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Energy Dissipation During Subglacial Abrasion at Nisqually Glacier, Washington, U.S.A.

  • Richard C. Metcalf (a1)

Abstract

This study examines the effect of subglacial abrasion on the basal sliding term of the gravitational energy balance of the dynamic, temperate Nisqually Glacier on Mount Rainier, Washington, U.S.A. Subglacial water flux is estimated as 3 × 107 m3 a–1 and suspended sediment flux as 3 × 107 kg a–1. Suspended-sediment flux is assumed to represent, within an order of magnitude, the annual mass eroded by subglacial abrasion.

Subglacial abrasion involves both brittle fracture and plastic deformation. Field observations of bas-relief and grooved depression striations appear to have exact counterparts in rock mechanics experiments approximating subglacial velocities and normal stresses. Boulton's ([Cl974]) abrasion model and a new attritivity model proposed herein are shown to predict subglacial abrasion-rates within the limits of natural variability and the error range of measurements. The first crude gravitational energy balance for lower Nisqually Glacier (1.96 km2) is attempted and probably has only order-of-magnitude accuracy. The importance of subglacial abrasion in dissipating basal sliding energy at Nisqually Glacier is confirmed.

Résumé

Cette étude examine l'effet de l'abrasion sous-glaciaire sur le terme de glissement à la base du bilan énergétique gravitationnel du glacier tempéré actif de Nisqually sur le Mont Rainier, Washington, U.S.A. Le débit d'eau sous-glaciaire est estimé à 3 × 107 m3/a−1 et le débit solide en suspension à 3 × 107 kg/a−1. On estime que le débit solide en suspension représente l'ordre de grandeur de la masse annuelle de l′érosion par abrasion sous-glaciaire.

L′érosion sous-glaciaire comprend des déformations plastiques et des ruptures brisantes. Les observations de bas reliefs et de stries par cannelures creusées semblent avoir des répliques exactes dans des expériences de mécanique des roches dans des conditions approximativement sous-glaciaires de vitesses et de contraintes normales. Le modèle d'érosion de Boulton ([c1974]) et un nouveau modèle d'attritivité ici proposé prévoient les vitesses d'abrasion sous-glaciaire dans les limites de la variabilité naturelle et du niveau de l'erreur sur les mesures. Le premier bilan brut d'énergie gravitationnelle pour la partie basse du Nisqually Glacier (1,96 km2) est tenté et n'a probablement que la précision d'un ordre de grandeur. L'importance de l'érosion sous-glaciaire pour la dissipation de l'énergie de glissement au fond du glacier Nisqually est confirmée.

Zusammenfassung

Diese Studie untersucht die Auswirkung der subglazialen Abrasion auf den Gleitanteil an der Bilanz der Gravitationsenergie des bewegten, temperierten Nisqually Glacier am Mount Rainier, Washington, U.S.A. Der subglaziale Wasserfluss beträgt schätzungsweise 3 × 107 m3 pro Jahr, der Gehalt an suspendierten Sedimenten 3 × 107 kg pro Jahr. Letzterer dürfte innerhalb einer Grössenordnung die von der subglazialen Abrasion pro Jahr erodierte Masse darstellen.

Subglaziale Abrasion schliesst sowohl spröden Bruch wie plastische Deformation ein. Aus Feldbeobachtungen von Kerbreliefs und geriefelten Schrammen wird ersichtlich, dass sie genaue Gegenstücke in felsmechanischen Versuchen haben, die annähernd unter subglazialen Geschwindigkeiten und Normal-spannungen stattfinden. Es wird gezeigt, dass Boulton ([c1974]) Abrasionsmodell und ein neues, hier vorgeschlagenes Verschleissmodell die subglazialen Abrasionsraten innerhalb der natürlichen Schwankungs-grenzen und der Beobachtungsgenauigkeit zu berechnen gestatten. Eine erste, rohe Bilanz der Gravitationsenergie für den unteren Nisqually Glacier (1,96 km2) wind aufgestellt; sie hält sich vermutlich nur innerhalb der richtigen Grössenordnung. Die Bedeutung subglazialer Abrasion für den Auf brauch der Gleitenergie am Untergrund bestätigt sich für den Nisqually Glacier.

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References

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