URBAN RESIDENTIAL RENOVATION AND RENEWAL: MODELING BY GRAPHIC SYNTHESIS

Houses in inner-city areas, because of their age, suffer the depredations of functional obsolescence and decay. Some, however, have sound locations and are able to assemble economic forces that precipitate rehabilitation by both renters and owners. Accompanying restoration is a concomitant increase in market value and in population density, the corollary of which is a reduction in lot size. This process is modeled by graphically synthesizing elements of location theory, urban growth theory, and principles of economics.     

Water and the density of silicate glasses

A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial molar volume of water is 12.0 ± 0.5 cm³/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H₂O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO₂–M₂O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H₂O is about 4 × 10⁻⁵ K⁻¹ in silicate glasses. Received: 30 June 1999 / Accepted: 5 November 1999     

The role of H2O in rapid emplacement and crystallization of granite pegmatites: resolving the paradox of large crystals in highly undercooled melts

Granite pegmatite sheets in the continental crust are characterized by very large crystals. There has been a shift in viewing pegmatites as products of very slow cooling of granite melts to viewing them as products of crystal growth in undercooled liquids. With this shift there has been a renewed debate about the role of H₂O in the petrogenesis of pegmatites. Based on data on nucleation of minerals and new viscosity models for hydrous granite melts, it is argued that H₂O is the essential component in the petrogenesis of granite pegmatites. H₂O is key to reducing the viscosity of granite melts, which enhances their transport within the crust. It also dramatically reduces the glass transition temperature, which permits crystallization of melts at hundreds of degrees below the thermodynamic solidus, which has been demonstrated by fluid inclusion studies and other geothermometers. Published experimental data show that because H₂O drastically reduces the nucleation rates of silicate minerals, the minerals may not be able to nucleate until melt is substantially undercooled. In a rapidly cooling intrusion, nucleation starts at its highly undercooled margins, followed by inward crystal growth towards its slower-cooling, hotter core. Delay in nucleation may be caused by competition for crystallization by several minerals in the near-eutectic melts and by the very different structures of minerals and the highly hydrated melts. Once a mineral nucleates, however, it may grow rapidly to a size that is determined by the distance between the site of nucleation and the point in the magma at which the temperature is approximately that of the mineral’s liquidus, assuming components necessary for mineral growth are available along the growth path. Granite pegmatites are apparently able to retain H₂O during most of their crystallization histories within the confinement of their wall rocks. Pegmatitic texture is a consequence of delayed nucleation and rapid growth at large undercooling, both of which are facilitated by high H₂O (±Li, B, F and P) contents in granite pegmatite melts. Without retention of H₂O the conditions for pegmatitic textural growth may be difficult to achieve. Loss of H₂O due to decompression and venting leads to microcrystalline texture and potentially glass during rapid cooling as seen in rhyolites. In contrast, slow cooling within a large magma chamber promotes continuous exsolution of H₂O from crystallizing magma, growth of equant crystals, and final solidification at the thermodynamic solidus. These are the characteristics of normal granites that distinguish them from pegmatites.     

Near-bed mass flux profiles in aeolian sand transport: high-resolution measurements in a wind tunnel

Vertical profiles of the streamwise mass flux of blown sand in the near-bed (< 17 mm) region are analysed from high-resolution measurements made using an optical sensor in a wind tunnel. This analysis is complemented by detailed measurements of mass flux and mean velocity profiles throughout the boundary layer depth (0·17 m) using passive, chambered sand traps of small dimensions and armoured thermal anemometers, respectively. The data permit a preliminary analysis of the relations between the observed forms of the profiles of near-bed fluid stress and horizontal mass flux within a carefully conditioned boundary layer. Profiles of mass flux density are found to be characterized by three regions of differing gradient with transitions at about 2 mm and 19 mm above the bed. The exponential decay of mass flux with height is confirmed for elevations above 19 mm, and when plotted as a function of u_*²/g (a parameter of mean vertical trajectory height in saltation), the gradient of mass flux in this region scales with the wake-corrected friction velocity (u), where u > 0·30 m s⁻¹. A separate near-bed region of more intense transport below 19 mm is identified which carries 80 per cent of the total mass flux. This region is evident in some previous field and wind tunnel data but not in profiles simulated by numerical models. Ventilated passive sand traps underestimate mass flux in this region by 37 per cent. At slow or moderate wind speeds a third significant region below 2 mm is observed. These regions are likely to be related to grain populations in successive saltation, low-energy ejections and intermittent bed contact, respectively. Optical measurements reveal locally high grain concentrations at some elevations below 5 mm; these heights scale with transport rate, mass flux gradient and wind speed. Copyright © 1999 John Wiley & Sons, Ltd.     

