Stratified flow in pyroclastic surges

Stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions. Particle transport is assumed to be by turbulent suspension, and calculations contained herein show that this is likely for many cases including the 18 May 1980 blast at mount St. Helens. The discussion centers on the Rouse number (Pn), which represents a ratio of particle settling velocity to scale of turbulence; the Brunt-Väisälä frequency (N), which is the maximum possible frequency of internal waves; the Froude number (Fr), representing the ratio of inertial forces to gravitational forces; and the Richardson number (Ri), a ratio of buoyant restoring forces to turbulent mixing forces. The velocity or flow power dependence of bed-form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge. This produces a decrease in dune wavelength with increasing distance from vent. Migration direction of bed forms is related toFr as it is defined for a continuously stratified flow. Proximal to distal facies variations in surge deposits reflect increasingPn andRi as the flows move away from their sources. This produces the progression from sandwave to massive to planar facies with increasing distance from vent. Where the long axis of topography is at low angles to the flow direction, massive facies in topographic lows may from concurrently with sandwave facies on highs, due to the higher particle concentration in the lows. Where long axis of topography is at high angles to flow direction, denser lower parts of the surge may be dammed or blocked. Blocked material tends to form massive flows that may move down slope independent of the overriding surge. A model incorporating turbulent transport, stratified flow, and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers. During the initial high energy (waxing) phase of the eruptive event,Pn is sufficiently low that only coarse, but poorly sorted, material is deposited to form relatively coarse bottom layers. As the event wanes, remaining finer material is deposited through a thin bed load to produce overlying bedded and cross-bedded veneer deposits. Throughout most of the event, blocking occurs to produce relatively thick and massive deposits in valley bottoms.     

On‐line Dangers?: Geographies of Parents’ Fears for Children's Safety in Cyberspace

Children are considered particularly important in debates about the possibilities and dangers of information and communication technologies (ICT). Discourses on ICT contain paradoxical representations of childhood. On the one hand, unlike most other understandings of child/adult relations, these discourses assume children to be equally, if not more, technologically competent than adults. On the other hand, children's very competence at using ICT is alleged to be putting them “at risk” of abuse or corruption. This paper addresses these moral panics about children and ICT by exploring to what extent and why parents are concerned about their children's safety in on‐line space. In doing so the paper reflects on the extent to which anxieties about children in cyberspace replicate concerns about public outdoor space and the way networked computers emerge as different tools in different households.     

The primacy of top-down effects in shallow benthic ecosystems

Individual scientists, scientific organizations, and government agencies have all concluded that eutrophication is among the most detrimental of all human activities in coastal ecosystems; very large amounts of funding have been earmarked to study the negative consequences of nutrient pollution. Most studies of eutrophication have been conducted long after the numbers and diversity of larger marine consumers were dramatically reduced by centuries of intense harvesting. It is now understood that these once abundant predators played pivotal roles in regulating ecosystem structure and function, and that the widespread overharvesting of large consumers can trigger indirect effects that alter species compositions in ways that are very similar to those reported to result from eutrophication. All of this suggests that we should reevaluate whether the many negative effects attributed to eutrophication are actually a result of nutrient additions or whether they may be the result of the indirect effects of dramatically altered coastal food webs. In this essay, we review experimental assessments of the degree to which changes in consumer abundances have indirectly altered the structure of benthic ecosystems in coastal waters, and on the relative importance of top-down and bottom-up effects on coral reefs, rocky shores, and seagrass meadows. We find that the evidence clearly indicates that indirect consumer effects are the primary drivers of coastal benthic ecosystem structure and function.     

Theorizing and Researching Intersectionality: A Challenge for Feminist Geography*

This article focuses on the concept of intersectionality, which is being used within the wider social sciences by feminists to theorize the relationship between different social categories: gender, race, sexuality, and so forth. Although research within the field of feminist geography has explored particular interconnections such as those between gender and race, the theoretical concept of intersectionality as debated in the wider social sciences has not been addressed. This article attempts to respond to that omission. It begins by tracing the emergence of debates about the interconnections between gender and other identities. It goes on to reflect on attempts to map geometries of oppressions. The emphasis then moves from theorizing intersectionality to questioning how it can be researched in practice by presenting a case study to illustrate intersectionality as lived experience. The conclusion demonstrates the contribution that feminist geography can make to advance the theorization of intersectionality through its appreciation of the significance of space in processes of subject formation. It calls for feminist geography to pay more attention to questions of power and social inequalities.     

A method to solve the paleotectonic analysis problem

The method to determine the rates of tectonic movements is based on the use of dynamic models of sedimentary basins. A standard dataset should be provided as input information: the present-day position of sedimentary layer interfaces (chronostratigraphic boundaries), land surface and basement, the layer ages, and the uncertainty limits within which the depth of sedimentation of each layer may have differed. In terms of dynamic models, the paleotectonic analysis problem is reduced to the determination of such tectonic rates that, at prescribed reference times, the model surface topography within the assumed limits would be constrained. At the final moment, the interfaces would be brought into agreement with the contemporary geologic cross-section. The analysis the problem of tectonic rates determination has shown that it has no unique solution. One of the ways to obtain the unique solution is to seek it within a prescribed class of functions, for example, the Fourier series. This method differs from the paleotectonic analysis methodology in that it treats the tectonic rates of motion as functions of time and spatial variables. Under certain conditions, it proves feasible to reconstruct the rates of tectonic movements not only within the time intervals represented in the deposited strata, but within periods of erosion as well. It also is possible to take into account the deformation-induced changes in thickness of the layers. The method's application is illustrated with an example of the Terek-Caspian Trough. As follows from the computation, the tectonic movements since the Middle Jurassic may be presented as a sum of two components: an overall slowing-down subsidence whose rate is proportional to the square root of the age, and local movements which follow a regular oscillatory time pattern with a period of 60–70 MY. The character of the local movements is such that the profile appears to break into a northern and southern segment. When one was being uplifted, the other segment was sinking, andvice versa. These two segments are separated by a deep-seated fault. This may have been a result of an external compression on the trough.     

