Geological History of the Iberian Peninsula

Precambrian Era (4000 - 600 million years ago)

A mountainous band of slate and gneiss (siliceous materials) emerges from the sea, comprising almost all of current-day Galicia. Elevations also emerged in parts of the Central System and the Montes de Toledo, which were later obliterated by erosion and covered by Paleozoic seas.

Paleozoic Era (600 - 225 million years ago)

During this era, the Hercynian orogeny takes place. From the seas that covered most of the Peninsula, the Hercynian mountains arise, composed of siliceous materials. The Hesperian Massif (or Meseta) appears to the west; to the northeast, the massifs of Aquitaine, Catalano-Balear, and the Ebro; and to the southeast, the Betic-Rif Massif. All of these were subsequently worn down by erosion during this era and converted into socles (base rock).

Mesozoic Era (225 - 68 million years ago)

This was a period of relative geological calm, dominated by erosion (which continued to wear down the Hercynian relief) and sedimentation. Calcareous materials were primarily deposited in the areas covered by the sea. On the eastern edge of the Meseta, which tilted towards the Mediterranean, a relatively thin layer of sediment was deposited. In contrast, huge thicknesses of sediment were deposited in the marine trenches located in the current Pyrenean and Betic areas.

Tertiary Era (68 - 1.7 million years ago)

During this era, the Alpine orogeny occurs, causing major changes to the peninsula's relief:

• Formation of Alpine Ranges: The Alpine mountain ranges rise as sediments from the Pyrenean and Betic trenches are folded between ancient massifs. This process formed the Pyrenees (between the Aquitaine, Ebro, and Hesperian massifs) and the Betic Ranges (between the Betic-Rif and Hesperian massifs).
• Creation of Depressions: Pre-Alpine depressions formed parallel to the new mountain ranges: the Ebro Depression (parallel to the Pyrenees) and the Guadalquivir Depression (parallel to the Betic Ranges).
• Tilting of the Meseta: The Meseta, or central plateau, began to tilt towards the Atlantic Ocean, which determined the orientation of many of the peninsula's rivers.
• Formation of Meseta Edges: The mountainous eastern and southern edges of the Meseta were formed. On the eastern edge, Mesozoic sediments folded, giving rise to the eastern part of the Cantabrian Mountains and the Iberian System. On the southern edge, the thrust from the Betic Ranges raised the Sierra Morena.
• Fracturing of the Meseta's Base: The base of the Meseta, composed of rigid Hercynian materials, fractured and faulted. This faulting created a Germanic-style relief of raised blocks (horsts) and sunken blocks (grabens).
  • The horsts formed the mountain ranges to the north of the Meseta (the Galician Massif and the western part of the Cantabrian Mountains) and the interior mountain ranges (the Central System and the Montes de Toledo).
  • The grabens formed the interior depressions, or sedimentary basins, of the Meseta (the Northern and Southern Sub-Mesetas).
• Volcanic Activity: The faulting also led to volcanic activity, particularly in areas like the Canary Islands and Campo de Calatrava.