Spain has some of the oldest as well as some of the youngest rocks of Europe. The entire western half of Iberia, with the exception of the extreme south, is composed of ancient (Hercynian) rocks; geologists refer to this Hercynian block as the Meseta Central. It constitutes a relatively stable platform around which younger sediments accumulated, especially on the Mediterranean side. In due course these sediments were pushed by major earth movements into mountain ranges. The term meseta is also used by geographers and local toponymy to designate the dominating relief unit of central Iberia. As a result, the Meseta Central defined by relief is subdivided by geology into a crystalline west (granites and gneisses) and a sedimentary east (mainly clays and limestones). The northern Meseta Central, which has an average elevation of 2,300 feet (700 metres), corresponds to the tablelands, or plateau, of Castile and León, although it is in fact a basin surrounded by mountains and drained by the Douro (Duero) River. The southern Meseta Central (the Meseta of Castile–La Mancha) is some 330 feet (100 metres) lower. Its relief is more diverse, however, owing to heavy faulting and warping caused by volcanic activity around the Calatrava Plain and to two complex river systems (the Guadiana and the Tagus) separated by mountains. Its southern plains rise gradually to the Sierra Morena. The southeastern side of this range drops almost vertically by more than 3,300 feet (1,000 metres) to the Guadalquivir depression. Dividing the northern and southern Mesetas are the Central Sierras, one of the outstanding features of the Iberian massif. Their highest points—Peñalara Peak at 7,972 feet (2,430 metres) and Almanzor Peak at 8,497 feet (2,590 metres)—rise well above the plains of the central plateau. In contrast, the granitic Galician mountains, at the northwestern end of the Hercynian block, have an average elevation of only 1,640 feet (500 metres), decreasing toward the deeply indented (ria) coast of the Atlantic seaboard.
Part of Alpine Europe, the Pyrenees form a massive mountain range that stretches from the Mediterranean Sea to the Bay of Biscay, a distance of some 270 miles (430 km). The range comprises a series of parallel zones: the central axis, a line of intermediate depressions, and the pre-Pyrenees. The highest peaks, formed from a core of ancient crystalline rocks, are found in the central Pyrenees—notably Aneto Peak at 11,168 feet (3,404 metres)—but those of the west, including Anie Peak at 8,213 feet (2,503 metres), are not much lower. The mountains fall steeply on the northern side but descend in terraces to the Ebro River trough in the south. The outer zones of the Pyrenees are composed of sedimentary rocks. Relief on the nearly horizontal sedimentary strata of the Ebro depression is mostly plain or plateau, except at the eastern end where the Ebro River penetrates the mountains to reach the Mediterranean Sea.
Cows grazing high in the central Pyrenees, Huesca province, Spain.age fotostock
A series of sierras trending northwest-southeast forms the Iberian Cordillera, which separates the Ebro depression from the Meseta and reaches its highest elevation with Moncayo Peak at 7,588 feet (2,313 metres). In the southeast the Iberian Cordillera links with the Baetic Cordillera, also a result of Alpine earth movements. Although more extensive—more than 500 miles (800 km) long and up to 150 miles (240 km) wide—and with peninsular Spain’s highest summit, Mulhacén Peak, at 11,421 feet (3,481 metres), the Baetic ranges are more fragmented and less of a barrier than the Pyrenees. On their northern and northwestern sides they flank the low-lying and fairly flat Guadalquivir basin, the average elevation of which is only 426 feet (130 metres) on mainly clay strata. Unlike the Ebro basin, the Guadalquivir depression is wide open to the sea on the southwest, and its delta has extensive marshland (Las Marismas). Drainage
Although some maintain that “aridity rivals civil war as the chief curse of [historic] Spain,” the Iberian Peninsula has a dense network of streams, three of which rank among Europe’s longest: the Tagus at 626 miles (1,007 km), the Ebro at 565 miles (909 km), and the Douro at 556 miles (895 km). The Guadiana and the Guadalquivir are 508 miles (818 km) and 408 miles (657 km) long, respectively. The Tagus, like the Douro and the Guadiana, reaches the Atlantic Ocean in Portugal. In fact, all the major rivers of Spain except the Ebro drain into the Atlantic Ocean. The hydrographic network on the Mediterranean side of the watershed is poorly developed in comparison with the Atlantic systems, partly because it falls into the climatically driest parts of Spain. However, nearly all Iberian rivers have low annual volume, irregular regimes, and deep valleys and even canyons. Flooding is always a potential hazard. The short, swift streams of Galicia and Cantabria, draining to the northwestern and northern coasts, respectively, have only a slight or, at most, modest summer minimum. The predominant fluvial regime in Spain is thus characterized by a long or very long summer period of low water. This is the regime of all the major arteries that drain the Meseta as well as those of the Mediterranean seaboard, such as the Júcar and the Segura: for example, from August to September the Guadiana River usually has less than one-tenth of its average annual flow. Only the Ebro River has a relatively constant and substantial flow—19,081 cubic feet (540 cubic metres) per second at Tortosa—coming from snowmelt as well as rainfall in the high Pyrenees. In comparison, the flow of the Douro is only 5,050 cubic feet (143 cubic metres) per second. The flow of many Iberian streams has been reduced artificially by water extraction for purposes such as irrigation. Subterranean flow is well-developed in limestone districts.
The Júcar River flowing past a 14th-century castle at Cofrentes, Valencia, Spain.Robert Frerck/Odyssey Productions
Soils
There are five major soil types in Spain. Two are widely distributed but of limited extent: alluvial soils, found in the major valleys and coastal plains, and poorly developed, or truncated, mountain soils. Brown forest soils are restricted to humid Galicia and Cantabria. Acidic southern brown earths (leading to restricted crop choice) are prevalent on the crystalline rocks of the western Meseta, and gray, brown, or chestnut soils have developed on the calcareous and alkaline strata of the eastern Meseta and of eastern Spain in general. Saline soils are found in the Ebro basin and coastal lowlands. Calcretes (subsoil zonal crusts [toscas], usually of hardened calcium carbonate) are particularly well-developed in the arid regions of the east: La Mancha, Almería, Murcia, Alicante (Alacant), and Valencia, as well as the Ebro and Lleida (Lérida) basins.
Soil erosion resulting from the vegetation degradation suffered by Spain for at least the past 3,000 years has created extensive badlands, reduced soil cover, downstream alluviation, and, more recently, silting of dams and irrigation works. Particularly affected are the high areas of the central plateau and southern and eastern parts of Spain. Although the origins of some of the spectacular badlands of southeastern Spain, such as Guadix, may lie in climatic conditions from earlier in Quaternary time (beginning 2.6 million years ago), one of the major problems of modern Spain is the threat of desertification—i.e., the impoverishment of arid, semiarid, and even some humid ecosystems caused by the joint impact of human activities and drought. Nearly half of Spain is moderately or severely affected, especially in the arid east (Almería, Murcia), as well as in much of subarid Spain (the Ebro basin). The government has adopted policies of afforestation, but some authorities believe that natural vegetation regrowth would yield more speedy and more permanent benefits. Climate