Long-term gain changes in mercury-level tiltmeters of the Stacey type

A uniaxial mercury-level short baselength tilt transducer constructed at the University of Alberta is described. The design closely follows the differential capacitance tiltmeter ofStacey et al. (1969), but differs in that aluminum is used for the beam, and how the mercury cups are supported beneath the beam. It also allows gain verification by comparison of the direct capacitances with a known capacitance. It has been found that the gain factor for the instrument changes with time, because of the relaxation of the mercury surfaces. This has been monitored for several years for four instruments at two sites. The gains change rapidly for several months after installation, but more slowly as time progresses. The magnitudes of such changes in instruments of this type may be as large as 3 percent.     

Massive sulfides in a sedimented rift valley, northern Juan de Fuca Ridge

A number of mounds each several hundred meters across and up to sixty meters high have been observed with SeaMARC II acoustic imagery and Seabeam bathymetry in the sediment-filled axial valley at the northern end of the Juan de Fuca Ridge. The mounds are located a few kilometers west of the eastern valley-bounding normal fault scarp where the local sediment fill is approximately 300 m thick. All of the mounds are believed to be of hydrothermal origin, and one is associated with anomalously high heat flow in excess of 1 W m⁻². A piston core collected from that mound comprises coarse clastic sulfide units interbedded with sulfidic muds. Hydrothermal minerals present in the 2.3 m section include pyrrhotite, pyrite, marcasite, sphalerite, chalcopyrite, iss (intermediate solid solution in the CuFeZnS system), chalcopyrrhotite, galena, talc, barite, and amorphous silica. Mineral fabrics of the clasts indicate that the material was precipitated at or near the sea floor by mixing of hot hydrothermal fluids with cold seawater. Low concentrations of Zn, Cu, Cd, and Ag relative to those found in unsedimented ridge hydrothermal deposits, and the presence of pyrrhotite as an early phase mineral indicates that the vent fluids have been modified by reaction with sediments beneath the mound. Rapid sedimentation in a rift valley is clearly conducive to the formation of large hydrothermal mineral deposits like those believed to be present within and beneath these mounds. The relatively impermeable sediment cover insulates the crust, inhibits groundwater recharge, promotes long-lived discharge at a restricted number of sites, provides a substrate for the efficient subsurface precipitation of minerals, and through continued sedimentation, protects surficial deposits from the corrosive effects of seawater. No reliable estimate of the bulk composition of the mounds can be made with existing data, but their size is comparable to major hydrothermal mineral deposits found on land; ancient settings in which many land deposits formed are in many ways similar to the one in which the features described here are currently forming.     

Three-dimensional temperature structure of a ridge-transform-ridge system

We have solved the problem of three-dimensional heat transfer by conduction and material transport within a ridge-transform-ridge system. The boundary conditions are those of a finite plate with temperatures fixed at the top and bottom. We do not solve for the pattern of flow, so must arbitrarily describe the material transport. Within triangular regions beneath the spreading centers, flow is assumed to be vertical and at constant velocity, with the volume of upwelling matching the volume of horizontal transport by the plates. The steady state solution is approached by repeated finite steps in time, each involving two stages. First, to represent material transport, temperatures in the grid of points are shifted horizontally and/or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field. Second, a three-dimensional Fourier Transform technique is used to calculate the cooling which would occur in the static case during the same period.

We find that the geometry of flow in the upwelling region, here parameterized by velocity of upwelling or width of the upwelling zone, is more important than spreading rate in determining the temperature structure. The results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone. For a 10 m.y. offset on the transform fault, conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault, in contrast to the deepening of the median valley over several tens of kilometers observed on the Mid-Atlantic Ridge.     

Central Arctic Ocean response to Pleistocene Earth-orbital variations

Three central Arctic Ocean sediment cores were sampled for percentage carbonate, number of foraminifera, and texture. These three parameters were used in spectral analyses to test the idea that the ice-covered Arctic Ocean may respond to orbital forcing in a different manner than has been indicated for lower latitude ice-free oceans. The record for two of the cores represents approximately 1 my, and the record for the third, approximately 400,000 yr. The 100,000-yr frequency is well represented in all of the cores. A 40,000-yr frequency may be present, as well. An unexpected 70,000-yr frequency occurs in most of the spectra and may reflect nonlinear sedimentation rates or the combined effect of obliquity and eccentricity. The strong 100,000-yr signal confirms that, in spite of ice feedback, the Arctic Ocean has responded to orbital forcing in much the same manner as have ice-free oceans.     

