Virtual Alps - Glaciers Noir & Blanc

Site Description - Glaciers Noir & Blanc

The French Alps are located in the south east of France. They cover an area of 15,000 square miles in France, but represent only a small part of a much larger mountain chain which extends across Europe. The High Alps (Hautes Alps) region in France is a region in the south east of the French Alps (east of Grenoble in Fig. 1). Towards the south are the sunny Mediterranean beaches and the arid region of Provence. In the north are the wetter and more wooded mountains of Savoie. To the west lies the Rhône valley and the cites of Grenoble and Lyon, in the east is the Italian border and the cities of Turin and Milan.

The High Alps is composed of a series of very high mountain peaks (several > 4000m) and deeply eroded valleys. The area has a large number of isolated rural settlements, tourist resorts and a few market towns. The base for the studies reported here was the small hamlet of Ailefroide (Fig. 2) [http://www.ailefroide.com/ accessed online 20-10-08] (1506m) where a large 20 ha. municipal campsite in a steep sided wooded valley offers the nearest accommodation to the field sites at Pre de Madame Carle which are 5 km to the NE up the valley Torrent de Gyr. Ailefroide is a popular centre for climbers and alpinists offering hotel, gite and camping accommodation, two small grocery stores, a Guide's Bureau and outdoor equipment/souvenir shops. The village of Vallouise is 7 km down the valley of the Torrent de Gyr at the confluence of the Gyr and L'onde, after which the river becomes La Gyronde. Banking services and supermarkets are available at Vallouise. After flowing for 8 km the Gyronde joins the Durance at Argentiere-la-Besse where further banking services, a larger supermarket, DIY store, kayak slalom course and campsite are located. Some 10km NE of Argentiere-la-Besse, upstream along the Durance, is the historic market town of Briancon which adjoins the Serre Chevalier ski complex. From here a short winding road over the Col de Montgenèvre leads into Italy (~15 km from Briancon).

The French Alps can be divided into four parallel north-south regions. The High Alps form a belt about 100km wide which lies along theFrench-Italian border. This belt contains a variety of sedimentary rocks in great overfolds and nappes. These sedimentary rocks also incorporate rocks such as granite, gneiss and schist.

The main mountains in this belt are the Mont Blanc, Mont Pelvoux and Mercatour (all 3000-4500 m). The Longitudinal Trench includes the vale of Chamonix and the valley of the Isère between Grenoble and Albertville. This trench is around 5km wide and is several thousand metres lower than the surrounding mountains. The trench is formed by a band of soft Liassic shale that has been rapidly eroded by fast-flowing rivers. The Pre-Alps (to the West) between Lake Geneva and the River Durance, are mainly composed of Limestones and are only about half the height of the High Alps. The Alpine Foreland (Dauphine) lies between the Rivers Isère and Rhone at the western foot of the pre-Alps. This zone is largely covered by fluvioglacial deposits, consisting of pebbles embedded in clay, swept down from the Alps during the glacial advances of the Quarternary period. The area also includes moraines, rock basins and sheets of outwash material which have since been terraced by river action.

Glaciation

Many of the landscape features of the High Alps owe their origins to ice movement and deposition of eroded material. The area was affected by the Ecrins ice cap during the last, Devensian, Ice Age. Ice flowed south then west down the Durance valley. The core of the high mountains are mainly erosional in form, whereas many lower valleys and regions towards the edge of the Ecrins ice mass have extensive areas of glacial and fluvio-glacial deposits.

Vast quantities of meltwater released after the ice age, which flowed towards lower land in the south and west, has further modified the landscape. Melting water from the Ecrins ice sheet flowed hundreds of miles towards the south of France. As the ice melted the mountains began an isostatic recovery which is still taking place today though the rates of uplift have slowed. The deep ravines and gorges seen today were created by accelerated river erosion and they provide evidence for these uplift movements.

The permanent ice and snow that remains in the region today is located in the highest and coldest parts of the mountain area. Recent studies of these valley, cirque and niche glaciers appear to suggest that most are retreating at present.

Glaciers Noir and Blanc
The Glaciers Noir and Blanc (see photos) are the largest in the area. They are approximately 6 km long and are very contrasting. Almost all precipitation above an altitude of 2500m is in solid form. The Glacier Blanc has its origin on north facing slopes of the highest peak in the area, The Barre des Ecrins (4102m), where the net accumulation of snow is around 2m per year. Snow accumulates here and recrystallises by internal deformation to form glacier ice.

At about 2950m on the Glacier Blanc, the net accumulation of snow gives way to net ablation, and melt is greater than the build up of snow. This transition is marked by the nève line, where there is no net accumulation or ablation. The line descends as low as 2750m on north facing slopes, but is 2950m on south facing areas.

At around 2000m, the glacier has a net ablation which can be as much as 8 metres per year [Virtual Montana web site accessed on-line 20-10-08]. Accumulation usually continues into May and around 1m of additional melt occurs for every 100m of descent. Ablation is also affected by sediment on the ice. Clean snow or ice will reflect more heat than a snow or ice surface with a thin layer of sediment (see study by Moore later in this report). If the layer of sediment is thick, then the glacier will be protected from melting by the stones (see Glacier Noir, next section).

A thick layer of deposits protects the snout of the Glacier Noir from melting (Fig. 3). There are 0.25 - 0.5m deposits over much of the lower part of the Glacier Noir (Fig. 3) and in some places the sediments are several metres thick. At an altitude the Glacier Noir has 2-3m of melt a year, while the Glacier Blanc has 5-6m of melt [Virtual Montana web site accessed on-line 20-10-08]. The Glacier Noir is thus protected from the suns rays by these sediments. Studies suggest that the glacial regimes of the Glaciers Noir and Blanc are explained by:-
1. The amount of winter precipitation (5%). There is little variation in this and so this only accounts for 5% of the changes in ice budget.
2. The amount of summer precipitation (16%).
3. July/August temperatures (56%), the most important factor.
The rest (22%) is unexplained. The total correlation explains 77% of the variation of the glacier mass. The lengths of the Glaciers Noir Blanc has been reconstructed from climatic data for the period 1880-1946 and measured since 1947. The survey shows the net loss of ice since 1947 and a retreat of the glacier (Figs 3-5).
In colder periods both glaciers used to be joined up (Fig. 5). The glaciers have been surveyed since 1815. At first only the position of the glacier snout was recorded, but since 1921 two cross profiles have been surveyed to measure annual changes in ice thickness.

References
Jaubert JP, Reynaud L. 1993. Le Glacier Blanc Le Glacier Noir, Publications de l'Ecole Moderne Francais, Park Nationaux de France, 39 pp.

Virtual Montana Web Site http://www.virtualmontana.org/virtualmontana/fr_alps/intro.htm accessed on-line 20-10-08.



Page last modified 21 December 2011.

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