Box 71 The Structure Of Glaciers

Glaciers contain a number of distinct structures. These can be divided into: (i) primary structures that result from accumulation, and: (ii) secondary structures that develop due to ice deformation during flow.

The principal primary structure is ice stratification, which results from the accumulation of snow each year. Summer surfaces are usually indicated by a refrozen melt-layer of bluish ice and by a concentration of debris that has fallen onto the glacier surface. Regelation layers are also primary structures, formed during the freezing of regelation ice to the base of the glacier (see Section 3.3.2). These primary structures become deformed during ice flow by internal deformation to produce secondary structures. The most important secondary structure is ice foliation, a layered fold structure of different sizes of ice crystals that develops by the deformation of primary ice structures during ice flow. It may develop either parallel to or transverse to the direction of glacier flow. Longitudinal foliation normally occurs at the ice margin and parallel to the direction of glacier flow. Most accumulation basins are wider than the channel that drains them; as a consequence of this, primary ice stratification is compressed and folded to form longitudinal folds or foliation. Transverse or arcuate foliation develops in regions with transverse crevasses or ice falls. Ice falls are densely crevassed regions of a glacier where ice descends a steep slope. Crevasses open as ice enters the ice fall, due to the extensional stress caused by flow acceleration in the ice fall. These crevasses then close at the base of the ice fall as extensional stresses change to compression. Former crevasses can be traced by the presence of different types of ice crystal that reflect light in different ways. These crevasse traces are deformed into an arcuate pattern by flow; extending further downstream in the centre of the glacier than at the sides. On some glaciers these crevasse traces become depicted by alternating bands of light and dark snow, known as ogives. It was suggested originally that each pair of dark and light ice bands represents a year's movement through the ice fall; the darker bands resulting from the concentration of dust and debris into crevasses during the summer months. However, their origin has now also been explained with reference to multiple shear zones in the ice, through which basal ice is uplifted to the glacier surface to produce the dark, foliated ogive bands. Finally, thrusts, sometimes referred to as shear planes, can develop in three situations: (i) in glaciers with a mixed or polythermal basal thermal regime; (ii) where glaciers flow against large subglacial bedrock obstacles or reverse bedrock slopes; and (iii) in surging glaciers.

Sources: Hambrey, M.J. and Lawson, W. (2000) Structural styles and deformation fields in glaciers: a review. Geological Society of London Special Publications, 176, 59-83. Goodsell, B., Hambrey, M.J.and Glasser, N.F.(2002) Formation of band ogives and associated structures at Bas Glacier d'Arolla, Valais, Switzerland. Journal of Glaciology, 48, 287-300.

on the glacier surface in the ablation zone will not be buried permanently by ice, unless it falls into a crevasse, and will therefore form a supraglacial cover.

Debris on the surface of a glacier may either be concentrated into down-glacier ridges known as medial moraines or will form an irregular layer over the glacier surface (Figure 7.2). There are two broad categories of medial moraine: ice-stream interaction; and ablation-dominant. Ice-stream interaction medial moraines are formed by the confluence of two lateral moraines at the junction of two glaciers (Figure 7.3). The medial moraine consists of a debris-covered ice ridge that extends down the trunk of the glacier and marks the line of suture between the two glaciers. It is important to note that the moraine is the surface expression of a vertical debris septum within the glacier that may extend to the glacier bed (Figure 7.3).

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Figure 7.2 Vertical aerial photograph showing medial moraines at the confluence of two Svalbard valley glaciers. The glacier is about 1 km wide here and flow is from the bottom of the photograph towards the top. [Photograph: N.F. Glasser]

Nunatak

Nunatak

Figure 7.3 The formation of a vertical debris septum and medial moraine by the confluence of two glaciers.

Figure 7.3 The formation of a vertical debris septum and medial moraine by the confluence of two glaciers.

Ablation-dominant medial moraines form where ridges of englacial debris are revealed down-glacier by surface melting. Such moraines appear to 'grow' out of the glacier surface in the ablation zone. This type of medial moraine may form in two ways.

