Iberian Peninsula: Relief and Morphostructural Units

Posted on Dec 17, 2024 in Geology

The Major Units of Peninsular Relief

The relief of the Iberian Peninsula can be grouped into the following morphostructural units:

The Plateau

The fundamental unit of the relief of the Iberian Peninsula arose during the Hercynian orogeny in the Primary Era. It was part of the Hesperian Massif, ravaged by erosion and became a socket. In the Tertiary Period, it was deformed and partially destroyed due to the Alpine orogeny. Thus, within the plateau, we can distinguish the old Paleozoic socket, the interior mountains, and the inland sedimentary basins.

The Old Paleozoic Socket

Appears in the west of the peninsula, where erosion has washed away the Tertiary sedimentary materials, revealing the primary materials: granite, slate, and quartzite. The relief consists of peneplains: erosion surfaces, very gently rolling, with little difference between valleys and watersheds. Sometimes, inselbergs are present: residual reliefs made of more resistant rocks, mainly quartzite. In these peneplains, the rivers form deep gorges as they are fitted on hard materials. Examples include the Zamora-Salamanca Peneplain and the Extremadura Peneplain.

The Inner Mountain Ranges of the Plateau

These are the Central System and the Mountains of Toledo. They formed in the Alpine Orogeny, resulting from the fracture and elevation of blocks of the plateau’s plinth. In both reliefs, primary crag dominates: granite, slate, gneiss. Their summits are smooth, as they are re-elevated erosion surfaces.

  • Central System: Divides the plateau in the middle. Formed by the mountains of Somosierra, Guadarrama, Gredos, Peña de Francia, Gata, and Estrela (Portugal).
  • Montes de Toledo: Divide the southern sub-plateau, separating the basins of the Tagus and Guadiana. The Sierra de Guadalupe stands out.

The Internal Sedimentary Basins of the Plateau

They formed in the Tertiary Period, because of the fracture and collapse of blocks of the Meseta’s socket. They were first covered by inland lakes and then filled with Tertiary sediments: soft in the lower layers (sands, clays, gypsum, marl) and hard on the surface (limestone). This originated a relief of moors, plains, and hills.

  • The moors: Flat and high structural surfaces, crowned by a limestone layer more resistant to erosion. The rivers carve “U” shaped valleys that cut and separate them into smaller tables. They are present in the north and east of the northern sub-plateau basin, and east of the southern sub-plateau (La Alcarria, Mesa de Ocaña, and La Mancha).
  • The countryside: Gently rolling lowlands crisscrossed by rivers, which form where the moors have been eroded and loams and clays outcrop from the lower levels. Witness hills or hillocks are common, crowned by the residual relief of limestone plains. The plains of the Duero, Tagus, and Guadiana stand out.
  • The slopes are steep areas between the highlands and the plains.

The Mountain Ridges of the Plateau

Their formation during the Tertiary Period is due to two causes: rejuvenation of the Plateau blocks and folding of material deposited by the sea during the Secondary Era in the ridges of the plateau. We can distinguish the following reliefs:

Galaico-Leonese Massif

A block of the Plateau socket fractured and rejuvenated during the Alpine Orogeny. Formed by Paleozoic materials. It features low, rounded mountains and high areas, cut by numerous faults. The Segundera, Cabrera, and Los Ancares mountain ranges stand out.

Cantabrian Mountains

It has two distinct sectors:

  • Western Sector and Asturian Massif: Formed by Paleozoic materials, part of the former base of the plateau, rejuvenated in the Alpine Orogeny. In the western zone, Appalachian areas appear.
  • Far East: A large primary limestone outcrop, which makes up the Picos de Europa, home to the greatest heights of the Cordillera: Torre Cerredo, Peña Vieja, and Naranjo de Bulnes.
  • Eastern Sector or Cantabrian Mountain: Formed by secondary limestone materials, deposited by the sea on the edge of the plateau during the Secondary Era and later folded during the Alpine Orogeny. In this sector, there are examples of Jurassic relief.

Iberian System

Made up of secondary material deposited by the sea on the eastern edge of the base of the plateau, then folded in the Alpine Orogeny. Paleozoic materials appear in some areas where Mesozoic sediments had less power, allowing the socket blocks to outcrop, rejuvenated by the Alpine Orogeny. There are two sectors:

  • Northern Sector: NW-SE direction, includes the highest peaks of the Cordillera: Urbión peaks. The Sierra de la Demanda (Paleozoic) and Moncayo (limestone) stand out.
  • From the southeast of Soria, the Iberian System splits into two branches:
    • Inner or Castilian branch: Sierra de Albarracín (Paleozoic) and Serranía de Cuenca (limestone).
    • Outer or Aragonese branch: Sierra de Javalambre and Sierra de Gúdar (limestone).

Both branches are separated by the Calatayud Rift Valley, filled with Tertiary materials.

Sierra Morena

An abrupt step that separates the plateau from the Guadalquivir valley. More than a gigantic fault, it is a great flexure of the socket, fractured in several places, produced by the thrust of the Andalusian ranges rising during the Alpine Orogeny. The rocks are of Paleozoic origin and dark in color. The Sierra Madrona, Pedroches, and Aracena stand out.

