Relief and Rock Formations in the Iberian Peninsula
The Iberian Peninsula and Relief Types
Siliceous Rock Areas
These areas are composed of siliceous rocks from the Precambrian and Paleozoic eras. Typically located in western regions like Galicia, León, and Extremadura, they also extend to the western Cantabrian Mountains, Central System, Montes de Toledo, Sierra Morena, and remnants of ancient massifs such as the Pyrenees’ Axial Zone and parts of the Iberian, Cordillera Costero-Catalana, and Penibético systems.
The predominant rock is granite, a crystalline and rigid rock susceptible to various forms of weathering. The characteristics of granite influence the modeling of the siliceous landscape.
Granitic Relief Elements:
- Pardoamarillentas Sands: These yellowish-brown sands result from the decomposition of granite crystals due to chemical alteration by water. They accumulate in valleys and low-slope areas.
Effects of Water on Granite:
- High Mountain: Water interacts with joints in the rock, and freeze-thaw cycles (gelifraction) break the rocks, forming sharp ridges and scree slopes.
- Lower Elevations: If the joints are parallel, the granite undergoes exfoliation, resulting in rounded domes. If the joints are perpendicular, they form balls, leading to various formations:
- Berrocales: Accumulations of granite balls, forming tors (stacks of balls), rocking stones (bolas), and castle koppies.
- Granitic Chaos: Erratic arrangements of granite balls on slopes or at the foot of mountains.
Limestone Areas
Formed by limestone sediments from the Mesozoic era, folded during the Cenozoic era. These reliefs include the Pre-Pyrenees, Basque Mountains, eastern Cantabrian Mountains, Iberian System, parts of the Cordillera Costero-Catalana, and Subbética Mountains.
Limestone is hard and forms cracks (diaclases) but dissolves easily in rainwater, creating karst landscapes.
Elements of Karst Relief:
- Sinkholes (Dolines or Lenar): Furrows or grooves formed by water in various ways:
- Slope Sinkholes: Runoff furrows on slopes or flat surfaces with cracks.
- Linear Sinkholes (Lapies): Narrow rows separated by sharp walls.
- Table Sinkholes: Large flat areas among furrows.
- Sea of Stone: Chambers separated by sharp walls where water collects and filters, creating deep holes.
- Canyons, Gorges, or Scythes: Narrow, deep valleys with steep slopes carved by rivers.
- Polje: Closed depressions or valleys, sometimes flooded, with streams that disappear into sinkholes (ponors) and flow underground.
- Caves: Formed by water infiltrating limestone fissures, often containing stalactites and stalagmites.
- Shafts (Sima): Narrow openings connecting the surface with underground galleries.
Clay Areas
Composed of less resistant materials (clays, marls, and gypsum) deposited during the late Cenozoic era. These areas include the depressions of the northern and southern sub-plateaus (Ebro and Guadalquivir), and Mediterranean coastal plains.
Clay relief is generally horizontal, easily eroded due to the softness of the materials. Rivers play a significant role, creating gently rolling landscapes. Alternating dry periods with heavy rainfall leads to the formation of gullies and badlands.
Clay Landforms:
- Differential Erosion:
- Horizontal Relief: Alternating hard and soft horizontal strata create mesas, buttes, and witness hills.
- Hilly Relief: Gently inclined strata with alternating hard and soft materials form cuestas with a steep front and gentle back slope.
- Appalachian Relief: Folded layers in ancient mountains create parallel ridges and valleys.
- Jurassic Relief: In younger folded mountains, erosion creates cluses, combes, and inverted relief.
- Sinkholes: Cavities formed in areas of stagnant water, sometimes merging to form larger depressions (uvalas).