Spanish Relief: Characteristics, Formation, and Lithology
Item 2: General Characteristics of Spanish Relief
Main Features of Peninsular and Insular Relief
The Geographical Area of Spain: Spain is a medium-sized European state located in the temperate northern hemisphere. It comprises a peninsula, the Mediterranean archipelago (Balearic Islands), the Atlantic archipelago (Canary Islands), Ceuta, and Melilla (in North Africa).
Peninsular Relief: Spanish relief configuration has three basic features:
The massive form is given by the great width of the peninsula from west to east coasts, hindering the penetration of the sea’s influence inland.
The high average altitude (660m) is due to the existence of high mountain ranges and an extensive core highland, the plateau, with altitudes between 660 and 800 meters.
The peripheral mountainous arrangement around the plateau stops the sea’s influence, resulting in substantial differences between the coast and the peninsular interior.
In short, the Iberian Peninsula’s central axis is the Plateau, divided into two sub-plateaus by the Central mountain range, with the Montes de Toledo in the Southern Sub-plateau. Around the plateau lies a series of peripheral reliefs (Galician-Leonese Massif, Cantabrian Mountains, Central System, and Sierra Morena). Outside the plateau are other reliefs (Basque Mountains, Pyrenees, Catalan Coastal Range, and the Betic Cordilleras). Between these large reliefs are valleys or depressions forming the backbone of peninsular river systems (Douro Valley between the Cantabrian and Central Systems, Tagus Valley between the Central System and Montes de Toledo, Guadiana Valley between Montes de Toledo and Sierra Morena, Guadalquivir Valley between Sierra Morena and the Betic Cordilleras, and the Ebro Valley between the Pyrenees and the Central System).
Major Morphostructural Units: The major morphological units found in Spanish relief are:
Sockets: Plains formed in the Paleozoic Era or as a result of the erosion of mountain ranges from that era. They occupy large areas in the western half of the peninsula.
Block Mountains: Old mountains formed in the Tertiary by the uplift of a base block due to Alpine orogenic movements. Examples include the Galician Massif and the Montes de Toledo.
Folded Ridges: Large mountain elevations formed during the Alpine orogeny by the folding of sedimentary material deposited by the sea in the Secondary Era. There are two types:
- Intermediate ridges, formed by the folding of material deposited on the sockets’ mountain ridges (Iberian System).
- Alpine ridges, formed by the folding of material deposited in long, deep trenches (Pyrenees, Betic Cordilleras).
Sedimentary Basins: Sunken areas or depressions formed in the Tertiary and filled with sediment. There are two types:
- Basins formed by the collapse of a base block due to Tertiary orogenic pressures (Duero depression).
- Pre-Alpine depressions, located on both sides of the Alpine ranges, originating from decompression following the uplift of the Cordilleras (e.g., the Guadalquivir depression).
Lithological Diversity of the Iberian Peninsula, Balearic, and Canary Islands
Silica Area: This area consists of siliceous rocks from the Precambrian and Primary Eras. It is mostly found in the western peninsula, extending to the western Cantabrian Mountains, the Central System, the Montes de Toledo, and Sierra Morena. It’s also found in the Axial Zone of the Pyrenees and parts of the Iberian System, the Coastal Range, and the Penibético System.
Granite is the prevailing rock. Crystal decomposition forms yellowish-brown sands. Granite alteration from joint networks varies with altitude:
- In high mountain areas, water freezes in fractures, breaking rocks and forming sharp peaks and scree.
- In lower areas, parallel joints cause scaling, resulting in a slightly undulating landscape (domes).
- Perpendicular joint disturbance forms granite boulders, tors, and chaotic knights.
Limestone Area: This area consists of limestone sediments from the Secondary Era, folded during the Tertiary. It forms an inverted “Z” spanning the Pre-Pyrenees, Basque Mountains, eastern Cantabrian Mountains, Iberian System, part of the Catalan Coastal Range, and the Subbetic Cordillera.
Limestone, easily dissolved by rainwater, creates karst landscapes:
- Sinkholes (Lenar) have varied forms: slope, linear, and table sinkholes; lapies (Mar stone).
- Gorges, canyons, or ravines are narrow, deep valleys with steep slopes, formed by rivers.
- Poljes are closed valleys with flat bottoms, traversed by waterways that disappear into sinkholes.
- Torques are cavities formed where water stagnates.
- Caves are created by water infiltrating limestone cracks, forming stalactites and stalagmites. Infiltrated water resurfaces through springs.
- Grikes are narrow openings connecting the surface with underground galleries.
Clay Area: This area consists of less resistant clay sedimentary materials deposited in the late Tertiary and Quaternary. It includes many depressions of the northern and southern sub-plateaus, the Ebro and Guadalquivir depressions, and the Mediterranean coastal plains.
The relief is substantially horizontal, with water erosion causing gullies on slopes due to lack of vegetation, leading to badlands landscapes.
Relief Caused by Differential Erosion
Within the three areas mentioned, erosion creates different reliefs based on strata inclination:
- Horizontal strata with a river network: sides erode faster, reducing tables to mesas and buttes.
- Gently inclined strata: slopes form; erosion retreat leads to mesas and buttes.
- Folded strata:
- Appalachian relief forms over eroded Hercynian mountains, creating parallel ridges, narrow valleys, and open depressions.
- Jurassic relief: alternating convex and concave folds. In anticlines, water erosion creates transverse valleys (cluses) and longitudinal valleys (combes). Anticline emptying leaves synclinal valleys raised, inverting the relief.
Rugged Terrain: Dynamics of Major Geological Eras in the Iberian Peninsula, Balearic, and Canary Islands
Evolution of Peninsular Morphostructural Units
Precambrian Era: An arched band emerged from the sea, northwest to southeast, including most of Galicia, along with some isolated points. This Precambrian mass was eroded and covered by Paleozoic seas.
Paleozoic Era: The Hercynian orogeny occurred. Hercynian mountain ranges (granite, slate) emerged. The Hesperian Massif rose in the west, eroded, and became a socket tilted towards the Mediterranean. The Aquitaine, Catalan-Balearic, and Ebro massifs appeared in the northeast, and the Betic-Rif Massif in the southeast, later eroded into sockets.
Mesozoic Era: A calm period of erosion and sedimentation. The Hercynian mountains continued to erode. The plateau’s slope towards the Mediterranean led to sea penetration and thick sediment deposition in Pyrenean and Betic trenches.
Tertiary Era: Alpine orogeny occurred.
- Alpine ranges uplifted by folding material in Pyrenean and Betic trenches. The Pyrenees, Betic Cordilleras, and Balearic Islands (a Betic Cordillera extension) emerged.
- Pre-Alpine depressions (Ebro and Guadalquivir) formed parallel to the Pyrenees and Betic ranges.
- The plateau was affected: tilted towards the Atlantic, orienting peninsular rivers; mountain ridges formed (eastern Cantabrian Mountains, Iberian System); the Betic Cordilleras’ thrust uplifted Sierra Morena; plateau base fractured, creating plateau mountain ranges, the Galician Massif, and sedimentary basins. The river network was established.
- Quaternary Period:
- Glaciation affected high mountain ranges, forming cirques and valley glaciers.
Cirque glaciers eroded valley heads, forming scarps and lakes.
Valley glaciers formed when thick ice moved from cirques, eroding valleys and forming lakes.
- The postglacial period saw river terrace formation due to climatic alternations. Glacial rivers deposited alluvium; increased flow in postglacial periods eroded channels, creating terraces.