Crystallization calculations for a binary melt cooling at constant rates of heat removal: implications for the crystallization of magma bodies

We report on thermodynamic non-equilibrium crystallization calculations for a unit volume of a binary melt subject to a constant, prescribed rate of heat loss. Crystallization histories and crystal size distributions for both melt components were calculated by accounting for the nucleation and growth of crystals. The crystal sizes were found to decrease with increasing rates of heat loss. The crystallization time defined as the time to crystallize 99% of the unit volume also decreased strongly with increasing rates of heat loss up to a critical rate. The critical rate was found to be somewhat smaller than the heat loss rate for the beginning of glass formation. At larger than critical rates, crystallization time increased again and for rates larger than the glass formation rate, crystallization time became infinite. The residual melt composition was found to increasingly deviate from the equilibrium composition with increasing rates of heat loss. But as long as the loss rate was less than the critical rate the crystallization path reverted to the eutectic composition during the final crystallization. For supercritical rates, no such reversion was observed. We compared the critical rate with estimates of the rates of heat loss in magmatic intrusions based on the Stefan solution for a freezing half space. It was found that rates of heat loss should be supercritical at distances of up to 0.5 m from the margin of an intrusion. In this region, non-equilibrium effects are expected to dominate and the texture of the crystallized rock should be characterized by small crystals and by glass. The glass and the crystals should be of non-equilibrium composition. Non-equilibrium effects should be negligible only at distances of more than 5 m from the margin where the rates of heat loss are less than 10⁻² times critical. At these distances, the crystallized rock should have an equigranular texture and an equilibrium composition.     

Ice marginal ramps: An indicator of semi-arid piedmont glaciations

Ice marginal ramps are depositional landforms that often developed over several tens of kilometres along the ice margins of large piedmont glaciers in semi-arid environments. The ramps extend with a 7–15° gradient several kilometres or even tens of kilometres into the forelands. They have front slopes that are tens of metres to several hundred metres high and frame the former terminal basins. The front slopes and the underlying bed consist of till; in the latter case, the till thins out towards the periphery. The overlying beds contain ice contact stratified drift, which, with increasing distance from the former ice margin, is succeeded by clearly sorted glaciofluvial layers of gravel. Under semi-ari d environmental conditions, the syngenetic contribution oftwo agents of transport (glacial, glaciofluvial) in formingone accumulation complex produces stratigraphic and phenotypical features that are rare in the glacial morphology of the temperate/humid zones. For this reason, they are often misinterpreted. Being indicators of ice margins, ice marginal ramps permit the accurate reconstruction of extensive piedmont glaciations in the semi-arid highlands of subtropical latitudes. Because of their high radiation values these play a key part in the global energy balance and, thus, in the origin and the evolution of ice ages.The author gratefully acknowledges the translation of this paper rendered by Mrs. Anne Beck, Berlin.     

Topography as a fundamental element of glacial systems

The equilibrium line of glaciers as a climate-sensitive parameter is indispensable for the assessment of changes in climate through time. The methods previously developed for calculating the equilibrium line do not obtain, however, satisfactory accuracy. Using the statistical evaluation of data collected from 223 glaciers it is shown here that the inaccuracy of the prevailing methods results from the negligence of the specific glacier geometry.In calculating realistic ELAs glaciers must be understood as dynamic systems whose variables, climatic environment and topography, are linked through feedback. The accompanying transformation in this dynamic system, which is expressed by the difference between a mathematical index and the ELA, can be exactly determined with a regression line. The climatically induced change in glacier geometry is the controlling factor, i.e. operator. The behaviour of glacial systems in view of long-term climatic variations can first be understood when the details of the interdependency between topographical and climatic parameters are fully known, as will be demonstrated here.ELA = Equilibrium Line Altitude     

The Sino-German Joint Expedition to S Tibet,Shisha Pangma and the N flank of chomolungma (Mt. Everest) 1984 — Expedition report

The Sino-German Joint Expedition consisted of a group of 8 scientists from the Lanzhou Institute for Glaciology and Cryopedology and the Institute for Plateau Biology in Xining (China) as well as 3 participants from the Geographical Institute of the University of Göttingen (FR Germany). The research expedition to S Tibet and the N slope of the Himalayas was undertaken from August to November 1984, supported by technical personel and yak herders.During the 87-day field campaign studies were done in the Transhimalayas. Tibetan Himalayas, and High Himalayas on Shisha Pangma (8046 m) and under specific aspects on Chomolungma (Mt. Everest, 8848 m). The participants of the expedition worked 63 days at altitudes above 6000 m and 6500 m. The highest altitude reached during the collection of data was 7100 m, i.e. the E face of Chang La leading to the N summit of Mt. Everest. Results were attained in the areas of Pleistocene research, recent glaciology (glacier movements, ablation, albedo and firn-ice temperature measurements), neoglacial and recent glacier history, cryopedology (debris drift measurements), botany and vegetation geography. Additionally, geoecological data on all significant climatic parameters at high altitudes were collected. This work was documented on 16-mm sound movie film in cooperation with the Institute for Scientific Films (IWF, FR Germany). The expedition results were presented and discussed at the International Symposium on Tibet and High Asia, October 8–11, 1985, in Göttingen.(Presented at the International Symposium on Tibet and High Asia, October 8–11, 1985, Göttingen, FR Germany)     

Hazard assessment of the Tidal Inlet landslide and potential subsequent tsunami, Glacier Bay National Park, Alaska

An unstable rock slump, estimated at 5 to 10 × 10⁶ m³, lies perched above the northern shore of Tidal Inlet in Glacier Bay National Park, Alaska. This landslide mass has the potential to rapidly move into Tidal Inlet and generate large, long-period-impulse tsunami waves. Field and photographic examination revealed that the landslide moved between 1892 and 1919 after the retreat of the Little Ice Age glaciers from Tidal Inlet in 1890. Global positioning system measurements over a 2-year period show that the perched mass is presently moving at 3–4 cm annually indicating the landslide remains unstable. Numerical simulations of landslide-generated waves suggest that in the western arm of Glacier Bay, wave amplitudes would be greatest near the mouth of Tidal Inlet and slightly decrease with water depth according to Green’s law. As a function of time, wave amplitude would be greatest within approximately 40 min of the landslide entering water, with significant wave activity continuing for potentially several hours.