A conceptual model of avalanche hazard

This conceptual model of avalanche hazard identifies the key components of avalanche hazard and structures them into a systematic, consistent workflow for hazard and risk assessments. The method is applicable to all types of avalanche forecasting operations, and the underlying principles can be applied at any scale in space or time. The concept of an avalanche problem is introduced, describing how different types of avalanche problems directly influence the assessment and management of the risk. Four sequential questions are shown to structure the assessment of avalanche hazard, namely: (1) What type of avalanche problem(s) exists? (2) Where are these problems located in the terrain? (3) How likely is it that an avalanche will occur? and (4) How big will the avalanche be? Our objective was to develop an underpinning for qualitative hazard and risk assessments and address this knowledge gap in the avalanche forecasting literature. We used judgmental decomposition to elicit the avalanche forecasting process from forecasters and then described it within a risk-based framework that is consistent with other natural hazards disciplines.     

RIPARIAN VEGETATION AND SANDBAR MORPHOLOGY ALONG THE LOWER LITTLE COLORADO RIVER,ARIZONA

The distribution of riparian vegetation in relation to channel morphology is poorly understood in canyon rivers, which are characterized by in-channel fluvial sediment deposits rather than flood plains. This study focuses on vegetation and sandbar characteristics in two reaches of the lower Little Colorado River canyon in Arizona–one reach with ephemeral flow from the watershed, and another with perennial baseflow from a spring. Both reaches have been colonized by the exotic Tamarix chinensis, a riparian species known for its geomorphic influence on river channels. On the basis of a sampling of 18 bars, results show that vegetation frequency and density is significantly greater in the perennial study reach. However, sandbar morphology variables do not differ between reaches, despite a significantly narrower and deeper ephemeral channel. Hydraulic calculations of flood depths and Pearson correlations between bar and vegetation variables indicate reach-specific biogeomorphic relationships. In the ephemeral reach, higher bars are less affected by flood inundation, support older vegetation, and may be more stable habitat for vegetation. In the wider perennial reach where bars are lower and more expansive, vegetation patterns relate to bar size, Tamarix being most common on the largest bars. Overall results suggest that (1) vegetation variation relates to baseflow hydrology, (2) bar formation relates to high discharge events, and (3) vegetation patterns respond to, rather than influence, sandbar form in this canyon riparian system.     

The Major Snow Avalanche Cycle of February 1986 in the Western United States

Snow avalanches are a significant hazard in mountainous environments around the world. This paper investigates the major February 1986 avalanche cycle that occurred in the western United States, and broadly analyzes the avalanche, snowpack, and weather conditions at twenty sites. These analyses suggest that the avalanche cycle resulted from the interaction of a relatively `normal' snowpack with an exceptional storm event, which was particularly noteworthy for the amount of precipitation it produced. Composited 500-hPa anomaly maps show the event resulted from an uncommonly persistent blocking pattern that resulted in a strong zonal flow and copious moisture being funneled over the western United States. Understanding severe and widespread avalanche cycles may improve our long-term forecasting of these events, and help mitigate theresulting avalanche activity.     

Concerning KAr dating of a basalt flow from the Tahoe-Tioga interglaciation,Sawmill Canyon,southeastern Sierra Nevada,California

New KAr ages for a basalt flow interbedded with Tahoe and Tioga tills in Sawmill Canyon, southeastern Sierra Nevada, slightly refine previously published ages for the flow and provide an estimate of 53,000 ± 44,000 yr for the Tahoe-Tioga interglaciation.     

Reevaluation of multiparameter relative dating techniques and their application to the glacial sequence along the eastern escarpment of the Sierra Nevada,California

Four valleys, recently studied by other workers, were examined along the eastern Sierra Nevada to refine relative-dating techniques. A variety of weathering parameters and soil properties fail to delineate more than two major post-Sherwin Pleistocene glaciations. We correlate these two glaciations with the Tahoe and Tioga Glaciations. Type Mono Basin Till, usually considered to be pre-Tahoe, exhibits the following weathering similarities with Tahoe Till, if both are under sagebrush: (1) grusification of subsurface granitic boulders; (2) degree of pitting, mineral relief, and rind development on surface granitic boulders; and (3) very slight clay increase in the B horizon. Type Casa Diablo Till also has weathering characteristics similar to Tahoe Till, except a slightly more developed Bt horizon is present. Hence, dates on basalt of 0.126 ± 0.025 and 0.062 ± 0.013 my Casa Diablo Till also has weathering characteristics similar to Tahoe Till, except a slightly more developed Bt horizon is present. Hence, dates on basalt of 0.126 ± 0.025 and 0.062 ± 0.013 my (Bailey et al. 1976), which bracket type Casa Diablo, may provide age control on the Tahoe glaciation. In addition, we are unable to demonstrate that the Tenaya is a separate glaciation. In three of the four valleys studied our weathering data for Tenaya Till are equivalent with those for Tioga Till, but with those for Tahoe Till in the fourth valley. We were not satisfied with our ability to differentiate the Casa Diablo, Mono Basin, and Tenaya as separate glaciations even though data were collected in the type areas for two of these deposits. Reasons for suggesting a change back to a two-fold Tahoe-Tioga glacial sequence, rather than the present five-fold sequence, are that we have measured a greater number of parameters than has been done previously, soils were submitted to detailed laboratory analyses, and surface weathering features were studied under consistent present vegetation cover to avoid possible problems induced by ancient forest fires. Nevertheless our relative-dating scheme does not rule out the possibility of a more detailed glacial sequence.     

