Alps, great mountain system of south central Europe, forming an arc some 1,200 km (750 mi) long from the Gulf of Genoa (see Ligurian Sea) to the River Danube at Vienna. The Alps are the highest and most densely settled mountain belt of Europe, occupying an area of about 240,000 sq km (92,700 sq mi) and inhabited by some 20 million people. The valleys of the Alps are areas of year-round settlement; the flatter upland tracts comprise pastures and seasonally inhabited settlements, and the zone above the treeline serves as pasture and for recreation. Important economic activities include tourism, dairy farming, forestry, the production of hydroelectric power, and the extraction of salt and iron ore. With its important pass routes between central and southern Europe, the Alps have been an area of transit trade since ancient times.

GEOLOGIC STRUCTURE AND FORMATION

     The Alps are a complex fold-mountain system. Sedimentary deposits (see Sedimentary Rock) of vast thickness, mainly limestone and dolomite, were laid down in the ancestral Tethys Sea during the Triassic and Jurassic periods. Subsequently, enormous pressure generated by a collision between the African and Eurasian tectonic plates thrust these rock strata upwards and northwards to form recumbent folds (nappes), which in the process of movement were detached from their roots.

     The four glaciations of the Quaternary Period (beginning about 2.5 million years ago) were of great importance in the sculpturing of the Alps. Vast ice masses moved through the valleys, transforming them into deep troughs with steep walls; the overflow of ice across the mountain divides shaped the passes. Glacial deposits in the form of moraines dammed the streams and rivers and produced the region's many lakes, the two largest of which are lakes Geneva and Constance (or Bodensee).

THE ALPINE SYSTEM

     Structurally, the Alpine mountain system is divided into the Western and Eastern Alps by a furrow that leads from the Rhine Valley in northern Switzerland, across Splügen Pass to Lake Como in northern Italy. The Western Alps average about 1,000 m (3,300 ft) higher, and are narrower and more rugged than the Eastern Alps. The highest peak of the Alps, Mont Blanc (4,807 m/15,771 ft), is on the French-Italian border. Among the principal ranges are the Maritime, Ligurian, Cottian, and Graian Alps in France and Italy and the Bernese, Glarus, and Pennine (or Valais) Alps in Switzerland. The Jura Mountains are a north-western extension of the French Alps. From Lake Geneva the Alpine ranges curve north-east and become more widely separated, attaining a width of 250 km (155 mi) in the centre of the arc. The ranges of the Eastern Alps diverge, finally plunging to the Danubian Basin near Vienna. Well-known mountain chains of the Eastern Alps are the Bavarian Alps, Algäuer Alps, Hohe Tauern, and Niedere Tauern in the north and the Dolomite and Carnic Alps in the south.

     Summit regions above 3,000 m (9,800 ft) are glaciated. Peaks and crests, however, rise above the ice, displaying jagged shapes (toothlike horns, needles, and knife-edged ridges). About 2 per cent of the total area of the Alps is covered by ice. The longest valley glacier, the Aletsch Glacier in the Bernese Alps, is 18 km (11 mi) long.

     Broad and deep longitudinal valleys, which hold the courses of the upper Rhône, upper Rhine, Inn, Salzach, Mur, and Drava (Drau) rivers, separate the structural units of the Alps, and contain the main settlements and the principal arteries for traffic. Deeply incised, transverse tributary valleys lead up to the pass regions. Passes at elevations above 2,000 m (6,600 ft) are blocked with snow during the winter months; these include the Mount Cenis, Great St Bernard, Simplon, and St Gotthard passes. Brenner Pass, at 1,370 m (4,495 ft), and Reschen Pass, at 1,508 m (4,948 ft), provide the easiest crossings. Engineering feats, such as tunnelling of the higher passes for roads and railways, have lessened the barrier effect of the Alps.

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