The External Depressions of the Plateau

Their formation dates from the Tertiary Period, when these ancient pre-Alpine basins or pits were located between the Alpine ranges and ancient massifs. Following the Tertiary orogeny, they formed subsidence basins, whose funds were gradually sinking during the Tertiary. Triangular in shape, they were filled by powerful thicknesses of Tertiary and Quaternary sediments. At present, there are virtually horizontal reliefs. These depressions are:

Depression of the Ebro

Parallel to the Pyrenees. Located between the Pyrenees, the Iberian System, and the Cordillera Costero-Catalana. It occupies the site of the old massif of the Ebro. After being heavily eroded, it sank while the Alpine ranges that border it rose. The depression was first occupied by the sea, but then it closed, turning into a lake until the end of the Tertiary Period, when the Ebro made its way to the sea through the Cordillera Costero-Catalana. The result of this evolution is thick marine and continental material deposits: conglomerates in the mountain ridges, finer materials in the center of the depression: sandstone, marl, gypsum, limestone, and salt. The hardness of different materials and the arid climate have given rise to various forms of relief:

Relief Forms of the Ebro Basin
  • Somontano or foothills: Present in both the Pyrenees and the Iberian System. They are flatlands, but slightly inclined, between the outer ranges and the central depression. They consist of conglomerates, thick and hard materials, from the mountainous relief, transported by rivers. Above them are different types of relief:
    • Meshes: Rocky towers identified by erosion exploiting the vertical diaclases.
    • Hoyas: Erosive depressions formed in the softer materials, may be small, forming salt ponds, shallow and temporary; or larger: Hoya de Huesca and Barbastro.
  • Center of depression: Horizontal layers, alternating hard limestones and clays, marls, and soft casts determine the appearance of a relief of flat surfaces on hard materials, and badlands on soft materials.

Guadalquivir Depression

Parallel to the Andalusian ranges. It was first opened to the sea, later becoming a coastal lake or lagoon, and, later, due to clogging by sediment flow, swampy marshes. The depression was filled with clay, limestone, and marl. The dominance of clay materials leads to gently rolling plains. When beds of limestone emerge, they form tables and witness hills or Alcores.

The Outer Ridges of the Plateau

Formed in the Tertiary Period, by folding the material deposited in the ocean trenches of the Pyrenees and Betic System between old beds that acted as buffers. They consist of the following mountain units:

Pyrenees

  • Axial Zone: Presents the higher altitudes of the Pyrenees. It consists of Paleozoic materials rejuvenated during the Tertiary orogeny. The Malditos Massif stands out with Aneto and Monte Perdido.
  • Pre-Pyrenees: Bordering the south side of the range. They were formed from limestone-type sedimentary materials deposited in the Pyrenean pit during the Secondary Era, strongly folded during the Alpine Orogeny. Its terrain is softer and lower in height than the axial zone. The Pre-Pyrenees are divided into inner and outer ranges.
  • Rating Depression: Loamy in character, it is situated between the two mountain ranges of the Pre-Pyrenees.

Basque Mountains

They are an extension of the western Pyrenean foothills. Limestone crag predominates, with gentle and moderate altitudes. In the easternmost outcrop, siliceous materials of the axial Pyrenees appear. The Aralar and Peña Gorbea ranges are its higher elevations.

Cordillera Costero-Catalana

It presents a complex structure. It is separated from the Pyrenees by a fault network that forms the volcanic region of Olot. Lithologically, it has two distinct areas:

  • Northern half: Paleozoic materials, remains of the ancient Hercynian massif Catalano-Balear, re-elevated in the Alpine Orogeny.
  • Southern half: Consisting of secondary limestones folded in the Tertiary Period.

Geographically, it splits into two alignments:

  • Area along the coast, low-height: Altos del Garraf.
  • Interior area with the highest elevations: Montseny, Montserrat.

Both alignments are separated by a graben, filled with Tertiary and Quaternary materials, a landscape of rolling hills and valleys.

Betic Cordilleras

  • Cordillera Penibética: Bordering the Mediterranean coast of the southern peninsula. Composed of Paleozoic materials, remnants of the old Betic-Rif Massif, rejuvenated by the Alpine Orogeny. It includes the higher elevations of the peninsula, in Sierra Nevada: Mulhacén and Veleta peaks.
  • Cordillera Subbética: Located in the hinterland of the Cordillera Penibética. It is composed of materials deposited in the Betic trench during the Secondary Era, folded during the Tertiary Period. The alternation of hard strata (limestone) and soft (marl) resulted in the appearance of shifting and thrusting sheets during the Alpine folding. The Sierras of Grazalema, Ubrique, and Cazorla stand out.
  • Intrabética Depression: Located between the Subbética and Penibética ranges. It is fragmented into several small depressions, where the basins of Ronda, Antequera, Guadix, and Baza stand out. It was filled with Tertiary materials, which, being eroded under an arid climate, have resulted in a landscape of badlands.