Ca II K Emission Line Asymmetries Among Red Giants

In the spectra of red giants the chromospheric emission feature found in the core of the Ca II K line often exhibits an asymmetric profile. This asymmetry can be documented by a parameter V/R which is classified as > 1, 1, or < 1 if the violet wing of the emission profile is of greater, equal, or lower intensity than the redward wing. A literature search has been conducted to compile a V/R dataset which builds on the large survey of bright field giants made by Wilson (1976). Among stars of luminosity classes II–III–IV the majority of those with V/R > 1 are found to be bluer than B-V =1.3, while those with V/R < 1 are mostly redder than this colour. Stars with nearly symmetric profiles, V/R≈ 1, are found throughout the colour range 0.8 < B-V < 1.5. There is no sharp transition line separating stars of V/R > 1 and < 1 in the colour-magnitude diagram, but rather a ‘transition zone’ centered at B-V ≈ 1.3. The center of this zone coincides closely with a ‘coronal dividing line’ identified by Haish, Schmitt and Rosso (1991) as the red envelope in the H–R diagram of giants detected in soft x-ray emission by ROSAT. It is suggested that both the transition to a Ca II K emission asymmetry of V/R < 1 and the drop in soft x-ray activity across the coronal dividing line are related to changes in the dynamical state of the chromospheres of red giants. By contrast, the onset of photometric variability due to pulsation occurs among stars of early-M spectral type, that are redward of the mid-point of the Ca II V/R ‘transition zone’, suggesting that the chromospheric motions which produce an asymmetry of V/R < 1 are established prior to the onset of pulsation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rheology of arc dacite lavas: experimental determination at low strain rates

Andesitic–dacitic volcanoes exhibit a large variety of eruption styles, including explosive eruptions, endogenous and exogenous dome growth, and kilometer-long lava flows. The rheology of these lavas can be investigated through field observations of flow and dome morphology, but this approach integrates the properties of lava over a wide range of temperatures. Another approach is through laboratory experiments; however, previous studies have used higher shear stresses and strain rates than are appropriate to lava flows. We measured the apparent viscosity of several lavas from Santiaguito and Bezymianny volcanoes by uniaxial compression, between 1,109 and 1,315?K, at low shear stress (0.085 to 0.42?MPa), low strain rate (between 1.1?×?10^?8 and 1.9?×?10^?5?s^?1), and up to 43.7 % total deformation. The results show a strong variability of the apparent viscosity between different samples, which can be ascribed to differences in initial porosity and crystallinity. Deformation occurs primarily by compaction, with some cracking and/or vesicle coalescence. Our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates. At lava flow conditions, no evidence of a yield strength is observed, and the apparent viscosity is best approached by a strain rate- and temperature-dependent power law equation. The best fit for Santiaguito lava, for temperatures between 1,164 and 1,226?K and strain rates lower than 1.8?×?10^?4?s^?1, is $ \log {\eta_{\text{app}}} = - 0.738 + 9.24 \times {10^3}{/}T(K) - 0.654 \cdot \log \dot{\varepsilon } $ where η _app is apparent viscosity and $ \dot{\varepsilon } $ is strain rate. This equation also reproduced 45 data for a sample from Bezymianny with a root mean square deviation of 0.19 log unit Pa?s. Applying the rheological model to lava flow conditions at Santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows, enabling highly viscous lava to travel long distances.