Supraglacial ice morphology

Supraglacial structures reflect glacier formation, deformation and flow. Glacier ice exhibits a wide variety of internal and superficial structures, such as crevasses, icefalls, ogive banding, and layering. Crevasses form where ice is pulled apart by tensile stresses that exceed the strength of the ice, and they are commonly oriented at right angles to the main stress direction (e.g. Paterson, 1994). Crevasses are therefore a reflection of the stress orientation in a glacier. Chevron crevasses are linear features oriented obliquely upvalley from the glacier margins toward the centre of the glacier. This type of crevasse forms in response to the drag caused by the valley walls. Due to velocity differences between the centre and the margin of the glacier, tensile stresses are formed that pull the glacier from the margins toward the central part, forming crevasses oriented at 45° to the valley walls. Splaying or marginal crevasses are formed due to compressive flow, causing the glacier to expand laterally. The crevasses are normally curved and parallel to the flow direction toward the centre. The crevasses normally bend outward to meet the margins at angles of less than 45°. Transverse crevasses form in a valley glacier as a result of extending flow. Near the centre of the glacier, the main tensile stress is parallel to the glacier flow. Transverse crevasses therefore open up at right angles to the centre line. Longitudinal crevasses are formed where the lateral stress increases, for example, as a result of widening of the valley glacier. On the upper part of cirque and valley glaciers, a randkluft is a fissure separating the glacier from the rock wall. Such crevasses form as a result of movement away from the rock backwall, and partly because of ablation adjacent to warm rock surfaces. Bergschrunds are deep, transverse crevasses near the heads of valley and cirque glaciers.

When crevasses are formed, they will be carried by the glacier into areas of different stress conditions. On valley glaciers and ice streams, marginal crevasses tend to rotate because of the higher velocity near the centre. Therefore, rotated crevasses tend to close unless the principal stress direction also rotates. When crevasses close, they normally leave linear scars, termed crevasse traces. These scars may either contain layers of blue ice formed by freezing of meltwater prior to closure, or consist of thin layers of white, bubbly ice, reflecting snow-filled crevasses. If the rate of extension is small, the stretching is taken up by the recrystallization of ice parallel to the fracture, forming a tensional vein (Hambrey and Müller, 1978).

Ice falls are steep parts of a glacier where the flow is rapid and intermittent avalanches are triggered by the collapse of ice blocks, termed séracs, piling up cones of broken ice at the base of the icefall. Where a glacier enters an icefall, the acceleration creates extreme extending flow causing stretching and thinning under tensile stresses. This leads to the formation of a large number of crevasses, breaking the ice up into séracs. At the base of icefalls, deceleration of the ice flow creates a zone of compressive flow, and crevasses are closed. In ice falls there are commonly ice-free rock slabs or cliffs.

Ogives are regular bands or waves on the surfaces of valley glaciers below icefalls. The bands are convex downglacier due to a higher velocity in the centre than at the margins. Banded ogives consist of alternating light and dark ice, the dark bands consisting of dirty ice and the light bands of more uniform bubbly ice. Wave or swell-and-wave ogives consist of alternating ridges and troughs. Ogives are considered to form annually, one light-dark/ridge-trough pair representing ice movement during one year. Ogives may reflect seasonal variations in the flow of ice down icefalls (Nye, 1958). Stretching and thinning, in addition to the high concentration of crevasses, give the ice flowing through the icefall a larger surface area than the rest of the ice mass. During the summer season, wind-blown dust and superficial material will concentrate on the glacier surface and form dark bands at the base of the icefall when velocity decreases. Ice moving through the ice fall in winter will, in contrast, collect excess snow and form crests or light bands.

Glacier ice commonly exhibits different types of layers or foliation. In the accumulation area, layering may reflect annual cycles of snow accumulation. The layers are mostly parallel to the glacier surface and can be observed in the side walls of crevasses. The light bubbly ice represents the winter snow, while the thinner blue strata represent summer ablation surfaces. Dark layers in the ice are formed when windblown dust is concentrated during the summer melting. Foliation is formed in deep ice in the accumulation area and is observed in englacial and supraglacial ice in the ablation area. Foliation develops from layers or crevasse traces subject to high strain and/or rotation by glacier flow. Transverse foliation normally consists of crevasse traces downglacier from transverse crevasses and ice falls. Longitudinal foliation, on the other hand, is parallel to glacier flow and formed by the rotation of ice layers or crevasse layers, most commonly near the ice margin. Ice breccias form below intensively crevassed areas or ice cliffs where disintegrated ice is reconsolidated.

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