Scaling impact melting and crater dimensions: Implications for the lunar cratering record

Abstract— The dimensions of large craters formed by impact are controlled to a large extent by gravity, whereas the volume of impact melt created during the same event is essentially independent of gravity. This “differential scaling” fosters size-dependent changes in the dynamics of impact-crater and basin formation as well as in the final morphologies of the resulting structures. A variety of such effects can be observed in the lunar cratering record, and some predictions can be made on the basis of calculations of impact melting and crater dimensions. Among them are the following: (1) as event magnitude increases, the volume of melt created relative to that of the crater will grow, and more will be retained inside the rim of the crater or basin. (2) The depth of melting will exceed the depth of excavation at diameters that essentially coincide with both the inflection in the depth-diameter trend and the simple-to-complex transition. (3) The volume of melt will exceed that of the transient cavity at a cavity diameter on the order of the diameter of the Moon; this would arguably correspond to a Moon-melting event. (4) Small lunar craters only rarely display exterior flows of impact melt because the relatively small volumes of melt created can become choked with clasts, increasing the melt's viscosity and chilling it rapidly. Larger craters and basins should suffer little from such a process. (5) Deep melting near the projectile's axis of penetration during larger events will yield a progression in central-structure morphology; with growing event magnitude, this sequence should range from single peaks through multiple peaks to peak rings. (6) The minimum depth of origin of central-peak material should coincide with the maximum depth of melting; the main central peak in a crater the size of Tycho should have had a preimpact depth of close to 15 km.     

The Late-Devensian proglacial Lake Humber: new evidence from littoral deposits at Ferrybridge, Yorkshire, England

Proglacial Lake Humber is of UK national significance in terms of landscape drainage and development of the British Ice Sheet (BIS) during Marine Isotope Stage 2 (MIS 2), yet it is poorly understood in terms of its dynamics and timing. Sands and gravels exposed at Ferrybridge, West Yorkshire, UK, are interpreted as part of the Upper Littoral sands and gravels related to a high-level Lake Humber, which inundated the Humber Basin to ∼30 m OD during MIS 2. Excavations exposed well-rounded gravels of local origin extending downslope from the 27.5 m OD contour and interbedded sands and fine gravels, which are interpreted as the coarse littoral deposits and nearshore associated deposits. A sample from the distal sands returned an Optically Stimulated Luminescence age of 16.6±1.2 kyr, providing the first direct age for the high-level lake and for when North Sea Basin ice must have blocked the Humber Gap. An underlying sequence included a diamicton dated to after 23.3 ±1.5 kyr and before 20.5±1.2 kyr, indicating that the Late Devensian ice reached at least 15 km south of the Escrick Moraine prior to the high-level lake. Previous to both the high-level lake and this ice advance, loess found at the two sites investigated indicates a long period of loess deposition earlier in MIS 2. These new data for the history of Lake Humber are discussed in the context of ice-marginal oscillations in both the Vale of York and the North Sea Basin.     

Exceptionally long, narrow drumlins formed in subglacial cavities, North Dakota

An array of about 200 exceptionally long and narrow drumlins occurs in north-central North Dakota. The most prominent of the drumlins is 27 km long, and their average length is between 2 km and 3 km. The drumlin field is closely associated with, and occurs immediately up-glacier from, an extensive area of ice-thrust topography. Most of the drumlin ridges have small glacier-thrust masses at their heads. Internal structures are complex, but typically include large numbers of gravity faults, which dip away from the center of the drumlin ridges. Materials contained in the drumlins were transported from higher pressure areas beneath the glacier, inward toward lower pressure areas (cavities) by flowing, thrusting, squeezing and other processes. The close and systematic association of the long drumlins with nearby areas of ice-thrust topography indicates a genetic tie; conditions that caused the glacier to form large ice thrusts also contributed to drumlin formation. Major factors involved in the formation of the drumlins were high porewater pressures in interbedded permeable and impermeable materials beneath a thin, swiftly flowing glacier, and the presence of areas of frozen ground near the margin of the glacier.     

Transmission electron microscope study of quartz microstructures produced by aeolian bombardment

The near-surface structure produced by laboratory aeolian bombardment of Brazilian quartz has been studied with transmission electron microscopy (TEM). Thin specimens of the abraded surface were prepared by a one-sided ion etching technique, and control specimens of unabraded material were prepared by standard ion etching methods. The abraded sample exhibited a vastly larger number of defects than the unabraded specimen. The abraded sample contained areas with very little surface disruption, presumably where pieces of material up to several tens of microns in diameter had broken away. Other areas of the abraded surface were extremely disrupted, with open and closed microcracks spaced from a few tens of angstroms to a few microns apart. Electron diffraction indicated significant rotation among submicron blocks bounded by the fractures. These observations suggest that surface disruption occurs primarily by a mechanism involving brittle fracture, rather than plastic deformation. This study demonstrates the feasibility of studying the effects of abrasion on near-surface structure with TEM. The degree and scale of fracturing also suggest a mechanism for forming submicron particles in quantity during aeolian abrasion.     

CONSTRUCTING,VISUALIZING, AND ANALYZING A DIGITAL FOOTPRINT*

ABSTRACT. Herein, we discuss the desire for new technology, the need for security, and the right to privacy; in doing so, we argue that each of these concerns comprises an important, tripartite debate. To highlight the complexities in this problem, we define our notion of a “digital footprint” and introduce Big Foot—specialized software created for the research described here to facilitate visualization and exploration of the data that comprise Stephen Weaver's personal digital footprint. Using Big Foot we demonstrate how multiple digital personae can be created from the data that constitute one unique digital footprint and provide a methodology for understanding the good and bad impacts that new technologies may have on future societies. One of the primary arguments of this work is that the debate—though not formally recognized—is currently before contemporary society and must receive sufficient attention.     

ORNITHOPHILIA: THOUGHTS ON GEOGRAPHY IN BIRDING

The deeper motives of bird‐watchers have rarely been subjected to geographical inquiry. Birders are sometimes dismissed as hobbyists bent on compensating for feelings of inadequacy and lack of control in their personal lives. In this article, utilizing textual references as well as experiences from my own participant‐observer status as geographer‐cum‐birder and bird‐tour leader, I construct a geographically oriented approach to understanding the fascinations of bird‐watching. I detail ethnographically the annual Christmas Bird Count and a bird walk in the Honduran rain forest. Then, drawing from the nest‐as‐home metaphors of Gaston Bachelard and the “becoming‐bird” relationships suggested by Gilles Deleuze and Félix Guattari, I position birding as extraordinarily intimate exploration of place, reinforced by anticipation, repetition, experience of beauty, and the culminating encounter of human self, bird or bird spectacle, and landscape.