Geological Evolution of the Iberian Peninsula Relief
The current relief of the Iberian Peninsula is the result of a geological history spanning millions of years, characterized by alternating orogenic phases and periods of calm dominated by erosion and sedimentation.
A. Archaic or Precambrian Era (4,000–600 Million Years Ago)
- An arched band emerged from the sea, extending from the northwest to the southeast, formed by schists and gneiss. This band included almost all of present-day Galicia.
- Elevations also arose in isolated points of the Central System and the Montes de Toledo.
- This Precambrian massif was later devastated by erosion and almost entirely covered by the Paleozoic seas.
B. Primary or Paleozoic Era (600–225 Million Years Ago): The Hercynian Orogeny
The Hercynian orogeny took place during this era. From the seas covering most of the Peninsula, the Hercynian mountain ranges arose, formed by siliceous materials such as granite, slate, and quartzite.
- To the west, the Hesperian Massif rose, which was eroded during the same era and converted into a plinth (basement) or plateau sloping towards the Mediterranean.
- Massifs also appeared to the northwest (Aquitaine, Catalan-Balearic, and the Ebro) and to the southeast (the Bético-Rifeño Massif).
- All these massifs were subsequently leveled by erosion and turned into plinths during the Primary Era.
C. Secondary or Mesozoic Era (225–68 Million Years Ago): Period of Calm
The Mesozoic Era was a period of calm dominated by erosion and sedimentation. The flattening of the Hercynian mountain ranges continued.
The inclination of the base of the Plateau towards the Mediterranean allowed, during periods of marine transgression, a deep penetration of the sea, which deposited on its eastern edge a relatively thin cover of plastic sedimentary materials (limestones, marls). Enormous thicknesses of sediment were also deposited in sea trenches located in the present-day Pyrenean and Betic areas.
D. Tertiary Era (68–1.7 Million Years Ago): Alpine Orogenesis
The Alpine Orogenesis profoundly reshaped the Peninsula. Key results include:
Formation of Alpine Mountain Ranges
The alpine mountain ranges were raised when the materials deposited in the Pyrenean and Betic trenches were folded between the ancient massifs, which acted as rigid blocks (butresses).
- The Pyrenees arose between the Aquitaine, Hesperian, and Ebro massifs (which eventually sank).
- The Béticas mountain ranges formed between the Bético-Rifeño and Hesperian massifs.
Creation of Pre-Alpine Depressions
Pre-alpine depressions formed parallel to the new mountain ranges, situated between these ranges and the old massifs:
- The depression of the Ebro (parallel to the Pyrenees).
- The depression of the Guadalquivir (parallel to the Béticas).
Transformation of the Plateau (Meseta Central)
The Plateau was significantly affected by the Alpine Orogeny:
- Inclination: It first began to incline towards the Atlantic, determining the direction of most peninsular rivers towards that ocean.
- Mountainous Edges: The mountainous edges of the Plateau were formed. On the eastern edge, plastic materials deposited in the Secondary Era were folded, giving rise to the eastern part of the Cantabrian mountain range and the Iberian System. On the southern edge, the push of the Béticas mountain ranges raised Sierra Morena.
- Fractures and Faults: The base of the Plateau, formed by rigid Paleozoic materials, experienced fractures and faults. This resulted in a Germanic structure, consisting of raised or rejuvenated blocks (horst) and sunken blocks (tectonic pits or graben).
The raised blocks formed the Galaico Massif and the interior mountain ranges of the Plateau (Central System and Montes de Toledo). The sunken blocks created the interior depressions or sedimentary basins of the Plateau (those of the northern and southern subplateaus).
The faults also triggered volcanic activity in areas such as Campo de Calatrava, Olot-Ampurdán, and Cabo de Gata.
River Network Establishment and Canary Islands Origin
Following the alpine orogenic movements, the river network was established. Rivers severely eroded the new mountain ranges, created alluvial fans (accumulations of pebbles at the foot of the mountains), and filled the inner and outer depressions of the Plateau, which are now practically flat areas.
The volcanic nature of the Canary Islands also originated in this era, when the alpine orogeny fractured the bottom of the Atlantic, allowing large masses of volcanic rocks to rise and form the islands.
E. Quaternary Era (1.7 Million Years Ago to Present)
Glaciation and the formation of river terraces were the primary processes of erosion and sedimentation that contributed most to modeling the current relief.
Glaciation and Glacial Landforms
Glacierism affected the highest mountain ranges (Pyrenees, Cantabrian Mountains, Central System, Iberian System, and Sierra Nevada), giving rise to cirque and valley glaciers.
- Cirque Glaciers: These are confined to the head of the valley (the cirque), where ice accumulates. Ice and thawing break the rocks of the cirque walls, causing the cirque to grow larger and its shapes to become steeper. Due to the Peninsula’s latitude, most glaciers were cirque glaciers, which formed small lakes when the ice melted.
- Valley Glaciers: These are rivers of ice, forming when the accumulated ice thickness is large enough to move down the valley. The ice contains rock fragments that dig into the valley, giving it the typical “U” shape. They also excavate small depressions that turn into lakes when the ice melts. Only the Pyrenees were covered by a powerful layer of ice, creating “U” valleys and lakes.
Formation of River Terraces
The Quaternary Era was characterized by the formation of river terraces—flat, elevated strips located on the banks of a river. These are ancient floodplains abandoned due to subsequent river incision (downcutting).
Terraces are the result of the climatic alternations of the Quaternary:
- During Glacial periods, when ice accumulated in the mountains, rivers lost erosive force and deposited alluvium in their channel.
- During Postglacial periods, as temperature increased and ice melted, the flow and erosive force of the rivers increased. They deepened their bed, leaving the previously accumulated alluvium suspended on their banks, thus forming terraces.
The successive glacial and postglacial cycles gave rise to stepped terraces. In the Peninsula, the most characteristic examples are those formed by the rivers Douro, Tagus, Guadiana, Guadalquivir, and the Ebro.