Glacial indicator dispersal processes: a conceptual model

Larson, Phillip; Mooers, Howard D.

doi:10.1080/03009480410001262

Cited by 10 publications

(2 citation statements)

References 24 publications

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“…He thought that, in an "ideal" situation, the concentration, c, of indicators would decrease with distance, x, from the source following a "negative exponential" pattern. Larson and Mooers (2004) showed analytically that the depth-averaged concentration of indicators should fall off hyperbolically with distance. Their relation can be written as…”

Section: Time Scale For Formation Of a Dispersal Plumementioning

confidence: 98%

Genesis of dispersal plumes in till

et al. 2013

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In regions formerly covered by continental ice, till sheets may contain distinctive clastic particles derived from local bedrock sources such as ore bodies. Such particles, especially in thicker tills, may be distributed in three-dimensional dispersal trains or plumes. Developments in our understanding of glacial erosion, entrainment, and deposition over the past two or three decades help clarify formation of these plumes. Much of the debris in the basal ice of ice sheets is likely incorporated at places where water is freezing onto the base of the ice sheet. This water, largely a product of melting of basal ice further upglacier, has migrated downglacier under the influence of a gradient in the hydraulic potential controlled primarily by the ice surface slope and secondarily by basal topography and thermal regime. Refreezing occurs over a substantial distance along flow, so as the ice moves away from an ore body, material eroded from the body is later elevated above the bed by refreezing of more meltwater, incorporating additional material derived from the country rock. Broadening and mixing of the plume, both vertically and horizontally, can occur by shear in the ice, by shifting ice flow directions, by collisions between particles, and by folding of basal ice layers. Further downflow, where net basal melting resumes, the debris is deposited by meltout or lodgement. Many idiosyncrasies of dispersal plumes are likely caused by non-steady-state changes in basal thermal regime.Résumé : Dans des régions anciennement recouvertes de glace continentale, les couches de till peuvent contenir des particules clastiques distinctives provenant du socle rocheux local, p. ex des amas minéralisés. De telles particules, surtout dans les tills à grande épaisseur, peuvent être distribuées dans des trains ou des panaches de dispersion à trois dimensions. Au cours des deux à trois dernières décennies, une meilleure compréhension de l'érosion glaciaire, du transport et de la déposition a aidé à expliquer la formation de ces panaches. Une grande partie des débris dans la couche de glace basale est sans doute incorporée à des endroits où l'eau gèle à la base de la nappe glaciaire. Cette eau, provenant en grande partie de la fusion de la couche de glace basale plus en amont du glacier, a migré vers l'aval sous l'influence d'un gradient du potentiel hydraulique, lequel est surtout contrôlé par la pente de la surface glaciaire et puis par la topographie à la base du glacier et le régime thermique. Le regel a lieu sur une grande distance le long de l'écoulement; donc, à mesure que la glace s'éloigne d'un amas minéralisé, le matériel érodé de l'amas est par la suite élevé au-dessus du lit par le regel d'autre eau de fonte, incorporant du matériel additionnel provenant de la roche encaissante. L'élargissement du panache et le mélange à l'intérieur de celui-ci, horizontalement et verticalement, peuvent se produire par le cisaillement de la glace, le changement de direction d'écoulement de la glace, des collisions entre les particules et le p...

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Section: Time Scale For Formation Of a Dispersal Plumementioning

confidence: 98%

Genesis of dispersal plumes in till

et al. 2013

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“…Nevertheless, although in some rock-types, a lower limit to block strength may be related to a minimum structural block size (Dreimanis & Vagners, 1971;Lim et al, 2004;Domokos et al, 2015) this is unlikely to pertain to granite which breaks-down to grus at the scale of the phenocrysts. None-the-less, fracture and surface wear, to an initial block population, tend to result in the observed block population consisting of those blocks which are strongly resistant to further comminution (Moss, 1972;Tavares and King, 1998;Larson & Mooers, 2004;Pfeiffer et al, 2022) which, in principle, enables some blocks to survive transport adjacent to the sole of the ice for great distances before being deposited during the waning phase of the easterly directed ice stream (Hallet, 1979). Thus, although there may be no lower effective block size, a statistical increase in resistance to fracture of the block population with distance is likely evident as witnessed by the increased rounding seen in the Teesdale population.…”

Section: General Considerationsmentioning

confidence: 99%

Coevolving edge rounding and shape of glacial erratics; the case of Shap granite, UK

Carling

2023

Preprint

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Abstract. The size distributions and the shapes of detrital rock clasts can shed light on the environmental history of the clast assemblages and the processes responsible for clast comminution. For example, mechanical fracture due to the stresses imposed on a basal rock surface by a body of flowing glacial ice releases initial ‘parent’ shapes of large blocks of rock from outcrop, which then are modified by the mechanics of abrasion and fracture during subglacial transport. The latter processes produce subsequent generations of shapes, possibly distinct in form from the parent blocks. Lacking is a complete understanding of both the processes responsible for block shape changes and the trends in shape adjustment with time and distance away from the source outcrop. Field data on edge rounding and shape changes of Shap granite blocks (dispersed by Devensian ice eastwards from outcrop) are used herein to explore the systematic changes in block form with distance from the outcrop. The degree of edge rounding for individual blocks increases in a punctuated fashion with the distance from the outcrop as blocks fracture repeatedly to introduce new fresh unrounded edges. In contrast, block shape is conservative, with parent blocks fracturing to produce self-similar ‘child’ shapes with distance. Measured block shapes evolve in accord with two well-known models for block fracture mechanics ─ 1) stochastic and 2) silver ratio models ─ towards one or other of these two attractor states. Progressive reduction in block size, in accord with fracture mechanics, reflects the fact that most blocks were transported at the sole of the ice mass and were subject to the compressive and tensile forces of the ice acting on the stoss surfaces of blocks lying against a bedrock or till surface. The interpretations might apply to a range of homogeneous hard rock lithologies.

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