Iberian Peninsula Geology: Rocks and Relief

Siliceous Rock Areas

Siliceous rock areas are found in the Precambrian era rocks of the western Iberian Peninsula (Galicia, León, Extremadura), with ramifications towards the western Cantabrian Dorsal, the Central System, the Montes de Toledo, and some areas in the Pyrenees Axial Zone, Iberian System, and the Penibetic coastal range. Rock types include quartzite and granite.

Limestone Karst Regions

Limestone sediments, folded during the Tertiary period, form an inverted 'Z' shape that extends through the Pre-Pyrenees, Basque Mountains, eastern Cantabrian Dorsal, part of the Iberian System, the Catalan Coastal Range, and the Subbetic limestone areas. This rock type is a fractured hard rock where water dissolves crevices, forming karstic relief. Its characteristic forms include:

• Gorges: Deep valleys carved by rivers.
• Dolines or Sinkholes: Cavities originating from areas of stagnant water.
• Caves: Created when water infiltrates limestone fissures, forming stalagmites and stalactites.
• Potholes: Narrow openings connecting the surface with underground galleries.

Sedimentary Clay Depressions

Sedimentary areas consist of less resistant materials like clay (clay loams and gypsum), deposited at the end of the Tertiary and Quaternary periods. These are found in the northern and southern sub-plateau depressions, the Ebro and Guadalquivir depressions, and the Mediterranean coastal plains. The relief is generally horizontal, characterized by soft materials susceptible to rapid erosion.

The Iberian High Plateau (Meseta)

The High Plateau (Meseta) is the ancient Hesperian Massif, a peneplain remnant. During the Tertiary period, it was affected by the Alpine Orogeny. It can be differentiated into three main parts:

Interior Mountain Ranges

These cross the Central Plateau, dividing it in half. The Central System stands out as the most vigorous, including the Guadarrama, Gredos, and Peña de Francia ranges. The Montes de Toledo, of lesser height, separate the Tagus and Guadiana basins, with the Sierra de Guadalupe being notable. These ranges formed during the Tertiary period due to the Alpine Orogeny, which caused fractures in the basement and uplifted blocks (horst structures).

Ancient Basement (Zócalo)

Located in the western part of the Peninsula (Zamora, Salamanca, and Extremadura). Erosion has removed the Tertiary sedimentary cover, exposing the ancient basement (granite and slate), which was affected by the Alpine Orogeny.

Sedimentary Basins

• Northern Sub-basin (Duero)

  An endorheic basin with an altitude of 800-850m, enclosed by the Galician Massif, Cantabrian Mountains, Iberian System, and Central System.

• Southern Sub-basin (Tagus & Guadiana)

  An exorheic basin with an altitude of 500-700m, open to the Atlantic.