Iberian Peninsula Relief: Formation, Features, and Types

Posted on Nov 25, 2024 in Geology

General Features of the Iberian Peninsula Relief

The main features of the Iberian Peninsula’s relief are:

  • High average altitude: 660 meters above sea level. 90% of the territory is over 200 meters above sea level. This elevation is due to the high plains and the numerous mountains in Spain. This altitude impacts agriculture.
  • Central plateau organizing the relief: This plateau is an old, high, solid, and slightly tilted-toward-the-Atlantic central block. It dictates the altitude and arrangement of surrounding ridges and mountains.
  • Peripheral reliefs: These reliefs are arranged in relation to the mainland plateau and the peninsula’s contour. The dominant west-east orientation of ridges influences precipitation distribution and the peninsula’s isolation.
  • Compact and solid peninsula: With few coastline accidents except in Galicia, the peninsula resembles a small continent protected by mountains that form a barrier between the plateau and the sea. This reduces maritime influence to the coastal strip, as the relief hinders communication between the interior and the coast, and across regions.

Formation of the Relief: Geological Eras

The Iberian Peninsula’s present relief results from a geological history spanning millions of years, alternating between orogenic phases and calmer periods of erosion and sedimentation.

a/ Archaic or Precambrian Era (4000-600 million years ago)

An arcuate band emerged from the sea, running from northwest to southeast and consisting of slate and gneiss, which included present-day Galicia. Elevations also appeared in parts of the Central System and the Montes de Toledo. This Precambrian mass was later eroded and almost entirely covered by Paleozoic seas.

b/ Primary or Paleozoic Era (600-225 million years ago)

The Hercynian orogeny occurred. Hercynian ranges, composed of materials like granite, slate, and quartzite, emerged from the seas covering much of the peninsula. The Hesperian Massif rose to the west, later eroded and becoming the Zócalo or plateau tilted towards the Mediterranean. The Aquitaine, Catalano-Balear, and Ebro massifs appeared to the northeast, and the Betic-Rif massif to the southeast. All were eroded during the Primary Era and became zócalos (basement rocks).

Secondary or Mesozoic Era (225-68 million years ago)

A period of orogenic calm, erosion, and sedimentation. Hercynian mountains were reduced to plateaus and plains by rain, wind, and snow. The plateau’s inclination towards the Mediterranean allowed, during marine transgressions, deep sea penetration, depositing plastic sedimentary materials (limestone, sandstone, loam) on the plateau’s edge. Thick sediments also accumulated in trenches located in today’s Pyrenean and Betic areas.

Tertiary Era (68-1.7 million years ago)

The most active orogenic period due to the Alpine orogeny, affecting secondary and primary materials. Due to the varying nature of these materials, the toughest broke, and the most plastic compressed, forming mountain ranges.

During the Alpine folding, major peninsular relief units emerged:

  • Alpine Ridges: Formed by the folding of materials deposited in the basins between the ancient Pyrenean and Betic massifs. The Pyrenees arose between the Aquitaine and Ebro massifs (the latter eventually subsiding), and the Betic Mountain Ranges between the Betic-Rif and Hesperian massifs.
  • Pre-Alpine Depressions: Formed parallel to the new ranges: the Ebro Basin alongside the Pyrenees and the Betic Depression alongside the Betic ranges.

The Alpine orogeny also affected the plateau:

  • It tilted towards the Atlantic, influencing the orientation of some peninsular rivers.
  • Mountain ridges formed on the plateau: the Iberian Mountains on the eastern edge, the Cantabrian Mountains above the eastern part, and the Sierra Morena on the southern edge.
  • The plateau’s Paleozoic base, composed of rigid materials, experienced fractures and faults, leading to the Germanic structure. This structure consists of uplifted blocks (horsts) forming the Galician Massif, Central System, and Montes de Toledo, and sunken blocks (grabens) creating interior depressions or sedimentary basins (North and South Submeseta). The faults resulted in volcanic activity in areas like Campo de Calatrava, Olot-Ampurdán, and Cabo de Gata.

e/ Quaternary Era (1.7 million years ago – present)

Silting of the largest inland depressions (Ebro and Guadalquivir) and erosion of the entire relief. The Ice Age saw glacial modeling (cirques, valleys, etc.).

This orogenic movement was accompanied by the separation of major continental plates, forming the continents we know today.

Variety and Lithological Relief

Considering the Iberian Peninsula’s soil composition, we can speak of “three Iberias,” each with distinct terrain types based on rock hardness and erosion behavior.

Silica Iberia

Occupies the western third, from Galicia to the Guadalquivir valley, also present in the Sierra Nevada, Pyrenees, Central System, and Sierra Morena. Composed of the oldest (Paleozoic) materials, including slate and granite, which erode into brownish-yellow sand. Granite weathering in joints or fractures creates various terrains:

  • Sharp peaks and screes (rock fragment accumulations) at the foot of high mountains.
  • Gently undulating landscapes with rounded shapes and berrocal (piles of granite balls).

Limestone Iberia

Composed mainly of limestone. Occupies the eastern half, forming an inverted “Z” through the Pyrenees, Cantabrian Mountains, Iberian, and Betic Cordilleras (Torcal de Antequera). Produces colorful landscapes. The permeable rocks accumulate water that seeps through joints, creating numerous springs at the foot of the mountains.

The resulting karst relief features various forms:

  • Sinkholes, gorges, poljes, caves, and chasms.

Clayey Iberia

Coincides with plains and depressions (Ebro, Guadalquivir), formed by erosion of mountainous areas. Composed of young, fertile Tertiary and Quaternary materials. Results in flat, easily eroded relief with excellent agricultural conditions. In areas with torrential rains alternating with droughts, gullies form “badlands.”

Relief Caused by Differential Erosion

Within the “three Iberias,” rocks of varying origin and resistance create diverse relief forms.

  • Horizontal strata result in mesas, buttes, and hills.
  • Gently inclined strata form slopes.
  • Folded strata create Appalachian and Jurassic relief.

Large Morphostructural Sets

Three major morphostructural groups exist:

  1. Ancient massifs: Medium-height mountains with flattened peneplain summits, composed of folded Paleozoic materials deformed by Hercynian and Alpine folding.
  2. Alpine mountains: Young, high mountains composed mainly of limestone.
  3. Depressions: Varying types:
    • Sunken fragments of the old Hercynian base (e.g., Cuenca del Duero).
    • Former inlets or bays silted up by sediments from Alpine ranges (e.g., Ebro and Guadalquivir depressions).