Spatial and diurnal distribution of invertebrate and fish fauna of a Zostera marina bed and nearby unvegetated sediments in Damariscotta River, Maine (USA)

Fish, epibenthos and macroinfauna were collected in a Zostera marina bed and nearby unvegetated sediments in the estuary of the Damariscotta River, on the mid-coast of Maine. Samples of epibenthic fauna and fish were collected at low tides both during day and night, and samples of infauna at low tides during the day. The mean density of Zostera shoots in the study area was 335 m⁻². Abundance and species number of fish were greater at night than during the day and greater in eelgrass beds (Z. marina) than in unvegetated habitats. Daytime fish collections were dominated by Atlantic silversides (Medinia medinia), while juvenile winter flounder (Pseudopleuronectes americanus) dominated night collections. Also Zostera-associated epifaunal abundances and number of species were significantly higher at night than during the day. Mysis stenolepis, Idotea balthica and Littorina obtusata were dominant species in the epifauna samples. Of the total of 37 invertebrate species encountered, only five occurred both in the infaunal and epifaunal samples. Nineteen different taxa were collected from the benthic core samples. The most abundant invertebrate infaunal taxa were sipunculids, the polychaete Nereis virens, and oligochaetes. Infaunal invertebrate abundances and species diversity were significantly higher in eelgrass beds than in unvegetated sediments. The abundance and number of species of benthic invertebrates were also positively correlated to seagrass biomass. Community diversity values (H′) were relatively low but fit well in the general pattern of decreasing diversity towards northern latitudes.     

Macroinvertebrate Production in the Submerged Aquatic Vegetation of the Mobile–Tensaw Delta: Effects of an Exotic Species at the Base of an Estuarine Food Web

This study, conducted in 1997, reports the first estimates of the impacts of the proliferation of an exotic submerged aquatic vegetation (SAV) species (Myriophyllum spicatum) on macroinvertebrate production via comparisons with two co-occurring native SAV species (Heteranthera dubia and Vallisneria americana) in the tide-influenced Mobile–Tensaw Delta (located in the north-central Gulf of Mexico, 30°40′ N, 87°55′ W). Production of macroinvertebrates was greatest on M. spicatum and H. dubia and least on V. americana. The key determinant of these differences was a greater abundance of amphipods (Gammarus mucronatus) found on the leaves of M. spicatum and H. dubia. Macroinvertebrate production on M. spicatum was three times greater (>1 kg m^?2 year^?1) than on either of the native SAV species. No-choice palatability tests showed that these differences could not be attributed to differences in invertebrate grazing on these plants. Instead, it is probable that the high production within the structurally complex M. spicatum and H. dubia was the result of reduced predator foraging efficiency. If true, then the presence of this exotic species probably renders this elevated production inaccessible to most high-order predators.     

Foreign Direct Investment in the Czech Republic

Children are considered particularly important in debates about the possibilities and dangers of information and communication technologies (ICT). Discourses on ICT contain paradoxical representations of childhood. On the one hand, unlike most other understandings of child/adult relations, these discourses assume children to be equally, if not more, technologically competent than adults. On the other hand, children’s very competence at using ICT is alleged to be putting them “at risk” of abuse or corruption. This paper addresses these moral panics about children and ICT by exploring to what extent and why parents are concerned about their children’s safety in on-line space. In doing so the paper reflects on the extent to which anxieties about children in cyberspace replicate concerns about public outdoor space and the way networked computers emerge as different tools in different households.     

Impacts of a Habitat-Forming Exotic Species on Estuarine Structure and Function: An Experimental Assessment of Eurasian Milfoil

It is widely believed that successful colonization of ecosystems by non-native species will have catastrophic consequences for the recipient system. Within the Mobile–Tensaw Delta, AL, exotic Eurasian milfoil (Myriophyllum spicatum) has been reported to trigger degradation of ecosystem structure and function. We evaluated the impacts of structurally complex milfoil on food web structure and predator-prey interactions via comparisons with two native grasses, structurally simple wild celery (Vallisneria americana) and the more complex water stargrass (Heteranthera dubia). While significant differences were not detected in the faunal compositions of milfoil and stargrass habitats, significant differences between milfoil and wild celery were found. Laboratory experiments showed that rainwater killifish, a key contributor to these differences, preferred milfoil over wild celery, but did not occupy milfoil more than stargrass. Subsequent experiments indicated that survivorship was drastically lower in wild celery. Though many of the documented impacts of Eurasian milfoil have been cast as detrimental, shelter-seeking organisms may perceive milfoil in the same way as other complex native species.