Gravimetric Study Of A Retrogressive Landslide In Southern Italy

Post-failure activity of the December 1993 Senerchia slump-earthflow was characterised by intermittent recession of the headscarp and earthflow movements. The retrogression showed considerable spatial variability, depending on the properties of the geological materials. The retrogressive failures were preceded by intense fissuring of the ground in the crown zone. Two microgravimetric surveys were carried out in order to detect possible spatial-temporal density variations in an area upslope of the headscarp. Although it was not possible to recognise any significant temporal density changes, this surveying revealed the presence of a negative anomaly which coincided with the area of maximum headscarp retreat. The gravity modelling was constrained by borehole information and new headscarp exposures produced by a series of retrogressive failures suggested that the origin of the anomaly might be associated with a hollow in an underlying clay-rich bedrock which had been subsequently filled by coarse colluvium. A possible concentration of groundwater in the hollow and its discharge towards the headscarp area controlled the local slope instability. The results of this study showed that microgravimetric surveys conducted upslope of retrogressive landslides can provide useful information on subsurface lithological heterogeneities that may control the amount and preferential direction of upslope landslide enlargement.     

Methodological frameworks for the geography of organizations

In this paper, we present three methodological frameworks for the geographic study of organizations. These are situated within three meta-theoretical perspectives in human geography: spatial science, critical realism, and post-structuralism. Each framework offers a different theorization of organizations, and each prompts different research questions that can be used to guide their geographic study. The research questions we offer are general, and are pertinent to all types of organizations. To supplement the methodological contributions of this paper, we suggest how each of these frameworks might inform empirical investigations of Appalshop, a media arts organization located in Whitesburg, Kentucky.     

Evaluating seismically induced mass movement hazard in Caramanico Terme (Italy)

Mass movements and earthquakes represent two major geological hazards in the municipal territory of Caramanico Terme (south-central Apennines). Available records revealed the contemporaneous occurrence of earthquakes and slope failures on four occasions in the last four centuries (1627, 1706, 1933, and 1984). These events, with local intensities ranging from VI to IX, generated mass movements varying from a rotational slope failure to rock/block falls. All occurred in the southern periphery of the town and involved a thick carbonate megabreccia caprock and coarse colluvia which overlie a clayey substratum. Field investigation and review of historical records helped to delimit the areas susceptible to seismically triggered rockfalls. The mapping of historic and pre-historic rockfall deposits revealed their dispersal patterns and provided the basis for a determination of potential hazard zones. We approximate the temporal hazard assessment by relating the rockfall occurrence to the probability of earthquake triggering. Considering the VI degree triggering threshold indicated by local historical data, the statistical analysis of the regional seismic activity shows that events capable of inducing rockfalls have an approximately decennial recurrence in Caramanico. The approach presented could be readily applied to other potential risk areas of Italy by exploiting the rich long-term record of historical seismicity. In general, temporal hazard estimates at relatively low intensity levels will be possible even where the seismic history of the site is only well documented for a relatively limited time interval, provided that this interval is much longer than the recurrence time of the events exceeding the threshold considered.     

Hornblende–garnet–plagioclase thermobarometry: a natural assemblage calibration of the thermodynamics of hornblende

Calibrations are presented for an independent set of four equilibria between end-members of garnet, hornblende, plagioclase and quartz. Thermodynamic data from a large internally-consistent thermodynamic dataset are used to determine the ΔG° of the equilibria. Then, with the known mixing properties of garnet and plagioclase, the non-ideal mixing in amphibole is derived from a set of 74 natural garnet–amphibole–plagioclase–quartz assemblages crystallised in the range 4–13 kbar and 500–800 °C. The advantage of using known thermodynamic data to calculate ΔG° is that correlated variations of composition with temperature and pressure are not manifested in fictive derived entropies and volumes, but are accounted for with non-ideal mixing terms. The amphibole is modelled using a set of ten independent end-members whose mixing parameters are in good agreement with the small amount of data available in the literature. The equilibria used to calibrate the amphibole non-ideal mixing reproduce pressures and temperatures with average absolute deviations of 1.1 kbar and 35 °C using an average pressure–temperature approach, and 0.8 kbar with an average pressure approach. The mixing data provide not only a basis for thermobarometry involving additional phases, but also for calculation of phase diagrams in complex amphibole-bearing systems. Received: 8 November 1999 / Accepted: 7 July 2000     