1. If there is a point source for debris supply from a rockwall or nunatak onto the glacier, debris may take a linear configuration down-glacier of that point. In the accumulation zone this debris will slowly descend through the glacier in the direction of flow as it becomes buried by snow fall. In the ablation zone melting will reveal the debris as a medial moraine on the glacier surface.

2. Medial moraines can develop by the folding of debris-rich ice in longitudinal structures such as foliation and stratification. In the accumulation zone debris may collect as a diffuse layer over much of the glacier surface, where it will become buried by snow to form debris strata. Debris strata may become folded across the glacier during ice flow, particularly if the ice flows from a wide basin into a narrow trunk and is compressed laterally (Figures 7.4 and 7.5). The axes of these folds are usually parallel to the direction of ice flow. When the debris-rich ice reaches the ablation zone, surface melting lowers the glacier surface to reveal the anticlinal crests of the longitudinal debris-rich folds. The intensity of folding is determined by the amount of transverse compression within the glacier. If the folding is relatively open then a series of small medial moraines may emerge along the axis of each fold. However, if the folding is tight then the debris in the individual folds may merge to form a single medial ridge (Figure 7.4). It is important to recognise that the englacial and supraglacial debris structure of any glacier may be very complex and is a product of the cumulative deformation history of the ice.

The debris structure on the surface of a glacier is also affected by the rate of debris supply relative to the glacier's flow velocity. If the rate of debris supply is high and the glacier velocity is low, a thick layer of debris can accumulate. If the rate of debris supply is low and the glacier velocity is high, then the debris cover will be spread across the glacier surface more rapidly and it will be thinner. The presence of debris on a glacier surface has an important influence on its mass balance because supraglacial debris acts as insulation and slows down surface melting. Thick patches or ridges of supraglacial debris may be associated with large ice-cored mounds, ridges and dirt cones on the glacier surface. Debris-free ice melts rapidly on the clean ice surface, but is retarded beneath the debris cover itself. This process is particularly important in the development of ice-cored moraines, where blocks of glacier ice become buried beneath debris as the glacier retreats (see Section 9.1.3).

Debris transported at a high level within a glacier is often referred to as passively transported because it remains largely unaltered during glacial transport. It therefore retains its primary characteristics; it is typically angular, coarse and contains little in the way of fine material (Figure 7.6). In fact the sedimentological characteristics of debris transported at high levels within a glacier are often similar to those of talus or scree, which reflects the common origin of both as rockfall debris.

190 Glacial Debris Entrainment and Transport „ PLAN

stratification

B. Cross-section of snout area snout

stratification

CROSS-SECTION

stratification with/without rockfall debris stratification with/without rockfall debris

^^supraglacial debris

^^supraglacial debris

debris layer from rockfall basal debris debris layer from rockfall basal debris supraglacial debris (medial moraines) / . \

supraglacial debris (medial moraines) / . \

B. Cross-section of snout area sandy diamicton gravel basal décollement proglacial landform/ sediment assemblage basally derived supraglacial debris sandy diamicton gravel basally derived supraglacial debris basal décollement

proglacial landform/ sediment assemblage basal debris englacial landform/ sediment assemblage © = thrust

Figure 7.4 Schematic representation of how glacier structures influence debris transport in a valley glacier with multiple accumulation basins. (A) Plan view of a valley glacier showing the formation of medial moraines by folding within the glacier. (B) Cross-section showing development of thrusts near the glacier snout. [Modified from: Hambrey and Glasser (2005) in Encyclopedia of Geology (eds R.C. Selley, L.R.M. Cocks and I.R. Plimer), Elsevier, Amsterdam, figure 13, p. 673]

basal debris englacial landform/ sediment assemblage © = thrust

Figure 7.4 Schematic representation of how glacier structures influence debris transport in a valley glacier with multiple accumulation basins. (A) Plan view of a valley glacier showing the formation of medial moraines by folding within the glacier. (B) Cross-section showing development of thrusts near the glacier snout. [Modified from: Hambrey and Glasser (2005) in Encyclopedia of Geology (eds R.C. Selley, L.R.M. Cocks and I.R. Plimer), Elsevier, Amsterdam, figure 13, p. 673]

snout

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