Post-positivist social geography

Although positivist methodology has been used so extensively in social geography, it is non widely recognized that it is inadequate for describing and understanding social processes as a whole because it eschews cultural values. The behavioural reaction recognized this but still used positivist methodology. Methods may reflect scale, a macro-approach demanding a positivist approach, a micro analysis being more qualitative. Applying both to problems of assimilation in an American ethnic group revealed different processes, both necessary for a full understanding.Aggregate methods and generalized models of man go part of the way only. The quantitative/macro approach must be supplemented by the qualitative/micro. A further departure has been a recognition that social geographers must look at society's needs. Involvement in social processes and policy making is accepted by most. Whether the answer is interpreted by a liberal/concensus approach, or by a radical/revolutionary, is for the individual to decide.Finally, as geographers, social geographers cannot ignore the significance of place and region, whose specificity is sometimes critical in our perception of problems and answers.     

The study of tourism in Britain — a geographical perspective

Conclusion This review must be seen alongside others which have attempted to explore the relationship between geography (and geographers) and the study of leisure and recreation (Coppock 1980, 1982a). It serves to confirm the wide-ranging and diverse contribution that geographers have made in this field and the ways in which the study of tourism has reflected developments in the wider discipline. It also confirms the very close and intimate connection between studies of tourism and social and economic policy in Britain and indicates the way in which geographers have attempted to shape policy and have in turn been influenced by the needs and demands of policymakers. The rewards of such interaction are clear and yet costs have also been incurred by the academic geographic community. Not the least of these costs is that tourism studies have been predominantly of an empirical nature and conceptual and theoretical advance has inevitably tended to lag behind these empirical investigations. It is to be hoped that the study of the geography of tourism, having now demonstrated its credentials to policymakers and decision-makers alike, can secure for itself a more central place within geographic teaching and research so that these lacunae can be remedied.     

Numerical simulation of divided-bar thermal conductivity measurements

Summary A three-dimensional numerical model of the divided-bar thermal conductivity measuring device that can include both solid and fluid sample portions has been developed. The model has been employed to investigate the effects of porosity in divided-bar measurements and the effects of the distribution of solid sample and contained fluids in porous material on the thermal conductivity calculated from the bar temperature measurements. The results show that the positions of the temperature sensors and thus the nature of the vertical column between the temperature measuring points is of prime importance, and that the sizes of the solid and fluid portions of the sample affect the sample conductivity estimates. The divided bar measures the thermal conductivity parallel to the axis of the bar. Thermal conductivity variations in the radial direction have little effect on the calculated conductivity. Whether or not the sides of the bar are insulated strongly affects the calculated conductivity.Presented at the International Meeting on Terrestrial Heat Flow and the Structure of Lithosphere, Bechyn Castle, Czech Republic, September 2 – 7, 1991.     

Sm−Nd dating of multiple garnet growth events in an arc-continent collision zone,northwestern U.S. Cordillera

Integrated petrologic and Sm–Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P–T studies indicate a single stage of metamorphism with final equilibration at 600°C and 8–9 kbar. The Sm–Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A⁴⁰Ar/³⁹Ar age for this hornblende indicates cooling through 525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm–Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent ages of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at 128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.     

The Neupert effect: What can it tell us about the impulsive and gradual phases of solar flares?

The Neupert effect is the name given to the correlation observed in many flares between the time-integrated microwave and hard X-ray emissions and the soft X-ray emission light curve. We have used hard X-ray data from the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) and soft X-ray data from the detector on GOES to determine what fraction of all events show this correlation and how the correlation changes from the impulsive to the gradual phase. We have found that of 66 HXRBS events observed in 1980 with a peak rate of > 1000 counts s^-1, 58 (80%) showed good correlations with peaks in the GOES time derivative plot corresponding to peaks in the hard X-ray (HXR) plots to within ±20 s. In 20 of these good-correlation cases (30%), the soft X-ray (SXR) time derivative stays high after the HXR emission has decreased suggesting that the later emissions result from energy release in a loop already affected by the initial energy release. In 8 of the 13 flares that showed poor correlation, the SXR time derivative shows no peak corresponding to the initial HXR impulsive peak that has structure on a time scale of 1 s. This suggests that in these events the initial impulsive energy release results primarily in electron acceleration, and that the secondary plasma heating from the accelerated electrons contributes relatively little compared to the more gradual plasma heating already taking place at the same time. The more gradually varying events, commonly referred to as type C flares, tend to show poorer correlation between the SXR time derivative and the HXR time profile. Of 26 GOES X1 or greater flares recorded between 1980 and 1989 with HXR peaks lasting over 10 rain, 13 (50%) showed poor correlation with the gradual HXR peaks either not registering at all in the SXR time derivative plots or showing up as very broad peaks. In one case, on 1981 April 26, the SXR time derivative peak was delayed by 13 rain. Only 17 (65%) of the 26 X-flares had an earlier, impulsive component and of those, 12 (71%) showed good correlation between the impulsive peaks.