Frost grain size metamorphism: Implications for remote sensing of planetary surfaces

An understanding of the rates of frost grain growth is essential to the goal of relating spectral data on surface mineralogy to the physical history of a planetary surface. Models of grain growth kinetics have been constructed for various frosts based on their individual thermodynamic properties and on the difference in binding energy between molecules on plane vs curved faces. A steady state situation can occur on planetary surfaces in which thermal elimination of small grains competes with their creation, usually by meteorite impact. We utilize predicted grain growth rates to explain telescopic spectral data on condensate surfaces throughout the solar system. On Pluto, predicted CH₄ ice grain growth rates are very high despite the low temperature, resulting in a multicentimeter optical path. This explains the strong CH₄ absorption band depths, which otherwise would require large amounts of CH₄ gas. On the Uranian and Saturnian satellites, extremely slow grain growth rates are predicted because of the low vapor pressure of H₂O at the existing average surface temperatures. This may explain evidence for fine grain size and peculiar microstructure. On Io, ordinary thermal exchange is more effective than sputtering in promoting grain growth because of the properties of SO₂. Over much of Io's disk, submicron size grains of SO₂ could plausibly reconfigure into a surface glaze on a timescale comparable to the resurfacing rate. This may explain the relatively strong SO₂ signature in Io's infrared absorption spectrum as opposed to its weaker manifestation in the visible spectrum. In spite of lower sputtering fluxes, sputtering plays a more important role in grain growth for Europa, Ganymede, and Callisto than on Io. This is a result of high rates of thermally activated grain growth and resurfacing on Io. The sequence of H₂O-ice absorption band depths (related to the mean grain size) is J₂(T) ～ J₃(T) > J₂(L) > J₃(L) ～ J₄(T) ～ J₄(L), where L = leading and T = trailing. This is to be expected if sputtering were dominant. The calculations show, however, that neither thermalized exchange fluxes nor sputtering exchange fluxes can produce the implied grain growth or the ordering by ice absorption band depths of the six satellite hemispheres. Only sputtering control by simple ejection of H₂O from the satellites, as the dominant cause of shorter mean lifetimes for smaller exposed grains, can satisfactorily explain the data. Some observations, which suggest that there are vertical grain size gradients, may result from a steady state balance between intense near surface production of fine frost by comminution, coupled with ongoing ubiquitous grain growth in the vertical column. In certain cases, e.g., Europa and Enceladus, the possibility exists that endogenic activity as well as comminution could affect grain size—at least locally. It is concluded that not only ice identification and mapping, but ice grain size mapping is an important experiment to be conducted on future missions.     

High altitude Venus haze from Pioneer Venus limb scans

High vertical resolution scans of the Venus limb made by the Pioneer Venus Orbiter Cloud Photopolarimeter at 365 nm and 690 nm wavelengths are used to investigate the level of the haze top, and haze particle properties and scale height. Haze particle vertical optical depth 0.01 occurs at altitude 80 to 85 km based on knowledge of instrument pointing. The lowest haze tops were observed close to subsolar longitudes but the data set supports a longitude dependence no more than a temporal variation. Single scattering computations for a spherical shell atmosphere show good agreement with observed intensities for particles smaller than 0.3 μm radius and refractive index less than 1.7, consistent with, but not limited to, concentrated sulfuric acid. Particle scale height in the 0.5 to 2 mbar pressure regions varies between 1 and 3 km over the season (12 of 92 days), latitude (15–45°N), and local time (0900–1800) ranges of the observations. Detached layers of haze are sometimes present. An average particle scale height of 2.2 km at 84 km altitude yields an eddy diffusion coefficient of 1.3 × 10⁵ cm² sec^?1.     

