The Inland Ice, climate and earlier glaciations

^{Gigantic iceberg moving slowly towards the mouth of the icefjord.}

Greenland’s Inland Ice has a surface area of 1.7 million square kilometres and is up to 3.2 km thick. The mass balance of the Inland Ice is maintained through the accumulation of snow equal to about 600 cubic kilometres of ice a year, and a similar loss through calving and melting in the marginal zone.

The Inland Ice occupies a bowl-shaped depression in the Earth’s crust, formed by the weight of the ice cover. If the Inland Ice disappeared, the land surface would rise, and form a landscape similar to that of Greenland prior to the formation of the ice sheet. Large rivers would drain the inner parts of Greenland, with some of the largest rivers draining westwards towards Disko Bugt and across the continental shelf off West Greenland.

^{About 21,000 years ago, during the last ice age, the large ice sheets of the Northern Hemisphere covered an area of 30 million km2, including much of North America. Ilulissat was centrally placed in the glaciated regions of the Northern Hemisphere.}

^{Left: The Inland Ice today. The dark blue region shows the drainage area of Sermeq Kujalleq.
Middle: Topography of the bedrock below the Inland Ice.
Right: Greenland without the present ice sheet, and with altitudes corrected for the present load of the ice sheet. Large parts of central Greenland are drained by rivers flowing into Disko Bugt in West Greenland. The continuation of the old drainage pattern can be seen as troughs in the offshore areas.
Click on the photo to enlarge it.}

After the onset of glaciation in Greenland, the river valleys were further eroded at the margin of the ice sheet; the old river valleys can still be traced beneath the margin of the Inland Ice. In some areas, the river valleys were over-deepened by the ice and functioned as drainage routes for the outlet glaciers during various stages of the development of the ice sheet. Calf ice production then, as today, was concentrated to the large glaciers occupying these former river valleys. The valleys are now found as troughs on the continental shelf, as fjords in the ice-free land areas and as deep depressions beneath the major ice streams in the marginal parts of the Inland Ice.

Although calving glaciers are widespread along the 6000-kilometre long perimeter of the Greenland ice sheet, calf ice production from the Inland Ice is concentrated in a few, large outlet glaciers. Many of the most important outlet glaciers are found along the coast of West Greenland, and about 84 cubic kilometres of calf ice stem from the five largest outlet glaciers. The rest of the calf ice originates from numerous smaller glaciers.

The transition from the ice-free landscape of pre-glacial times to Greenland’s present glacial landscape is not known in detail. Data from sediment cores from the sea-bed indicate that there was a pronounced global cooling about 38-24 million years ago, which led to the formation of an ice sheet in the eastern Antarctic. After a relatively warm period, which culminated 15 million years ago, the climate started to get colder again about 10 million years ago. Sediments deposited in the North Atlantic about 7 million years ago contain material that was probably transported by icebergs, and this is taken as the first indication of widespread glaciation in southern Greenland. Sediments with ice-rafted material deposited about 2.5 million years ago, are considered to be indicative of even more widespread glaciation in Greenland.

The earliest evidence of a new ice-free period dates from the Kap København Formation, a sedimentary sequence in North Greenland laid down about 2.4 million years ago. At that time, Greenland was without the Inland Ice, which was probably formed during the ice ages that started about one million years ago.

In the central part of the Inland Ice the accumulated snow is gradually buried, and as it sinks deeper and deeper into the ice sheet, the increasing pressure transforms the snow into glacier ice. This transformation takes place in the upper hundred metres of the ice sheet. Concurrent with the vertical movement downwards through the ice sheet, the ice also very slowly moves down slope towards the sides of the ice sheet. In general terms this movement starts from the ice divide in the centre of the Inland Ice, and ends at its outermost margins. From the ice divide out towards the ice margin a series of zones are recognised, depending on the altitude of the surface of the ice sheet above sea level.

^{Cross section of Greenland, showing the thickness of the Inland Ice. In the central part the ice is more than 3 kilometres thick. Note that the deepest part of the ice is lying below sea level. Flow lines show how the ice is moving from the inner parts towards the margin. Click on the image enlarge it.}

The highest zone of the Inland Ice is dry, and snow accumulates without any melting. At a lower level there is a zone with some melting and percolation of melt water. In the next zone, the snow can become waterlogged in the course of the summer, but the percolating water re-freezes in the deeper-lying snow layers. Closer to the ice margin the refrozen snow creates a new zone at the snow line, which in the Ilulissat area is 1300 metres above sea level; here, the refrozen melt water is added to the glacier ice. This zone ends 1200 metres above sea level at the equilibrium line, where the addition of snow and frozen melt water equals the net loss through melting. The lowest level, extending out to the boundary with the land, is the ablation zone, where melting exceeds the net gain.

At the summit of the Inland Ice the movement of the ice is slow and is mainly downwards. The degree of sideways movement increases the further one moves towards the ice margin. Outside the limits of the ice streams, the average horizontal flow of the ice reaches velocities of 50-200 metres per year near the equilibrium line, after which the movement falls to near zero at the ice margin.