated to be 0.3 × 10¹¹ W if linear mantle rheology is assumed or more if mantle rheology is non-linear. The assumption that the gravitational constant decreases with time imposes an implausible rate of decrease in dynamo energy. With conventional thermodynamics it also requires radiogenic heating of the mantle considerably in excess of the likely content of radioactive elements.     

A new technique for precise uranium-series dating of travertine micro-samples

Secondary carbonate formations, such as travertine and calcareous tufa deposits, are important archives for quaternary continental climate studies and archaeology. The extremely complex growth mechanisms result in some serious problems for precise mass spectrometric uranium-series dating. Often, detrital and organic particles contaminate the carbonate and large pore volumes yield a great potential for open system behavior. We utilized microscopic, mineralogical and geochemical methods prior to sample selection to determine the abundance of primary calcite, i.e. micrite and spar. Furthermore, the state of alteration was characterized by cathodoluminescence and trace-element analysis. We conclude that travertine and calcareous tufa are appropriate for precise U-series age determination if a) micrite and/or spar are the dominant phases; b) cathodoluminescence of both phases is weak or absent; c) Fe and Al levels are low; and d) Sr concentrations are close to the average of the studied site. We mapped and sampled solely areas of major micrite/spar abundance having minor alteration for accurate U-series dating. When this new method was applied, travertines located in eastern Germany (sites Bad Langensalza, Burgtonna and Weimar-Ehringsdorf) gave single ²³⁰Th/²³⁸U-ages consistent with the lithological growth sequence and greatly improved compared to previously published chronologies. In addition, we determined ²³⁰Th/U isochron ages on bulk samples that confirm our single ages. In contrast to primary calcite, pore cements are homogeneously distributed throughout the travertine fabric and reflect early diagenetic processes and/or weathering.     

Canary in a coal mine: perceptions of climate change risks and response options among Canadian mine operations

A survey documenting how climate change is perceived and responded to by Canadian mine operations was administered to a random sample of practitioners working at mine sites across Canada. Key findings include: (1) Mines are sensitive to climatic hazards; (2) There is concern about climate change among mine practitioners, but the majority have not yet noticed climate change to be affecting operations; (3) Future climate change is expected to have negative impacts for mine operations; (4) Mines are responding to climate change mainly through efforts to reduce greenhouse gas emissions but also through adaptation, although to a lesser degree; and (5) Knowledge of future climate change projections and impacts is limited, potentially constraining understanding of the nature of climate change risks. The survey compliments previous work documenting perceptions among upper level management on climate change in the Canadian mining industry. The results from both surveys are largely consistent, establishing that the mining sector perceives climate change as an emerging risk and is developing response options, but needs to invest more time and resources for adaptation to what are inevitable changes in climate. The results support the need for targeted in-depth research to assess the vulnerability of mining to climate change and to evaluate response options.     

Deriving rules from activity diary data: A learning algorithm and results of computer experiments

 Activity-based models consider travel as a derived demand from the activities households need to conduct in space and time. Over the last 15 years, computational or rule-based models of activity scheduling have gained increasing interest in time-geography and transportation research. This paper argues that a lack of techniques for deriving rules from empirical data hinders the further development of rule-based systems in this area. To overcome this problem, this paper develops and tests an algorithm for inductively deriving rules from activity-diary data. The decision table formalism is used to exhaustively represent the theoretically possible decision rules that individuals may use in sequencing a given set of activities. Actual activity patterns of individuals are supplied to the system as examples. In an incremental learning process, the system progressively improves on the selection of rules used for reproducing the examples. Computer experiments based on simulated data are performed to fine-tune rule selection and rule value update functions. The results suggest that the system is effective and fairly robust for parameter settings. It is concluded, therefore, that the proposed approach opens up possibilities to derive empirically tested rule-based models of activity scheduling. Follow-up research will be concerned with testing the system on empirical data. Received: 31 January 2001 / Accepted: 13 September 2001