Spain’s Geological Diversity: Plateaus, Mountains, and Depressions
March. The Rocky Peninsula and Relief Types
The peninsula can be divided into three areas according to the rock type (silica, limestone, and clay), each with distinct relief types (granite, karst, clay, or differential erosion).
3.1 The Silica Area
This area comprises siliceous rocks from the Precambrian and Paleozoic eras. It’s primarily located in the western peninsula but also appears in the Pyrenees’ Axial Zone, parts of the Iberian System, the Catalan Coastal Range, and the Penibetic System.
Granite is the dominant rock. Chemical alteration by water can break down its crystals, creating yellowish-brown sand. Alteration along joints and fractures yields different results based on altitude:
- In high mountains, water freezing in fractures creates scree, rock fragment accumulations at mountain bases.
- In lower elevations, alteration along surface-parallel joints causes exfoliation or slow granite disintegration, resulting in a gently rolling landscape with rounded domes.
Perpendicular joint alteration forms tors, stacked granite balls creating berrocals, knights, and balancing rocks. Slopes or foothills may feature granite chaos.
3.2 The Limestone Area
This area consists of Mesozoic limestone sediments folded during the Tertiary period. Limestone forms an inverted “Z” across the Pre-Pyrenees, Basque Mountains, Iberian System, and Subbetic Range.
Limestone, a hard yet water-soluble rock, fractures into cracks or joints, creating complex karst relief with features like:
- Sinkholes or lapiés: Open furrows for water runoff on slopes or level surfaces with cracks, or cavities separated by sharp partitions.
- Gorges, canyons, or ravines: Narrow, deep valleys with steep slopes formed by rivers.
- Poljes: Closed depressions or valleys with horizontal floors.
- Caves: Underground circular formations created by water infiltrating limestone cracks.
- Chasms: Narrow openings connecting the surface with underground galleries.
3.3 The Clay Area
This area comprises less resistant clay sedimentary materials, including many depressions in the northern and southern sub-plateaus, the Ebro and Guadalquivir depressions, and the Mediterranean coastal plains.
The terrain is mostly horizontal, unaffected by subsequent folding.
In areas with alternating dry and hot periods, torrential rains, and sparse vegetation, water erodes watersheds, forming gullies. Extensive gully development creates badlands.
3.4 Relief Caused by Differential Erosion
Within each area, rocks of varying origin and resistance exist. Selective erosion creates diverse relief based on strata inclination:
- Horizontal, alternating hard and soft strata: River networks separate platforms called mesas or buttes, with the top horizontal layer being hard rock.
- Gently inclined, alternating hard and soft strata: Cuestas are formed.
- Folded strata: Appalachian and Jura relief types are created.
- Appalachian relief forms on a leveled Hercynian mountain range.
- Jura relief forms on young ridges with alternating convex and concave folds, found in the Iberian System, Cantabrian Mountains, Pyrenees, and Betic Range.
4. Major Relief Units of the Peninsular Morphostructure
4.1 The Plateau
The plateau, a high plain formed during the Primary Era orogeny, highlights Spain’s fundamental unity. It features an old basement in mountain ranges and interior sedimentary basins.
- The old basement outcrops in the western peninsula, featuring peneplains (gently rolling erosion surfaces) with occasional hills of resistant rocks. Rivers have carved deep gorges.
- Plateau mountain ranges (Central System and Montes de Toledo) formed during the Tertiary period by the Alpine orogeny, creating rocky ranges with primary soft peaks.
- The Central System, a Germanic structure, divides the Meseta in half.
- The Montes de Toledo, an ancient massif, divides the southern sub-plateau into two watersheds separated by the Tagus River.
- Internal sedimentary basins formed during the Tertiary period when the Alpine orogeny caused block subsidence, creating moors, plains, and hills.
- Moors are flat, high structural surfaces with U-shaped river valleys.
- Countryside refers to gently rolling lowlands traversed by eroded highland rivers.
- Slopes are steep areas between moors and countryside.
The northern sub-plateau basin is higher and uniform, belonging to the Duero river basin and almost entirely enclosed by mountains.
The southern sub-plateau basin is lower, rugged in the middle by the Montes de Toledo, dividing it into two watersheds open to the Atlantic Ocean.
4.2 Plateau Mountain Ridges
These ridges (Macizo Galaico-Leonés, Cantabrian Mountains, Iberian System, and Sierra Morena) formed during the Tertiary period by block rejuvenation or folding of Mesozoic materials.
- Macizo Galaico-Leonés: Formed by Paleozoic materials, featuring rounded, low-lying mountains.
- Cantabrian Mountains: Two distinct sectors:
- The western Asturian Massif, with Paleozoic materials rejuvenated during the Alpine orogeny, displays Appalachian relief in the west and primary limestone (Picos de Europa) in the east.
- The eastern sector, with Mesozoic limestone, exhibits Jura relief.
- Iberian System: Formed mostly from Mesozoic materials, with Paleozoic materials in some areas. Two sectors:
- The northern sector includes the highest elevations.
- The southeastern sector splits into two branches: the interior branch with Mesozoic materials and the exterior (Aragonese) branch with Paleozoic limestone, separated by a rift valley filled with Tertiary materials.
- Sierra Morena: A sharp step separating the plateau from the Guadalquivir valley, interpreted as a gigantic fault. The crag is Paleozoic, with prominent mountains like Madrona, Aracena, and Pedroches.
4.3 External Plateau Depressions
The Ebro and Guadalquivir depressions were pre-Alpine basins filled with Tertiary and Quaternary sediments, resulting in nearly horizontal reliefs.
- Ebro Basin: Parallel to the Pyrenees, enclosed by the Pyrenees, Iberian, and Catalan Coastal Ranges. Varied relief forms due to differing material hardness and arid climate:
- Pyrenean and Iberian foothills (slightly inclined piedmont plains) with tors formed by erosion.
- Central depression with horizontal strata of alternating limestone and clay, creating toothache relief and badlands on soft materials.
- Guadalquivir Depression: Parallel to the Betic Ranges, with predominantly clay materials leading to gently rolling countryside.
4.4 Outer Plateau Ridges
These ridges include the Pyrenees, Basque Mountains, Catalan Coastal Range, and Betic Ranges.
- Pyrenees: Complex structure:
- Axial zone: Rocky Paleozoic massif from the Hercynian orogeny, rejuvenated during the Alpine orogeny, featuring steep, high areas.
- Pyrenean foothills: South of the axial zone, with Mesozoic limestone deposited in the Pyrenean basin and uplifted during the Alpine orogeny.
- Median depression: A long, narrow depression between the inner and outer Pre-Pyrenean marl mountains.
- Basque Mountains: Mostly preface the Pyrenean foothills, with limestone rocks, low height, and smooth shapes.
- Catalan Coastal Range: A transformation of the eastern Pyrenees, with Paleozoic materials in the northern half and limestone in the southern half. Divided into coastal and higher interior ranges.
- Betic Ranges: Geologically complex with the peninsula’s highest elevations, divided into two sets:
- Penibetic Range: Along the coast, with Paleozoic materials, including Sierra Nevada with Mulhacén and Veleta peaks.
- Subbetic Range: Interior, with alternating hard and soft Mesozoic materials, including Grazalema, Ubrique, and Cazorla mountains.
- Intrabetic Depression: Between the two ranges, fragmented into small depressions filled with Tertiary materials, resulting in badlands due to the arid climate.