Mark H. Bulmer

The roughness of natural terrain: A planetary and remote sensing perspective

We examine the various methods and parameters in common use for quantifying and reporting surface topographic “roughness.” It is shown that scale-dependent roughness parameters are almost always required, though not widely used. We suggest a method of standardizing the parameters that are computed and reported so that topographic data gathered by different workers using different field techniques can be directly and easily intercompared. We illustrate the proposed method by analyzing topographic data from 60 different surfaces gathered by five different groups and examine the information for common features. We briefly discuss the implications of our analysis for studies of planetary surface roughness, lander safety, and radar remote sensing modeling and analysis.

Patterns of movement in rotational and translational landslides

The movement of rotational and translational landslides occurs as a result of either sliding on discrete shear surfaces or ductile deformation within a shear zone. In this paper we examine the movement histories of a number of landslides that occurred in a variety of materials in a range of settings, and demonstrate that one of two movement styles is evident during accelerating phases for all landslides observed. The first style, which has previously been noted, has a linear form in a plot of 1/v against time (v is velocity). The second style has an asymptotic form in the same plot, trending toward steady-state movement rates. We propose that the linear form occurs in landslides in which crack propagation (i.e., shear surface generation) is the dominant process, whereas the second style occurs where movement is taking place across existing planes of weakness or as a result of ductile deformation processes. This study demonstrates that the evaluation of plots of 1/v against time for landslides is a useful technique for forecasting movement styles and rates, and for interpreting ongoing deformation processes in real time

Development of progressive landslide failure in cohesive materials

The development of progressive failure in slopes is a poorly understood process, and relatively few advances in terms of the mechanics of the development of failure have been made since 1967. However, advances in the understanding of the development of fractures in brittle materials provide new insights into landslide mechanics. In addition, the link between the deformation mechanism and movement type allows interpretation of displacement records to determine the mechanisms acting within a slope. This paper utilizes these insights into deformation processes, combined with “reinflation” stress-path triaxial experiments, to propose a new model for the development of a progressive, first-time failure within a slope. This model is able to explain the development of failure when the factor of safety is greater than unity, the existence of “Saito” linearity (a linear trend when the reciprocal of velocity prior to failure is plotted against time) during tertiary creep, and the development of failures during periods of apparently increasing normal effective stress.

Small volcanic edifices and volcanism in the plains of Venus

The most widespread terrain type on Venus consists of volcanic lowland plains. Several styles of volcanism are represented in the plains. The most extensive volcanic units consist of flood lavas, the largest of which have volumes of the order of thousands of cubic kilometers. As with terrestrial flood lavas, they are inferred to have erupted at high effusion rates. They show a range of radar backscatter characteristics indicating different surface textures and ages. Small edifices on the plains occur mainly in clusters associated with fracture belts. The majority are shield volcanoes that may be up to a few tens of kilometers across but are generally 10 km or less in diameter. Volcanic cones have the same size range. Volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 km3. These are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava. For many domes, the flanks were unstable during and afte eruption and suffered gravity sliding that produced steep, scalloped outer margins. Because of the high atmospheric pressures on Venus, explosive activity is less likely to occur than on Earth. However, n a few plains areas there is evidence of pyroclastic deposits surrounding craters, indicating that volatile contents in some of the magmas may be high in comparison to Earth. The clusters of small volcanic edifices are considered to be analogous to plains volcanism, similar to that of the Snake River Plain of Idaho. There may also be analogues with terrestrial volcanic clusters associated with mid-oceanic ridges.