Pb---Nd---Sr isotopic constraints on the origin of the Caledonian Bindal Batholith, central Norway

The Bindal Batholith is the largest granitoid batholith in the Scandinavian Caledonides, emplaced prior to or during the Scandian collision in a complex scenario of Ordovician to Middle Silurian nappe assembly. The Bindal Batholith ranges in compositon from mafic gabbro to leucogranite, but granites and granodiorites are by far the most abundant rock types.

Pb---Pb, Sm---Nd and Rb---Sr isotopic results from plutons of the batholith constrain the origin of the Bindal Batholith magmas. The isotope results suggest the presence of several source reservoirs, giving rise to the granitoid magmas. Both a source relatively depleted in U, Th and Rb and enriched in Sm, a source enriched in U and Rb and depleted in Sm, a source enriched in Th and Rb, but depleted in Sm, and, finally, a source enriched in Th and Sm, but depleted in Rb, is indicated by the initial compositions of the radiogenic isotope ratios. It is suggested that the depleted source reservoirs were contemporaneous depleted mantle and mantle derived rocks in the nappe sequences, that the enriched source reservoir was sediments derived from Proterozoic upper crust of Baltic Shield affinity and that the Th-enriched source reservoir was various Proterozoic rocks, in a lower crustal position, of either Baltic or Laurentian affinity.     

Integrating soils and geomorphology in mountains—an example from the Front Range of Colorado

Soil distribution in high mountains reflects the impact of several soil-forming factors. Soil geomorphologists use key pedological properties to estimate ages of Quaternary deposits of various depositional environments, estimate long-term stability and instability of landscapes, and make inferences on past climatic change. Once the influence of the soil-forming factors is known, soils can be used to help interpret some aspects of landscape evolution that otherwise might go undetected.The Front Range of Colorado rises from the plains of the Colorado Piedmont at about 1700 m past a widespread, dissected Tertiary erosion surface between 2300 and 2800 m up to an alpine Continental Divide at 3600 to over 4000 m. Pleistocene valley glaciers reached the western edge of the erosion surface. Parent rocks are broadly uniform (granitic and gneissic). Climate varies from 46 cm mean annual precipitation (MAP) and 11 °C mean annual temperature (MAT) in the plains to 102 cm and −4 °C, respectively, near the range crest. Vegetation follows climate with grassland in the plains, forest in the mountains, and tundra above 3450 m. Soils reflect the bioclimatic transect from plains to divide: A/Bw or Bt/Bk or K (grassland) to A/E/Bw or Bt/C (forest) to A/Bw/C (tundra). Corresponding soil pH values decrease from 8 to less than 5 with increasing elevation. The pedogenic clay minerals dominant in each major vegetation zone are: smectite (grassland), vermiculite (forest), and 1.0–1.8 nm mixed-layer clays (tundra). Within the lower forested zone, the topographic factor (aspect) results in more leached, colder soils, with relatively thin O horizons, well-expressed E horizons and Bt horizons (Alfisols) on N-facing slopes, whereas soils with thicker A horizons, less developed or no E horizons, and Bw or Bt horizons (Mollisols) are more common on S-facing slopes. The topographic factor in the tundra results in soil patterns as a consequence of wind-redistributed snow and the amount of time it lingers on the landscape. An important parent material factor is airborne dust, which results in fine-grained surface horizons and, if infiltrated, contributes to clay accumulation in some Bt horizons. The time factor is evaluated by soil chronosequence studies of Quaternary deposits in tundra, upper forest, and plains grassland. Few soils in the study area are >10,000 years old in the tundra, >100,000 years old in the forest, and >2 million years old in the grassland. Stages of granite weathering vary with distance from the Continental Divide and the best developed is grus near the sedimentary/granitic rock contact just west of the mountain front. Grus takes a minimum of 100,000 years to form.Some of the relations indicated by the soil map patterns are: (1) parts of the erosion surface have been stable for 100,000 years or more; (2) development of grus near the mountain front could be due in part to pre-Pennsylvanian weathering; (3) a few soil properties reflect Quaternary paleoclimate; and (4) a correlation between soil development in the canyons and stream incision rates.     

Electron microscope study of peridotite xenoliths in kimberlites

Dislocation structures in naturally deformed olivine from garnet peridotite xenoliths from South African kimberlites have been studied by electron microscopy. The substructure consists mainly of straight subboundaries of dislocations with Burgers vectors [001]. Most of the dislocations have both edge and screw components, and the slip planes are mainly (100). The dislocation density between the subboundaries is low.The slip planes in olivine are discussed in relation to the olivine structure. The observed dislocation structures seem to indicate that the large difference in strain rate between natural and experimental deformation will produce a difference in the slip mechanisms.The nature of the deformation lamellae visible in optical microscope is discussed.