Evolution of Peninsular Relief Morphostructures: Geological Eras
Evolution of Relief Morphostructural Peninsular Units
a) Precambrian Archean (4000 – 600 million years)
A band from northeast to southwest emerged from the sea, an arqueda formed by shale and gneiss, occupying almost all of current Galicia. Elevations also arose in individual points of the Central and Toledo Mountains. This Precambrian massif was later eroded and almost entirely covered by Paleozoic seas.
b) Primary or Paleozoic Era (600 – 225 million years)
The Hercynian orogeny took place. From the seas that covered much of the peninsula, Hercynian mountain ranges emerged, formed by materials such as granite, slate, and quartzite. To the west rose the Hesperian Massif, which was razed during the same era, becoming a tilted plateau towards the Mediterranean. The Aquitaine, Catalano-Balearic, and Ebro Massifs appeared to the northeast, while the Betic-Rif came to the southeast. All these massifs were eroded and converted into plateaus during the Primary Era.
c) Secondary or Mesozoic Era (225 – 68 million years)
A period of calm prevailed, dominated by erosion and sedimentation. The leveling of Hercynian mountain ranges continued. The eastward tilt of the plateau base towards the Mediterranean allowed for the deposition of plastic limestone, sandstone, and marl sediments on the eastern rim during marine transgressions. Simultaneously, enormous sediment thickness accumulated in the Pyrenean and Betic trenches.
d) Tertiary Period (68 – 1.7 million years)
The Alpine orogeny occurred, resulting in the formation of Alpine ranges by folding materials deposited in the Pyrenean and Betic basins between ancient massifs, which acted as buffers. The Pyrenees arose between the Aquitaine and Ebro-Hesperian masses (the latter eventually sinking). The Andalusian ranges emerged between the Betic-Rif and Hesperian masses. Pre-alpine depressions formed parallel to the new ranges: the Ebro Depression parallel to the Pyrenees and the Guadalquivir to the Betic Ranges. The plateau was affected by the Alpine orogeny, tilting towards the Atlantic (directing many peninsular rivers towards the ocean). Mountain ridges formed on the Plateau: at its eastern edge, folded plastic materials deposited by the sea during the Secondary Era created the eastern Cantabrian Mountains and the Iberian System. At the southern edge, the thrust of the Andalusian ranges uplifted the Sierra Morena.
The plateau base, formed by rigid Paleozoic materials, experienced fractures and faults, creating a Germanic structure of uplifted blocks (horsts) and sunken blocks (grabens). Uplifted blocks formed the Galaico Massif, the Central System, and the Montes de Toledo, while sunken blocks created inland depressions or sedimentary basins within the Plateau.
Faulting led to volcanic activity in areas like Campo de Calatrava, Olot-Ampurdán, and Cabo de Gata. Finally, after the Alpine orogeny, the river network was established. River erosion mainly affected the mountains, creating scree (rock debris at the foot of mountains) and filling the interior and exterior depressions of the plateau, resulting in virtually flat areas.
e) Quaternary Period (1.7 million years to present)
Glaciers and river terraces formed. Glaciation affected the highest mountain ranges: the Pyrenees, Cantabrian Mountains, Central System, Iberian System, and Sierra Nevada. Cirque glaciers were confined to valley heads (cirques). The melting ice and rock fracturing widened and steepened the cirque contours. Disappearing glaciers left behind lakes. Cirque glaciers were the most common type in the Peninsula.
Valley glaciers formed when ice overflowed the cirques and flowed down valleys, dragging rocks that eroded the valley floor, creating a characteristic “U” shape. Glacier tongues also over-excavated small basins, which became lakes upon ice melt. These trough-shaped valleys only occur in the Pyrenees.
The Postglacial Era was characterized by the formation of river terraces, ancient floodplains left by rivers as they downcut. River terraces result from Quaternary climatic alternations. During glacial periods, rivers lost erosive force, depositing alluvial sediments on their banks. In post-glacial periods, rivers regained erosive force, deepening their beds and leaving the accumulated alluvium as terraces. This created successive terrace tiers. The most characteristic examples are found along the Douro, Tagus, Guadiana, Guadalquivir, and Ebro rivers.