www.geus.dk > Popular science > Ilulissat Icefjord > This page

Giant iceberg

The Inland Ice, climate and earlier glaciations

The Greenland Inland Ice is the only major remnant of the gigantic ice sheets that covered large parts of the continents and the continental shelves of the Northern Hemisphere during the ice ages. The Inland Ice, together with the far larger Antarctic ice sheet, make up 99% of the volume and 97% of the area of the world’s glacier cover

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.

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.

Gigantic iceberg moving slowly towards the mouth of the icefjord.

Ice cover during the last ice age
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.

Map showing greenland with and without icecap
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 here to enlarge.


Earlier glaciations

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.

Greenland about 2.3 million years ago
Forests of larch, spruce and birch covered most of Greenland about 2.3 million years ago, with heaths on high-lying areas.

2.4 million year olsd logs
About 2,4 million year old trunk from larch and spruce from Washington Lan in Northwest Greenland. The longest trunk is 176 cm which indicates that the trees were rather small.
(Photo: Peter Warna-Moors)
Click here to enlarge.


The Inland Ice from top to margin

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.

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.

Cross section of Greenland
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 here to enlarge.

The highest part of the inland ice
The highest part of the Inland Ice is a large plain, where the snow rarely melts. (Photo: Henrik Højmark Thomsen)

Large meltwater rivers on the ice
On the lower part of the Inland Ice all the winter snow and some of the ice are melting every summer. This creates large meltwater rivers on the ice surface. (Photo: Henrik Højmark Thomsen)

Climate activities in the Inland Ice

The Inland Ice preserves a unique archive for the climatic evolution and atmospheric chemistry of the past 250,000 years. Snow that falls on the interior part of the ice sheet in one year is buried by the snows of successive years, and is gradually converted into ice by the pressure of the overlying snow. In the course of thousands of years the layers of ice sink deeper and deeper into the ice sheet.

The remarkable thing is that each year’s layer of ice ‘remembers’ the temperature from the time that the snow fell. The reason for this is that the relationship between two oxygen isotopes in the snow reflects the air temperature during the snowfall. The isotopes are stable and are preserved in the ice, and it is therefore possible to follow the variations in climate backwards in time by analysing progressively older and older ice layers.

To date, five deep holes have been drilled in the Inland Ice. The ice cores recovered are kept frozen, and flown back to laboratories where the annual ice layers are analysed and dated. In this way information about the climate in Greenland during the past 250,000 years has been documented, a continuous record that reaches back through the last ice age.

The drill cores show that during the last ice age the climate was extremely unstable. Periods of intense cold were interrupted many times by milder periods that sometimes lasted several thousand years. Typically, the climate suddenly became 10-20 degrees warmer in the course of a few decades, and then became slowly colder again in the course of a few millennia. It is thought that the marked oscillations in climate reflect changes in global ocean currents, especially the North Atlantic Current.

The last ice age ended abruptly about 11,550 years ago, and the climate in the present warm interglacial period has been relatively stable. The warm interglacial culminated from 8000 to 4000 years ago, and was followed by falling temperatures with two particularly cold periods about 1600 and 1850, the so called ‘Little Ice Age’; average temperatures were then 1°C colder than today.

It is not only information about past temperatures that is preserved in the Inland Ice. Small air bubbles caught in the ice as the snow was gradually buried preserve the former composition of the atmosphere. It is thus possible to determine the content of the so called ‘greenhouse’ gases in the atmosphere as far back in time as the ice core extends.

Major volcanic eruptions can be traced in the Inland Ice as layers of ice with relatively high acidity, since large amounts of sulphuric acid are formed during volcanic eruptions. There may also be small particles of volcanic ash in the ice. It is known that large, or prolonged, volcanic eruptions have a cooling effect on global cli-mate, since the ash particles and the small sulphuric acid drops in the atmosphere formed during a volcanic eruption reflect sunlight and thus energy from the sun back into space. The concentration of dust particles in the ice reflects the number and size of past storms. Dust from deserts and steppes is transported widely

in the atmosphere, and it is known that dust that reaches Greenland today can come from central Asia. Dust content in the ice from the last ice age are substantially higher than in ice from the current interglacial period.

Ice cores drilling through the Inland Ice
Ice cores drilled through the Inland Ice preserve an archive of past climate variations. The Nord-GRIP boring in northern Greenland reached the bottom of the ice sheet, and yielded a 3085 m long ice core that goes back 120,000 years in time.

Curve showing climate changes
Changes in the Greenland climate over the past 120,000 years.
Click here to enlarge.

Climate curve for the past 1500 years
Climate curve for the past 1500 years, and important events in the history of Greenland.
Click here to enlarge.

Eruption of the volcano Pinatubo in the Philippines
Eruption of the volcano Pinatubo in the Philippines on the 12th june 1991. the ash column is 23 kilometres high. 30 million tons of ash and gas was ejected, of which sulphur dioxide made up 20 million tons.

PREVIOUS PAGE [ 1, 2, 3, 4, 5, 6, 7, 8 ] NEXT

[Til top]   Sidst ændret: 11. november 2007 © Geological Survey of Denmark and Greenland - GEUS
Øster Voldgade 10, DK-1350 Copenhagen K - Tel.: +45 38142000 - Fax: +45 38142050 - E-mail: geus@geus.dk