Ice sheet formation and glacial and fluvioglacial processes

Overview

Mass balance, ice sheet and warm-based and cold-based ice.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   How does falling snow become glacier ice?

   Model answer

   Freshly fallen snow is an open lattice of crystals that is about 90 per cent air. As more snow buries it, the weight compacts it, the delicate points of the crystals melt and refreeze and the crystals become rounded granules, producing firn or neve, a dense granular snow surviving from one year to the next. Continued burial squeezes out most of the remaining air, the granules grow and interlock, and at a depth of a few tens of metres the mass becomes solid glacier ice in which air survives only as isolated bubbles. Density rises from around 100 kilograms per cubic metre for new snow to around 900 for glacier ice. The process takes decades where snowfall is heavy and thousands of years in the cold, dry interior of Antarctica.

2. 2

   What is mass balance, and what has it to do with whether an ice sheet grows?

   Model answer

   Mass balance is the difference between accumulation, the inputs of snowfall, avalanche and hoar frost, and ablation, the losses by melting, evaporation, sublimation and the calving of icebergs, measured over a year. A positive balance means inputs exceed losses, the ice thickens and the margin advances; a negative balance means the reverse. An ice sheet begins to build where the balance stays positive over many years, which requires enough snowfall and, more importantly, summers cool enough that the winter snow is not removed. The two are combined by the accumulation area ratio, and a glacier in equilibrium has a stationary snout even though the ice within it is still moving forward.

3. 3

   What is an ice sheet, and how does it differ from a valley glacier?

   Model answer

   An ice sheet is a mass of glacier ice exceeding roughly 50,000 square kilometres which is thick enough to bury the underlying relief almost completely, so that it flows outwards radially from one or more high central domes rather than being steered by valleys. Only Antarctica and Greenland qualify today; during the last glacial, ice sheets covered Ireland, Britain, Scandinavia and much of North America. A valley glacier, by contrast, occupies and is confined by a pre-existing valley, flows in one direction down a fixed course, and is far smaller and thinner. The difference matters because an ice sheet can move across a lowland regardless of the topography and can lay down landforms over huge areas, which is what produced the drumlin fields and till plains of Ireland.

4. 4

   At what point does ice begin to flow, and by what mechanisms does it move?

   Model answer

   Ice starts to deform under its own weight once it is roughly 50 to 60 metres thick, because below that depth the pressure exceeds the strength of the ice and it behaves as a very viscous plastic solid. Movement then occurs in two ways. Internal deformation is the slow creep of ice crystals sliding over one another, which happens in all glaciers including those frozen to their beds. Basal sliding is the whole mass sliding over the bed on a film of meltwater produced by pressure melting and friction, and it happens only where the base is at the pressure melting point; where the bed is soft, deformation of the saturated sediment beneath adds a third component. Basal sliding is much the faster process and is what produces erosion.

5. 5

   Distinguish between warm-based and cold-based ice, and explain why it matters for erosion.

   Model answer

   Warm-based, or temperate, ice is at the pressure melting point at its base, so meltwater is present at the bed. Cold-based, or polar, ice is well below freezing throughout and is frozen firmly to its bed. The distinction is decisive because warm-based ice can slide, and it is sliding that drags embedded debris across the rock and allows meltwater to penetrate joints and freeze; cold-based ice moves almost entirely by internal deformation, hardly slides, and therefore erodes very little, sometimes preserving the landscape beneath it intact. Warm-based ice is also the ice that produces meltwater, so the fluvioglacial landforms and the great bulk of glacial erosion and deposition are the work of warm-based ice.

6. 6

   Explain the process of abrasion.

   Model answer

   Abrasion is the wearing away of bedrock by rock fragments held in the base of moving ice. The ice acts as a conveyor, pressing angular debris against the bed with the weight of the whole overlying mass and dragging it forward as the glacier slides. Harder fragments scratch and gouge the softer rock beneath, leaving parallel scratches called striations aligned with the direction of ice movement, and grind finer material to a flour-like powder called rock flour, which gives meltwater streams their characteristic milky appearance. Rates depend on the hardness and abundance of the debris, on the velocity of the ice and on the pressure at the bed, and clean ice with no debris cannot abrade at all but merely polishes.

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