Movement of Water in Glaciers

Abstract

ABSTRACT. A ne twork of p assages situa ted along three-grain intersections enabl es water to p ercolate through temperate glacier ice. The d eformability of the ice a llows the passag es to expand and con tract in response to ch a nges in pressure, a nd melting o f the p assage walls by heat generated by viscous dissip a tion and ca rri ed b y above-freez ing water causes the larger passages gradually to in crease in size a t the expense of the sma ll er ones. Thus, th e behavior of the passages is prima rily the result o f three basic ch a racteristics: ( I) the capacity o f th e system continua ll y adj usts, though not instantly, to flu ctu ations in the suppl y o f melt water; (2) the direc tion of movement of the wa ter is d etermin ed mainly by the a mbient pressure in the ice, which in turn is governed primarily b y the slo pe of the ice surface a nd secondaril y b y the local topogra ph y of the glac ier bed ; and, most importan t, (3) th e ne twork of passages tends in tim e to become a rborescent, with a superglacia l part much like an ordin a r y river sys tem in a karst region , an englacial part com prised of Iree-like systems of passages penetrating th e ice from bed to surface, and a subglacia l p a r t consisting of tunn els in the ice carrying water a nd sedimen t a long the glacier b ed. These ch a rac teristics indica te that a sh ee t-like basa l water layer under a glacier wou ld normally be unstab le, th e stable form being tunnels: a nd they expl a in, among o ther things, why ice-marginal melt-water stream s and la kes a re so com mon , why eskers, which are generally considered to ha ve form ed in subglac ia l passages, trend in the general direction of ice fl ow with a tendency to follow va ll ey fl oors and to cross divides at their lowes t points, why they a re typicall y discontinuous where t hey cross ridge crests, why they sometimes con tain fragments from bedrock outcrops near the esker but no t actuall y crossed by it, a nd why they seem to be formed mostly during th e later stages of glaciation.RES UME. Ces ca ra cteristiques indiquen t qu 'u n n iveau d 'eau e tendu comme un drap su r le lit d ' un gl acier d evra it n orm a lement etre une formation insta ble, la form e stabl e e ta nt cell e d es tunnel s; ceci explique, en tre a utres, p ourquoi les torrents et lacs d 'eau d e fu sio n sont si communs le long d es ri ves d es glaciers, pourquoi les eskers, qui son t genera lement consideres comm e ayant e te formes par d es canaux sous-glaciaires, tend en t a s'aligner dans la direction genera le d e I'ecoul ement d e la glace avec une tendance a suivre le fond d e la vallee et a traverser les o bstacles en leur point le plus bas, pourquoi ils sont typiqu em ent interrom pus lo rsq u ' ils traversent la crete d'une ond ul a ti on, p ourquoi ils contien nen t parfois d es fragments pre leves sur le lit pres de I'esker mais non exactemen t sur sa trajectoire et pourquoi il s semblen t se former surtout a u cours des d erniers stades d 'un e glac iation. ZUSAMMENFASSUNG . B ew...

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