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Koh-i-Sultan is a volcano in Balochistan, Pakistan. It is part of the tectonic belt formed by the collision of the Eurasian Plate and Indian Plate: specifically, a segment influenced by the subduction of the Arabian Plate beneath the Asian plate and forming a volcanic arc which includes the Bazman and Taftan volcanoes in Iran. The volcano consists of three main cones, with heavily eroded Volcanic crater running west-northwest and surrounded by a number of Parasitic cone. Its summit is high, and the crater associated with the Miri cone has a smaller crater inside.
The volcano is formed by andesite and dacite rocks, with fragmentary rocks prevailing over lava flows. The rocks have typical arc-volcano chemistry and composition, with a progression from andesite to dacite in the eruption products with younger age. Potassium-argon dating has indicated an age range from 5,900,000 to 90,000 years. Subsequent erosion has generated a large debris apron around the base of the volcano and carved rock formations which impressed early explorers; one well-known rock formation is the staff-like Neza e Sultan (trans. "Sultan's Spear").
Geothermal activity and the emission of are ongoing, and the volcano has been prospected for the possibility of obtaining geothermal energy. The geothermal activity has resulted in widespread rock alteration and the formation of sulfur deposits, which were mentioned in a 1909 report and later mined. Koh-i-Sultan also has deposits of other minerals.
It is tectonically influenced by the Arabian Plate Subduction beneath the Eurasian Plate, forming the Chagai volcanic zone. Tectonic activity related to this subduction is ongoing. Before the onset of volcanic activity at Koh-i-Sultan in the early Quaternary, tectonic uplift occurred in the region. Koh-i-Sultan and the Iranian volcanoes Bazman and Taftan volcano form the Sultan volcanic arc. Also known as the Makran or Baluchistan volcanic arc, it is long and stretches in an east-northeast direction. Magma generated by the relatively shallow subduction of the Indian Plate rises to the surface and forms the andesitic rocks, among others, of Taftan and Koh-i-Sultan. The related Makran subduction zone is one of the few Cretaceous zones still active, and has formed a large accretionary prism.
The volcano consists of , flows and tuff. Lava flows make up about 10 percent of the volcanic pile, and the remainder is volcaniclastic material. Other layers alternate between ash, lava flow and . Andesitic-dacitic materials dominate, including block-and-ash flows, and tuff. Andesitic lava flows have thicknesses of ; two at Miri are and thick. The dacites form and subsidiary centres. Rock fragments are widely buried by long lava flows, and an Eocene mélange makes up part of the volcano's basement.
The rocks follow the calc-alkaline trend of magmatic differentiation, containing silicic lavas such as dacite. Their overall composition ranges from basaltic andesite to dacite, and the dominant lava rocks are andesite and basalt. Andesites range from green to gray in colour, and dacites are pink- and light-gray. The andesitic lavas contain of andesine, hornblende, plagioclase and hypersthene. Dacites have biotite, hornblende, plagioclase and quartz . Textures range from porphyritic to Vitrophyre. Andesites are more common in Pliocene rocks, and Pleistocene rocks tend to be dacitic. At least five cycles of andesite rock formation have been found on the southwest side of Koh-i-Sultan. Its rocks are typical volcanic-arc rocks in elemental chemistry, and may stem from sub-continental mantle-derived magma. The satellite centre rocks differ from the main centre rocks in composition, suggesting that different processes generated the magmas which constructed the cones. There is a compositional trend from Bazman over Taftan to Sultan, with the latter having more potassium in its rocks than the other centres. Such compositional trends may come from different partial melting ratios and different fluid contributions to magma generation at each volcano. Hydrothermal alteration has given rise to alunite, kaolinite, phengite, propylite and sericite.
Potassium-argon (K-Ar) dating of the Miri summit has yielded an age of 200,000 years, and an older date (5,900,000 ± 2,800,000) has been obtained from the northwestern centre. Between the two are andesite dates of 5,630,000 ± 90,000 years and 2,390,000 ± 50,000 years. Although the last activity occurred during the Pleistocene (probably a large eruption), volcanism began earlier. After a phase of erosion, an ash fall occurred. The youngest date, obtained from K-Ar dating of pumice, is 90,000 ± 10,000 years. Koh-i-Sultan's Pleistocene activity is probably related to the formation of travertine deposits, which are mined.
Although the volcano has experienced relatively little dissection, the western cone has been eroded to the base and there is widespread hydrothermal alteration of rocks. Post-volcanic erosion has created an apron of rock fragments at Koh-i-Sultan's base, with radially incised valleys bearing traces of energetic stream erosion emanating from the volcanic cones. Koh-i-Sultan's remaining rock formations often have irregular shapes with a striking appearance; a 1909 report noted the presence of a monolith-shaped rock and a dome-shaped rock formation, Koh-i-Kansuri. Neza e Sultan ("Sultan's Spear") is a major spear-shaped rock formation, about high with a basal diameter of . Weathering has created longitudinal fissures in the rock. Similar pillars are found elsewhere on the volcano, reminding early explorers of Gothic architecture and . Neza e Sultan (possibly named after the mythical Sultan-i-Pir-Khaisar, who is reportedly buried nearby), at the westernmost crater, was discovered by Europeans in 1877. The rock formations may be Volcanic plug of old volcanic centres. Gravel, pebbles and sand form Dry lake and dunes, also found in dry valleys.
Previous volcanic activity in the area includes the Cretaceous Sinjrani volcanics, resembling those of Koh-i-Sultan, and activity which produced the Chagai . The Sinjrani volcanics, about thick, consist primarily of lava. Other rocks are agglomerates, limestone and tuff, and the Cretaceous rocks are mainly andesitic. The monzonitic Chagai Intrusive rock are accompanied by other intrusions with additional minerals. Other formations are the mostly sedimentary Humai formation of the late Cretaceous and the probably Paleocene Jazzak formation. The Chagai topography is dominated by a sand-covered plain, rising to an altitude of . Wind-eroded rocks and dry lake beds are also present. Other tectonic objects are the Ras Koh range, the Mirjawa range and the Chagai Hills east of Koh-i-Sultan. The Sinjrani and Chagai volcanics crop out primarily west of Koh-i-Sultan, and are part of the older Chagai volcanic arc. Some geologists consider Koh-i-Sultan part of the Chagai arc. Two nearby volcanic centres are Damodin and Koh-i-Dalel, which may be part of Koh-i-Sultan together with Koh and Koh-i-Malik-Shah. Koh i-Kannesin is northeast of Koh-i-Sultan. The crust beneath the volcano reaches a thickness of about .
Emanations of hydrogen sulfide have been reported around the volcano, preventing the mining of underground sulfur deposits, and the gas is also present in the hot springs. The confirmed presence of sulfur dioxide would indicate hot magma beneath the volcano.
The sulfur deposits, south of Koh-i-Sultan's crater, are named Batar, Miri, Nawar and Zond and are within a area. In the deposits, the sulfur is in the form of lenses within the volcanic rock. The Koh-i-Sultan deposits are the principal native source of sulfur in Pakistan. They were probably formed by the interaction of sulfuric acid and hydrogen sulfide or from sulfur-containing hot springs when the volcano was still active. Another theory suggests that they formed during the Holocene. Other mineral deposits found at Koh-i-Sultan are copper-containing alterations with alunite and quartz; alunogen, which can be used to obtain aluminium; limonite, used as a yellow dye, and a red ochre rock containing gypsum and anthophyllite. Copper deposits which also contain gold are part of an epithermal-sulfidic mineralization at Washaab.
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