Geophysical surveys on the East Pacific Rise — Galapagos Rise system

Summary. The East Pacific Rise at 12–15° S is topographically smooth with a crestal horst or linear volcanic peak marking the present axis of spreading. The Galapagos Rise at 14–17° S is topographically rough with a possible central graben marking the extinct spreading axis. The seafloor spreading magnetic anomalies on the East Pacific Rise are of low amplitude, but fracture-zone anomalies at 13–14° S have amplitudes of up to 1250 nT. Anomalies of this amplitude at the magnetic equator must be formed within the fracture zone by some combination of block reversal boundaries, anomalously-high magnetic intensities, and/or anomalously-large thicknesses of the magnetic layers within the fracture zone. Magnetization and major-element chemical analyses of basalts dredged from four locales along the fracture zone indicate that the large magnetic-anomaly amplitudes are caused by the high iron and titanium content of these ferrobasalts. The magnetic-anomaly profiles from the Galapagos Rise and its fracture-zone system are of normal amplitude and are extremely difficult to correlate internally or with the geomagnetic timescale.
Eighty-one heat-flow measurements indicate that the values measured are controlled by sediment thickness. Where the thickness of the sediment blanket is greater than 100 m, high heat flow is measured and possibly is representative of the total heat transfer at the seafloor. Where the sediment thickness is less than 100 m, seawater circulation in the oceanic crust is thought to remove most of the heat convectively; thus causing low conductive heat-flow values to be measured by the usual heat-flow apparatus. The heat loss by convective processes is probably a function also of topographic roughness and sediment permeability.     

Temperature conditions for chondrule formation

Abstract— The liquidus temperatures of chondrules range from about 1200 °C to almost 1900 °C, based on the calculation of Herzberg (1979). Dynamic melting and crystallization experiments with no external seeding suggest that some chondrule textures formed with initial temperatures below the liquidus (e.g., porphyritic, granular) and some were completely melted (e.g., excentroradial, glassy). Type I and III chondrules in carbonaceous chondrites in this interpretation consist of incompletely melted magnesian chondrules, completely melted silica-rich chondrules and intermediate composition chondrules with both porphyritic and nonporphyritic textures. A similar pattern for ordinary chondrites, with data also for Type II porphyritic and barred olivine chondrules, suggests that few chondrules with liquidus temperatures over 1750 °C were completely melted and few with under 1400 °C were incompletely melted. The range of liquidus temperatures for barred olivine chondrules, for which initial temperatures appear to have been essentially at the liquidus, is similar. Most chondrules may therefore have been heated to temperatures of 1400–1750 °C and, because of a peak in the distribution of barred olivine chondrule temperatures at 1500–1550 °C, the temperatures appear normally distributed within this range. Given a narrow range of temperatures, bulk composition is at least as important as initial temperature in controlling chondrule textures. Truly granular (not microporphyritic) Type I and truly glassy Type II and III chondrules appear under-represented in nature according to this model, based on internal nucleation experiments. External heterogeneous nucleation, or seeding due to droplet-dust collisions, is likely to occur in a dusty nebula and has been shown to reproduce chondrule textures experimentally. Generally high initial temperatures (1600–1800 °C), coupled with dust-seeding of superheated droplets of less refractory composition is an alternative explanation of chondrule textures. Cooling rates of 100–1000 °C/hr are required for chondrules, which must have been mass produced in clouds with sufficient particle density to buffer cooling rate and perhaps also initial temperature. Melting precursor particles in a thick clump and/or the nebular mid-plane would provide evaporation and thus explain the high oxidation state and volatile content of chondrules, relative to the bulk hydrogen-rich nebula, as well as the nature of the cooling.