Spain’s Peninsula: Relief and Geological Evolution
1. Peninsula Relief Features
The Iberian Peninsula is characterized by its massive form, given its width from west to east (1094 km), and its relatively straight coastlines. This limits the sea’s influence inland. The high average altitude (660 m) is due to the central high plateau, a series of antiplanicies. The peripheral mountainous schema around the plateau further restricts the sea’s influence, creating a stark contrast between the coast and inland areas.
2. Morphostructural Units
Several major morphostructural units exist on the peninsula:
- Sockets: Plains and plateaus formed by the erosion of ancient mountain ranges. These areas are composed of rigid, siliceous rocks that fracture under orogenic pressure, resulting in horizontal reliefs.
- Ancient Massifs: Mountains formed during the primary era and rejuvenated by Alpine orogenesis. These massifs now have rounded, flattened tops due to erosion (e.g., Sistema Central, Montes de Toledo).
- Fold Mountains: Large mountain ranges formed during the Alpine orogenesis by the folding of sedimentary materials, primarily limestone. These mountains exhibit steep and rugged slopes (e.g., Pyrenees, Andalusian ranges).
- Sedimentary Basins: Sunken areas or depressions filled with clay and limestone sediments. These basins exhibit horizontal reliefs and can be formed by block subsidence or pre-alpine depressions.
3. Peninsula Geological Evolution
The current relief is a product of a long geological history:
- Precambrian Era (4000 million years ago): A mountainous arc emerged in the southeast, composed of shale and gneiss. This formation included elevations in Galicia and isolated points in the Sistema Central and Montes de Toledo. It was later eroded and covered by seas.
- Paleozoic Era (600 million years ago): The Hercynian orogeny occurred, forming the Hercynian Mountains from siliceous materials. The Hesperian Massif arose to the west, later eroded to become a zócalo. The Aquitanian, Cantabro-Balear, and Betico-Rif massifs appeared to the northeast and southeast, respectively. All were subsequently eroded.
- Mesozoic Era (225 million years ago): A calm period dominated by erosion of the Hercynian reliefs and sedimentation, with mineral deposits in sea-covered areas.
- Tertiary Period (68 million years ago): The Alpine orogeny caused significant changes. The Alpine Mountains were uplifted, and alpine depressions formed. The plateau tilted towards the Atlantic, and mountainous edges formed in the east and south. Fractures and faults in the plateau’s base created the northern mountain range and inland depressions. These faults also led to volcanic activity.
- Quaternary Period (1.7 million years ago – present): Key developments included glaciers affecting the highest mountain ranges (Pyrenees, Cantabrian Mountains, Sistema Central) and river terraces forming along river margins due to alternating Quaternary climates.
4. Rock Types and Relief
The peninsula’s geological evolution has resulted in three areas with distinct rock types:
4.1 Siliceous Area
Located in western areas, with branches extending to the western Cantabrian Mountains, Sistema Central, Montes de Toledo, and Sierra Morena. The primary rock is granite, a rigid crystalline rock. Granite landscapes include chemically altered areas and formations resulting from jointing and fracturing, such as ridges, scree, domes, berrocales, and other granite formations.
4.2 Limestone Area
Forms an inverted ‘Z’ shape across the Pre-Pyrenees, Basque Mountains, and Iberian Range. Limestone is the dominant rock, leading to karst relief features such as:
- Lenar, sinkholes, or grooves
- Gorges or canyons
- Poljes (elongated depressions)
- Torques (large sinkholes)
- Caves
- Troughs (narrow openings connecting the surface to underground galleries)
4.3 Clay Area
Located in the northern and southern sub-plateaus and the Ebro and Guadalquivir depressions. Clay is the dominant rock, resulting in horizontal reliefs, including:
- Campos (gently rolling plains)
- Badlands (landscapes with narrow alleyways and ridges)
4.4 Differential Erosion
Erosion varies due to the different rock strengths:
- Horizontal strata with alternating hard and soft layers create flat-topped hills and mesas.
- Gently inclined strata form cuestas.
- Folded strata create Appalachian relief (alternating ridges and valleys) and Jurassic relief (convex and concave folds).