Geografía
The current relief of the Peninsula is the result of a geological history of millions of years in which orogenic phases alternated with others of calm, in which erosion and sedimentation predominated.
a) During the Archaic or Precambrian Era (4,000-600 million years ago) an arched band emerged from the sea from northwest to southeast, formed by slabs and gneiss, which included almost all of present-day Galicia. Elevations have also arisen in some isolated points of the Central System and Montes de Toledo. This Precambrian massif was later devastated by erosion and covered almost entirely by the Paleozoic seas.
b) In the Primary or Paleozoic Era (600-225 million years ago) the Hercynian orogeny took place. From the seas that covered most of the Peninsula arose the Hercynian mountain ranges, formed by siliceous materials such as granite, slate and quartzite. To the west rose the Hesperian Massif, eroded by erosion during the same primary era and converted into a plinth or plateau sloping towards the Mediterranean. To the northwest appeared the massifs of Aquitaine, Catalan-Balearic and the Ebro, and to the southeast, the Bético-Rifeño Massif. All of them were also razed by erosion and during the primary era turned into plinths.
c) The Secondary or Mesozoic Era (225-68 million years ago) was a period of calm in which erosion and sedimentation predominated. The flattening of the Hercynian mountain ranges continued. The inclination of the base of the Plateau towards the Mediterranean allowed, in the periods of
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marine transgression, a deep penetration of the sea, that I deposit in its Eastern edge a cover not very powerful plastic sedimentary materials (limestones, marls). Enormous thicknesses of sediment have also been deposited in sea trenches located in the present-day Pyrenean and Betic areas.
d) Alpine Orogenesis took place during the Tertiary Era (68-1.7 million years ago). As a result of this:
– The alpine mountain ranges were raised, when the materials deposited in the Pyrenean and Betic pits were folded between the ancient massifs, which acted as tops. The Pyrenees arose between the Aquitaine, Hesperian and Ebro massifs (which eventually sank), and the Béticas mountain ranges, between the Bético-Rifeño and Hesperian massifs.
– The pre-alpine depressions formed parallel to the new mountain ranges, and between these and the old massif: the depression of the Ebro, parallel to the Pyrenees, and that of the Guadalquivir, parallel to the Béticas.
– The Plateau has been affected by alpine orogeny. In the first place it happened to incline towards the Atlantic, determining the direction towards this ocean of good part of the peninsular rivers. Second, the mountainous edges of the Plateau were formed. On its eastern edge, the plastic materials deposited by the sea in the secondary era were folded, giving rise to the eastern part of the Cantabrian mountain range and the Iberian System. On the southern edge of the Plateau, the push of the Béticas mountain ranges raised Serra Morena. Finally, the base of the Plateau, formed by rigid Paleozoic materials, experienced fractures and faults. The latter gave rise to a Germanic structure, consisting of raised or rejuvenated blocks (horst) and sunken blocks (tectonic pits or graben). The raised blocks formed the Galaico Massif and the interior mountain ranges of the Plateau (Central System and Montes de Toledo), and the sunken ones created the interior depressions or
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sedimentary basins of the Plateau (those of the northern and southern subplateaus). The faults also led to volcanic activity in areas such as Campo de Calatrava, Olot-Ampurdán and Cabo de Gata.
From the alpine orogenic movements the river network was established. The rivers have severely eroded the new mountain ranges, created ravines (accumulations of pebbles at the foot of the mountains) and filled the inner and outer depressions of the Plateau, which are now practically flat areas.
The volcanic nature of the Canary Islands also originated in this era, when the alpine orogeny broke the bottom of the Atlantic and, through its fractures, large masses of volcanic rocks rose that gave rise to the islands.
e) During the Quaternary Era (1.7 million years to the present) glaciation and the formation of river terraces constituted the processes of erosion and sedimentation that most contributed to the modeling of the current relief.
– Glacierism has affected the highest mountain ranges (Pyrenees, Cantabrian Mountains, Central System, Iberian System and Sierra Nevada), giving rise to circus and valley glaciers.
– Circus glaciers are reduced to the head of the valley (circus), where ice accumulates. Ice and thawing break the rocks of the circus walls, so that the circus grows larger and its shapes become steeper. In the Peninsula, due to its latitude, most of the glaciers were circus and when the ice melted they gave rise to small lakes.
Valley glaciers are rivers of ice. They form when the thickness of the ice accumulated in the circus is large. Then the ice in the lower layers moves down the valley. The ice contains rock fragments that dig into the valley, giving it the typical “U” shape. They also dig small buckets that, when the
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ice melts, turn into lakes. On the Peninsula, only the Pyrenees were covered by a powerful layer of ice, from which glaciers started that created “U” valleys and lakes.
– The glacial period was characterized by the formation of river terraces, flat and elevated strips located on the banks of a river, which are ancient floodplains abandoned by the subsequent river embankment. The terraces are the result of the climatic alternations of the Quaternary. In the Gallic periods, when ice water was found in the mountains, the rivers lost their erosive force and deposited alluvium or materials in their channel. In the postglacial periods, as the temperature increased and the ice melted, the flow and erosive force of the rivers increased, so that they deepened their bed and left the previously accumulated alluvium suspended on its banks, thus forming terraces. The successive glacial and postglacial cycles of the Quaternary gave rise to graduated terraces. In the Peninsula, the most characteristic are those formed by the rivers Douro, Tagus, Guadiana, Guadalquivir and the Ebro.