Geological Evolution of the Iberian Peninsula
The Plateau
An elevated plain at 600-800 m, formed by the erosion of the Hesperian Massif. It was reshaped by the Alpine orogeny, creating three distinct units:
a) The Former Paleozoic Base
Located west of the peninsula, erosion exposed raw materials like granite, slate, and quartzite. The relief is primarily peneplain, with occasional residual mountains. Rivers crossing the plateau have carved deep gorges.
b) The Mountain Ranges of the Plateau
Formed during the Tertiary period by the Alpine orogeny. Fractures and faulting raised some blocks, creating primary, rocky peaks:
- Central System: Divides the plateau in two, with prominent mountains like Somosierra, Guadarrama, Gredos, and Gata.
- Montes de Toledo: Lower, dividing the southern sub-plateau, with Guadalupec as its most prominent peak.
c) Inland Sedimentary Basins of the Plateau
Formed in the Tertiary period when collapsing blocks filled with sediments. This created moorlands (The Alcarria, Mesa de Ocaña, and La Mancha) and plains with hills (Duero, Tagus, and Guadiana basins).
- Northern sub-plateau: Slopes belonging to the Duero Basin.
- Southern sub-plateau: Hilly, divided by the Montes de Toledo into the Tagus and Guadiana basins.
Peninsular Geological Evolution
The current relief is the result of millions of years of geological history:
1) Archaic or Precambrian Era (4000-600 million years)
Slate and gneiss elevations emerged. Isolated mountains and the central system were eroded and covered by the sea.
2) Primary or Paleozoic Era (600-225 million years)
The Hercynian orogeny formed mountain ranges: the Hesperian Massif (northwest), the Aquitaine, Catalonia, the Balearic Islands, the Ebro Massif, and the Betic-Rifeño Massif (southwest).
3) Secondary or Mesozoic Era (225-68 million years)
Erosion of the Hercynian ranges and sedimentation in covered areas.
4) Tertiary Era (68-1.7 million years)
The Alpine orogeny caused significant changes: Alpine ranges were lifted, pre-Alpine depressions formed, the plateau tilted, and the plateau’s base fractured and faulted.
5) Quaternary Era (1.7 million years – present)
Key events include:
- Glaciers: Cirque glaciers (ice accumulations) and valley glaciers (rivers of ice).
- Fluvial terraces: Flat strips raised on river fringes.
Types of Morphostructural Units
a) Sockets
Plains and plateaus formed in the Paleozoic era. Composed of rigid silica materials (granite, slate, quartzite), they fracture rather than fold under pressure. They occupy large areas in the western peninsula.
b) Solid Ancients
Mountains formed in the Tertiary by the uplift of base blocks. They have rounded peaks and are found in the plateau’s ranges (Central System and Montes de Toledo) and the western Cantabrian Mountains.
c) Folding Ridges
Large mountain elevations formed by the folding of sedimentary materials. Two types:
- Intermediate ranges: Formed by the folding of material deposited on the baseboard edges (Iberian System and eastern Cantabrian Mountains).
- Alpine ranges: Formed by the folding of materials deposited in geosynclines (Pyrenees, Betic Cordilleras).
d) Sedimentary Basins or Depressions
Sunken areas filled with sediments. Two types:
- Basins formed by the collapse of a base block (interior depressions of the plateau).
- Pre-Alpine depressions, located on either side of the Alpine ranges (Ebro and Guadalquivir).
Mountain Flanges of the Plateau
a) The Massif Galaico-Leones
Located northwest, it consists of Paleozoic materials and features rounded, low-lying mountains (Segundera, Cabrera, and Los Ancares).
b) The Cantabrian Mountains
Composed of two parts:
- Western sector: Paleozoic materials, rejuvenated during the Alpine orogeny.
- Eastern sector: Limestone materials folded during the Alpine orogeny. Highest elevations are in the Picos de Europa.
c) The Iberian System
Composed of secondary materials folded during the Alpine orogeny. Two distinct areas:
- North: NW-SE ranges (Moncayo, Picos de Urbión, Demanda, and Cebollera).
- South: Branches from Teruel (Castilian and Aragonese).
d) Sierra Morena
A step separating the plateau from the Guadalquivir Valley. A fractured deflection with Paleozoic rocks. Prominent mountains include Madrona, Aracena, and Pedroches.
Depressions
Pre-Alpine basins filled with Tertiary and Quaternary sediments. Two highlights:
a) The Ebro Depression
Parallel to the Pyrenees, with marine and continental deposits. Varied relief includes foothills, piedmont, and badlands.
b) The Guadalquivir Depression
Parallel to the Betic System, filled with clay, limestone, and marls. Relief includes rolling plains, hills, and mesas.
External Cordilleras
Formed during the Alpine orogeny.
a) The Pyrenees
Divided into two areas:
- Axial zone: Paleozoic rocks, steep and high (Pico de Aneto).
- Pre-Pyrenees: Limestone, less high and softer.
b) The Basque Mountains
Continuation of the western Pyrenees, with secondary rocks (Aralar, Gorbea).
c) The Catalan Coastal Range
Transformation of the eastern Pyrenees, divided into two alignments separated by a longitudinal depression.
d) The Betic Cordilleras
Geologically complex, with the highest peaks. Two major folds separated by the intra-Betic depression:
- Penibética: Along the coast, Paleozoic materials (Sierra Nevada).
- Subbética: Inland, secondary materials (Sierra de Grazalema, Ubrique, Cazorla).