With this name, the great plateau, surrounded by mountainous reliefs, which rises between Mesopotamia and the Indus valley, facing the Caspian and Turanian depressions to the north and the Persian Gulf and the Arabian Sea to the south, and forming the connection between Anterior Asia and Central Asia. Three times larger than Anatolia and Armenia taken together, it partakes of the characters of both on a larger scale. The surface can be estimated at 2,700,000 sq km, the population at 15,000,000.
The name originates from the Indo-European populations, which at the time of the great migrations came to settle in that plateau and, although certainly already differentiated into various lineages, they bore the common name of “Arî”. This name (* ar i̯ o -) certainly dates back to the period of the Indo-European community and most likely originally had the meaning of “good”, “noble”; in the period of the Indo-Iranian community it came to designate more properly the dominant caste, the nobility; and therefore, in opposition to the populations of other races and languages of the occupied territories, the ethnic-cultural unity of the new residents. In the Avesta with the name of Airyan ə m va ē æ ō it indicates a legendary region from which the Iranians would have drawn origin, and the name of Arius (airya -) indicates the peoples of Iranian lineage in opposition to the Turani. In the inscriptions of the Achaemenids there is also ariya in this sense. From the ancient Iranian airyana derives the Middle-Persian form ē r ā n (“Iranian” as opposed to an ē r ā n “non-Iranian”, “foreigner”), and from this the modern denomination of Īrān. In addition to the geographical value indicated above, the name of Īrān indicates the ethnic, linguistic and cultural complex that belongs to Persia and is the official name of the Persian nation (v.).
The morphology of Iran is closely related to its structure. The great bundle of the tertiary folds, which was strongly tightened in the Armenian plateau, slows down further east, separating into two main groups of chains that diverge and curve into wide festoons, to then reunite and close again towards the Pamir. Squeezed between the Eurasian mass on one side, and the Indo-Arab-African mass on the other, the wrinkles reveal a maximum of compression in the east and west of Iran under the action of orogenic pressures coming more intensely from the north. The northern marginal ranges run for a length of 2700 km. They are still confused in Azerbaijan, which the Arasse furrow divides from Armenia, and where the folds of ancient land (crystalline and paleozoic) are half buried, with the exception of the high Kara Daǧ chain, under the tertiary saliferous deposits and the large volcanic masses dominated by the very broad cones of the Sahend and Savalan (4800 m.). A large chain, clearly shaped and perfectly identified, is that of Elburs, which surrounds the Caspian depression to the south: a relatively narrow barrier (from 60 to 130 km.) And very high, describing an open arc towards the north, with several crests parallel to each other and with a height ranging between 2000 and 4000 m. It is constituted by a sedimentary series that goes from the upper Devonico to the Oligocene, with a prevalence of Jurassic limestones and with a granite core; on it the large recent trachyte mass of the Demavend volcano (today in the state of solfatara) extoles, which reaches 5670 m. Due to the climate rich in rainfall and the steepness and height of the slopes, erosion is intense, and most of the water flows into the Caspian through narrow gorges that the rivers have managed to cut across the chains; wide is only the valley of the Kizil-uzun (Sefīd rūd), the only one that crosses the whole harsh barrier.
Further east, the other very broad festoon of the Khorāsān chains follows the arch of Elburs. They are arranged in a much wider bundle (up to over 200 km.); in the first section, up to the gorges of the middle Herū rīd, they are less elevated, often discontinuous, with easy passes, with vast depressions between one and the other; further to the east they rise and close together in the complex, elevated and still poorly known Afghan system (see afghānistān), whose median chain culminates in the Hindu-kush with heights over 7000 m. This is where the two major eastern rivers of Iran originate: the Herī rūd, which crosses the northern barrier of the plateau and gets lost in the Turkestan desert, and the Hilmend, which descends into the inland basin of the Seistan.
The southern marginal ranges stretch for about 4000 km. From the Armenian plateau they branch off with the predominantly granite mountains of Kurdistan (2500 to 3000 m high), which surround the 0rmiyah depression to the west and continue towards the SE. flanked by crystalline schists and Jurassic and Cretaceous limestone plateaus. Thus one passes insensibly to the great Zagros system, which holds for a thousand kilometers. the same direction in the form of a wide raised belt, thick and continuous, made up of more recent rocks the more one proceeds towards Mesopotamia. In the internal wrinkles granite and crystalline schists emerge, on which paleozoic limestones rest; but the most important part of the system is given by Jurassic and Cretaceous limestone masses, curved in simple and regular folds, which determine very long walls (high up to 4700 m. in the Ochtoran) and long longitudinal valleys (with rarely depressed bottom below 1000 m.), in which the rivers are fed by copious karst springs; they then cross the chain through narrow and deep transversal valleys (teng), which connect the longitudinal trunks with frightening locks, such as those that end up conveying most of the system’s river waters to the Kerkha. The foothill edge towards Mesopotamia is formed by reliefs of ruiniform Nummulitic limestones and by hills of Miocene marl and sandstone with gypsum and rock salt.
The Zagros chain continues to the SE., Up to the Gulf of Hormvz, in the Fārs mountains, less elevated (the peaks above 3000 m are rare), arid, mainly made up of Eocene limestone and Miocene sandstone, curved in long simple folds weakly eroded and forming a maze of crests and closed depressions, preceded towards the coast by the weak reliefs of the marly oil and gypsum-sulphurous formation.
Between the gulf of Hormùz and Quetta, the marginal chains are arranged according to a very broad concave arch to the north. Along a coastal strip from 150 to 200 km. Wide, the Makrān mountains follow one another for about 800 km. in the W.-E direction, with a uniform and interminable series of mountains and arenaceous ridges, under which depressions dug in clayey stretches and forming a very complicated hydrographic network in a semi-arid climate.
Approaching the Indian massif, the folds rise, tighten, and bend towards the North and the NE. Eocene, Cretaceous and Jurassic limestones reappear on a large scale, even in extensive massifs cut by deep gorges. Beyond Quetta, the same grounds curve in a bundle of folds arranged in a system of concentric asymmetrical arches, which rise up to 3500 m. in the acute limestone ridges of the Suleiman, alongside the Afghan system.