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Basal Sliding and Conditions at the Glacier Bed as Revealed by Bore-hole Photography

  • H. F. Engelhardt (a1), W. D. Harrison (a1) and Barclay Kamb (a1)

Abstract

Bore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. Basal sliding velocities measured by bore-hole photography, and confirmed by inclinometry, range from 0.3 to 3.0 cm/d and average 1.0 cm/d, much less than half the surface velocity of 15 cm/d. Sliding directions deviate up to 30° from the surface flow direction. Marked lateral and time variations in sliding velocity occur. The glacier bed consists of bedrock overlain by a ≈ 10 cm layer of active subsole drift, which intervenes between bedrock and ice sole and is actively involved in the sliding process. It forms a mechanically and visibly distinct layer, partially to completely ice-free, beneath the zone of debris-laden ice at the base of the glacier. Internal motions in the subsole drift include rolling of clasts caught between bedrock and moving ice. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. The gap may result from ice—bed separation due to pressurization of the bed by bore-hole water. Water levels in bore holes reaching the bed drop to the bottom when good hydraulic connection is established with sub-glacial conduits; the water pressure in the conduits is essentially atmospheric. Factors responsible for the generally low sliding velocities are high bed roughness due to subsole drift, partial support of basal shear stress by rock friction, and minimal basal cavitation because of low water pressure in subglacial conduits. The observed basal conditions do not closely correspond to those assumed in existing theories of sliding.

Résumé

Les forages furent réalisés au Blue Glacier, Olympic National Park, Washington, dans une partie où le glacier a une épaisseur de 120 m, une inclinaison de 12° à la surface et une vitesse Superficielle de 15 cm/d. Les images photographiques qui ont été prises au fond des trous montrent que l'on a pu atteindre le lit du glacier dans cinq occasions, en utilisant une perforatrice à câble. Les vitesses de glissement obtenues à l'aide de la photographie ont été confirmées par la mesure des inclinaisons dans les trous. Les valeurs observées varient de 0,3 à 3 cm/d. La moyenne est 1,0 cm/d, ce qui contredit les conclusions antérieures qui ont estimé la vitesse subglaciaire à une valeur voisine de la moitié de la vitesse superficielle (15 cm/d). Les directions du glissement divergent jusqu'à 30° de la direction du mouvement glaciaire à la surface. On a pu observer d'évidentes variations latérales et temporelles dans la vitesse du glissement. Le lit du glacier consiste en roc solide, couvert d'une couche active d'apport d'une épaisseur approximative de 10 cm. Cette couche joue un rôle important dans le mécanisme du glissement. Elle forme une rouelle bien visible et mécaniquement individualisée, partiellement ou complètement dépourvue de glace, en-dessous de la zone de glace chargée en moraine à la base du glacier: les mouvements internes dans cette couche comportent le roulage de blocs coincés entre le lit et la glace. Les vitesses les plus grandes interviennent lorsqu'un décollement à la base de quelques centimètres se produit entre la base de la glace et l'apport morainique. Ce décollement peut provenir d'une séparation lit/glace due à la mise en pression du lit par l'eau de sondage. Le niveau de l'eau dans les forages atteignant le lit s'abaisse jusqu'au fond lorsque de bonnes connections hydrauliques s'établissement avec le réseau sous-glaciaire; la pression de l’eau dans ce réseau est essentiellement la pression atmosphérique. La vitesse du glissement est relativement faible à cause de trois phénomènes: (a) l'aspérité considérable du lit glaciaire créée par la couche active d'apport qui est intercalée entre le roc et le fond du glacier, (b) les cissions près du sol qui sont partiellement absorbées par la friction entre les roches, (c) la formation peu fréquente de cavités à cause de la faible pression d'eau dans les canaux sous-glaciaires. Les phénomènes observés au fond du glacier ne correspondent pas aux conditions présumées par les théories du glissement.

Zusammenfassung

Bohrungen wurden am Blue Glacier, Washington, vorgenommen, in einem Gebiet, wo der Gletscher eine Dicke von 120 m, eine Oberflächenneigung von 12° und eine Oberflächengeschwindig-keit von 15 cm/d hat. Photographische Aufhahmen vom Grund der Bohrlörher zeigen, dass der Felsunter-grund, über den der Gletscher gleitet, in fünf Fällen erreicht werden konnte, und zwar mit Hilfe eines Kabelbohrgeräts. Die Gleitgeschwindigkeiten, die sich aux diesen Aufnahmen erniitteln lassen, werden durch Neigungsmessungen in den Bohrlöchern bestätigt; sie reiehen von 0,3 bis 3 cm/d und sind damit nicht, wie bisher angenommen wurde, ungefähr gleich der halben Oberflächengeschwindigkeit, sondern wesentlich geringer. Die Gleitrichtungen weichen bis zu 30° von der Fliessrichtung des Gletschers an der Oberfläche ab. Ausgeprägte laterale und zeitliche Änderungen der Gleitgeschwindigkeit treten auf. Das Gletscherbett besteht aus gewachsenem Fels, der mit einem etwa 10 cm dicken Schuttgeschiebe bedeckt ist. Dièse Schicht unterhalb des Gletschers, welche teilweise oder völlig eisfrei ist, besitzt besondere mechanische Eigenschaften und ist aktiv am Gleitvorgang beteiligt. Bewegungsvorgänge innerhalb dieser Schicht schliessen ein Rollen von Felsbrocken ein, die zwischen dem gewachsenen Fels und dem sich bewegenden Eis eingespannt sind. Die höchsten Gleitgeschwindigkeiten treten an Stellen auf, wo sich ein Spalt von einigen Zentimetern Breite zwischen der Sohle und dem subglazialen Geschiebe öffnet. Der Spall entsteht möglicherweise durch eine Trennung des Eises vom Brett infolge des erhöhten Wasserdrucks am Grunde des Bohrlochs. Der Wasserspiegel in Bohrlöchern sinkt bis auf den Boden, wenn gute hydraulische Verbindung zu subglazialen Kanälen vorhanden ist; der Wasserdruck in den Kanälen entspricht im wesentlichen dem Luftdruck. Für die im allgemeinen geringe Gleitgeschwindigkeit sind verantwortlich (a) die hohe Bettrauhigkeit, die durch das zwischen Fels und Gletschersohle eingelagerte Schuttgesehiebe erzeugt wird, (b) die bodennahen Schersparnnungen, die zum Teil durch Gesteinsreibung aufrecht erhalten werden, (c) die sehr geringe Hohlraumbildung aufgrund des niedrigen Wasserdrucks in den subglazialen Kanälen. Die beo-bachteten Erscheinungen am Grund des Gletschers entsprechen weilgehend nicht den Annahmen, die in den hestehenden Theorien des Gleitens angenommen werden.

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Basal Sliding and Conditions at the Glacier Bed as Revealed by Bore-hole Photography

  • H. F. Engelhardt (a1), W. D. Harrison (a1) and Barclay Kamb (a1)

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