Spain’s Morphostructural Units: A Geological Overview
Morphostructural Unit Types
Large-Scale Continental Relief Units
Paleozoic Orogenic Materials: These are plains formed in the primary era as a result of erosion of mountain ranges. The materials are siliceous rocks such as granite, slate, quartzite, and schist. These are very rigid, prone to fracturing rather than bending. The oldest mountains were formed in the Tertiary by the rise of a new block due to Alpine orogenic movements. These mountains have rounded peaks and are found in the inner mountain ranges of the Meseta, the Galician Massif, and the western part of the Cantabrian Mountains.
Folding Mountain Ranges: These ranges were formed during the Tertiary orogeny by the folding of sedimentary materials deposited by the sea in the Secondary era. There are two types:
- Intermediate Ranges: Formed by the folding of materials deposited on the edges of the baseboards.
- Alpine Ranges: Formed by the folding of material deposited in long, deep trenches.
Sedimentary Basins or Depressions: These are sunken areas formed in the Tertiary and filled with sediments like limestone, clay, and sandstone. There are two types:
- Basins formed by subsidence of a base block.
- Prealpine depressions on both sides of alpine ranges.
Development of Morphostructural Units
The current relief of the Iberian Peninsula is the result of alternating orogenic and calm phases throughout geological history.
Precambrian Era: An arc emerged from the sea, running northeast to southwest, consisting of slate. This included elevations in Galicia, isolated points in the Central System, and the Montes de Toledo. This was later destroyed by erosion and covered by seas.
Paleozoic Era (Primary Orogeny): Hercynian orogeny took place. Seas covered part of the peninsula, and mountain ranges formed from slate, granite, and quartzite. The Hesperian Massif arose, was eroded, and became the base. This included areas in Aquitaine, Catalonia, the Balearic Islands, and the Betic-Rifian Massif.
Mesozoic Era (Secondary): This era was dominated by erosion and sedimentation. The tilting of the Meseta base towards the Mediterranean led to deep penetration of the sea.
Tertiary (Alpine Orogeny): This period saw the rise of the Alpine ranges (Pyrenees and Betic Cordillera), the formation of prealpine depressions (Ebro and Guadalquivir), and the tilting of the Meseta towards the Atlantic, forming the Meseta mountain ranges.
Quaternary Era: Glaciers formed and fluvial terraces developed. Glaciation affected the highest mountains, resulting in cirques (bowl-shaped valleys) and valley glaciers.
Siliceous Areas
Composed of ancient rocks from the Precambrian and Primary eras, located in western mainland Spain, with ramifications to the Cantabrian Mountains, Central System, Montes de Toledo, and Sierra Morena. Granite is the prevailing rock. Chemical weathering by water can decompose crystals into yellowish-brown sand, accumulating in valleys and low-slope areas. Joints in the rock can lead to sharp peaks and scree formations in high mountain areas. Parallel joints cause flaking, resulting in undulating landscapes. Perpendicular joints form tors and rock formations.
Limestone Areas
Formed by folded secondary sediments, forming an inverted Z across the Pyrenees and near the Cantabrian Mountains. Limestone is the predominant rock, fracturing and dissolving in rainwater, creating karst relief features:
- Lenar, Sinkholes: Furrows formed by water runoff. Spacing of cracks determines furrow width and separation.
- Throats, Foses, Sickles: Narrow, deep valleys framed by strands, caused by rivers.
- Poljes: Horizontal depressions or valleys, sometimes flooded.
- Torcas: Circular or funnel-shaped cavities where water stagnates.
- Caves: Created by water filtering through limestone fissures.
- Potholes: Narrow openings connecting the surface with underground cavities.
Clay Areas
Composed of less resistant sedimentary materials deposited in the late Tertiary and Quaternary periods. Includes parts of the Ebro and Guadalquivir depressions and the Mediterranean coastal plains. Relief is characterized by horizontal, easily eroded fields. Alternating dry and wet periods, combined with lack of vegetation, lead to gully formation.