1 Crevasses

Ice flows under pressure, but cracks under tension creating crevasses. They form particularly in areas of extending flow, or where the bedrock under the glacier is irregular. Between crevasses, large towers can form, called seracs.

Crevasses can be covered over by winter snow and are very dangerous. Usually they are about 70 metres deep and indicate variations in ice flow.

Crevasses can go across a glacier from side to side – transverse crevasses. Some point 40° upstream, indicating change in speed close to the margin, and are called marginal crevasses. Whereas longitudinal crevasses are found near the terminus and they show that the glacier is spreading sideways, indicating ice loss along its edges. The fourth crevasse is the special bergschrund.

2. Moraine and Till

Rock debris carried in some manner is known as till. Features made of till are called moraines. Moraines are categorised according to their “place” in the glacier system.

Lateral moraines: are found at the sides of a glacier and the rock and debris usually originate from the rockfalls and avalanches that come from the valley walls. The Tasman Glacier has impressive stranded lateral moraines along Ball Hut Ridge.

Medial moraines: are formed by coalescing lateral moraines that occur when a tributary glacier flows into the main glacier.

Surface moraines: are evident more at the terminus, where they are exposed because ice melt is greatest at this point. The surface moraine of the Tasman Glacier is only about 1-2 metres deep, but covers a significant portion of the lower part of the glacier.

Englacial moraine and subglacial moraine: are carried within and under the glacier. The greatest bulk of debris carried by a glacier is in either of these forms.

Terminal moraine: When a glacier’s terminus or snout remains at the same place for a long time the till from the snout forms a curved ridge. Terminal moraines are useful for people who study glaciers because they indicate the glacier’s down valley maximum extent or magnitude. The ridges that remain are called terminal moraine loops. A series of these indicates the glacier is retreating because it hasn’t eroded the loops away as it would if it was advancing.

3. Meltwater Features

Near the terminus of the Tasman Glacier the surface is characterised by surface moraine 1-2 metres deep covering the ablating ice and sinkholes or moulins. Moulins start off as small potholes that eventually deepen to join up with meltwater tunnels that form the maze of the glacial plumbing system. On the Tasman, these moulins have expanded into large grey circular lakes.

Eventually these systems find their way to the base of the glacier and out to the terminus where they discharge themselves into the fluvioglacial system, usually carrying rock flour. The amount of debris discharged is enormous. The present Tasman River is at work on a bed of outwash gravels 500 – 1000 metres deep. The river constantly shifts its pattern, often across tussock land, filling Lake Pukaki. The Hooker River alone transports 20,000 m3 of sediment.

4. Icefalls