Lithological Domains and Geological Formations in Spain
Lithological Domains of Spain
In Spain, four lithological domains are recognized: Siliceous, limestone, clay, and volcanic.
The Clay Domain
In this area, the predominant rocks are clays, marls, and gypsum. It extends through Tertiary depressions and Mediterranean coastal plains. In semiarid areas devoid of vegetation, water streams generate gullies, ravines, or fissures that form vertical walls. Extensive plains formed by the alteration of soft and hard materials give rise to forms of modeling that are the result of differential erosion:
- In some cases, horizontal reliefs alternate between two levels: the stop level and the level of the countryside.
- The stop level corresponds to structural plains where erosion has exposed layers of hard limestone.
- The countryside level consists of plains of erosive origin, carved by rivers on soft materials.
- If reliefs alternate with inclined materials, it results in reliefs on costs (cuestas).
The Volcanic Domain
Identified with the Canarian archipelago, volcanic eruptions and subsequent erosion model various forms of relief. These depend on the viscosity of the lava and the ratio of pyroclastics to lava. Some of the most characteristic volcanic landforms are:
- Volcanic cone: Popularly known as the volcano.
- Boilers (Calderas): Circular or elliptical depressions.
- Domes: Volcanoes where the lava is so viscous that it cools and solidifies within the crater itself.
- Badlands (Malpaíses): Clusters of solidified viscous lava resulting in barren landscapes that look like heaps.
- Dams (Dikes): These are the pythons, masses of lava-hosted vents of a volcano which have been eroded.
- Ravines: Formed by streams of rainwater that run through steep slopes toward the sea.
The Siliceous Domain
The siliceous domain includes silicate rocks, such as igneous rocks like granite, and some metamorphic rocks such as slate, schist, and gneiss. These rocks are hard, rigid, and resistant to erosion; however, given tectonic pressures, they can become fractured. The siliceous area occupies almost the entire western third of the peninsula. Among silicate rocks, granite is the most abundant on the Earth’s surface.
Dominant forms of modeling result from alteration by the action of water, ice, and snowmelt, which acts through the joints (lines of weakness in the granite where erosion begins). The resulting forms are varied:
- If water acts on shallow joints in a horizontal arrangement, desquamation occurs, shaping a landscape of rounded forms and smooth domes.
- If water seeps through joints in an orthogonal network, it can model a granite tor; when erosion reaches these blocks, they are identified as bowling (berrocales).
- In cold high-mountain areas, ice action results in a landscape of peaks, ridges, and scree.
The Limestone Domain
This domain extends in the form of a large inverted Z from the Catalan coast to the Strait of Gibraltar. It is the dominant lithology in the Balearic archipelago, except in the central plains. Limestone is a hard and permeable rock that forms karst modeling, which is the result of the dissolution of limestone in contact with water. The most characteristic karst forms are:
- Canyon: A narrow valley carved with deep vertical sidewalls by a river.
- Lapiaz: A surface formed by furrows or grooves of different sizes.
- Potholes (Sinkholes): Narrow openings that connect the surface with underground galleries.
- Dolines: Closed depressions formed on the surface with an oval shape and sinuous contours.
- Polje: A generally flat-bottomed valley surrounded by steep slopes, where a stream is lost through a sinkhole.