Ciomadul
Ciomadul is a dormant volcano in Romania. It is in the Eastern Carpathians, between the spa towns of Băile Tușnad and Balvanyos. Ciomadul lies at the southeastern end of the Carpathian volcanic chain and it is the youngest volcano of the Carpatho-Pannonian region. Ciomadul consists of several lava domes with two embedded explosion craters known as Mohoș and Sfânta Ana, the latter of which contains a crater lake, Lake Sfânta Ana. The dominant volcanic rock at Ciomadul is potassium-rich dacite.
Volcanic activity at Ciomadul commenced with effusive activity about one million years ago. Most of the volcano was constructed between 650,000 – 500,000 years ago.
Between 56,000 and 32,000 years ago explosive volcanic activity occurred at Ciomadul. Both the exact dates of the various eruptions and of the formation of the Sfânta Ana and Mohoș craters are unclear, partly because dates obtained by potassium-argon dating and other dating techniques deviate from each other. Some eruptions may have reached sub-Plinian strength, ejecting volcanic ash as far as the Black Sea.
The last eruption took place between 32,600 and 27,500 years ago. Its date is likewise unclear. Ongoing seismic and geothermal activity, and exhalations of volcanic gas and evidence of a still existing magma chamber indicate that Ciomadul is a potentially active volcano.
Geography and geology
Regional setting
With the exception of Greece and Italy, the most recent volcanic activity in Continental Europe occurred between 40,000 and 6,500 years ago in Garrotxa, the Massif Central and the Vulkaneifel.Volcanism in the region of Carpathia and Pannonia has been ongoing since 20 million years ago but has decreased during the Quaternary. No eruptions took place in the Holocene. The last volcanism occurred at Ciomadul in the last glacial age. Sparse basaltic volcanism has also taken place in the area, forming monogenetic volcanic fields.
A volcanic arc lies in the Carpathians. In its southern segment, also known as the Călimani – Gurghiu – Harghita chain, volcanism has migrated between 9 and 0.22 million years ago southward, forming a volcanic chain. Magma output progressively decreased during time, with early volcanoes being large stratovolcanoes sometimes featuring caldera-forming eruptions, while more recent activity includes monogenetic volcanoes although more precise dating and volume estimation efforts at Ciomad have found an increase of eruption rates over time.
This volcanism occurs in a setting where the collision between the Eurasian Plate and the Tisza-Dacia microplate took place, preceded by a stage of subduction involving a narrow ocean. This is part of the collision between the African Plate and the Eurasian Plate; subduction may still be underway in the area of the Carpathians. The, which is away from Ciomadul, features ongoing earthquake activity; deep earthquakes suggest that a remnant of a slab exists beneath the Vrancea Zone. This tectonic setting may also be responsible for ongoing exhumation in the southeastern Carpathians, volcanism at Ciomadul and the Perșani volcanic field, south of Ciomad, which was concurrently active to the older Ciomadul activity. Other theories on Ciomadul's volcanic activity imply delamination of the lithosphere or roll-back of the subduction zone.
Volcanism in this chain is calc-alkaline, yielding both andesite, dacite, and rhyolite. Three million years ago, a change in the chemistry of volcanism occurred, with an increased content of potassium in the rocks. This change in composition geographically coincided with the volcanic activity crossing a lineament known as the Trotuș line.
Volcano
Ciomadul is located in the southeastern Carpathians, at the end of the Călimani – Gurghiu – Harghita volcanic chain, and is also known as Csomád in Hungarian. The gorge of the Olt River separates Ciomadul from the Harghita Mountains. The towns of Băile Tușnad and Bixad are close to the volcano, and a road leads up the volcano from the southeast and goes past the Mohoș swamp to Lake Sfânta Ana. The basement of the volcano is formed by flysch of Cretaceous age and by older volcanics; in some places volcanic rocks overlie fluvial deposits.Places around Ciomadul were first mentioned in 1349; the Saxon mineralogist Johann Ehrenwert Fichtel was the first to interpret it as a volcano, in 1780. The idea that Ciomadul could be a still active volcano was first proposed in the same year on the basis of its young appearance and the release of gas. These discoveries drew scholars and visitors to the volcano and the first scientific analysis of the volcano was published just eight years later. While a publication in 1964 postulated that the tuffs of Ciomadul were reworked Pliocene volcanites, the late Pleistocene age was established soon afterwards. The volcano is the youngest volcanic centre in the Carpathians and has a more rugged appearance than the surrounding mountains.
Ciomadul is formed by a complex of lava domes and other volcanic material that form a south-tilting ridge that rises above the surrounding Lower Ciuk Basin. Individual lava domes form cone-shaped hills, which reach heights of and widths of. Individual domes include Haramul Ierbos, Haramul Mare, Haramul Mic, Vf. Cetății, Vf. Comloș, Vf. Surduc and Dealul Mare southeast from the main complex. The central cluster of domes is elliptical and tectonic faults influenced their growth. The highest point of the complex is Ciomadul Mare with an altitude of. Some domes were later affected by erosion, explosive activity or fumarolic alteration. The whole volcanic complex covers a surface area of, and is surrounded by a circular/semicircular plain made of volcanic debris.
The lava dome complex contains two craters, named Mohoș and Sfânta Ana. They were formed in the previously existing lava domes which form the western margin of the craters, while products of explosive eruptions crop out in the east. The Sfânta Ana crater is wide and deep beneath the rim, comparable with the crater of El Chichón volcano in Mexico. This crater lacks a breach and is relatively unaffected by erosion. It contains a crater lake, which once may have been over deep. This lake is known as Lake Sfânta Ana and lies at an altitude of ; its ecosystem and environment has drawn the attention of scientists for two centuries.
The Mohoș crater lies at an altitude of. It is larger than Sfânta Ana with a diameter of and not as deep with its bottom lying above sea level. It is filled with a and Sphagnum peat bog and its rim is cut by the Sfânta Ana crater.
Unlike Sfânta Ana, the Mohoș crater has been breached by erosion, causing the formation of an outlet valley. Both craters were formed by explosive eruptions and distinguishing between the deposits of both is difficult. The existence of an even larger crater with a diameter of has been suggested, encompassing both Sfânta Ana and Mohoș.
Pyroclastic flow deposits generated by Ciomadul have been found on its northeastern, southern and western slopes. They reach a distance of as much as from the volcano. At Tușnad road, one of the flows has a thickness of . Tephra fall bed, lapilli, and surge deposits are also found, and the flow deposits contain pumice blocks. One lapilli layer, thick, from Ciomadul has been identified east of the volcano. The whole pyroclastic formation has been subdivided into three classes known as "Early Phreatomagmatic + Plinian Activity", "Middle Plinian Activity" and "Latest Sfânta Ana Phreatomagmatic Activity". Each comprise a number of individual tephra layers that were erupted 42,000—40,000, around 31,500 and 29,000—28,000 years ago. Some of these eruptions may have dammed the Olt river; when the river returned on its course it produced lahar deposits.
Other landforms at Ciomadul include coulees and lava flows. The total volume of the complex is about dense rock equivalent. Drilling has identified the existence of an intrusion at a depth of. Finally, volcanic erosion products and tephra occur all over the volcanic complex and up to east of it.
Older volcanic centres extend northwest of Ciomadul. With increasing distance they are the 2.5—1.5 million-year-old Pilisca centre, the 2.8—2.2 million-year-old Cucu centre and the 4.3—3.6 million-year-old Luci-Lazu and Șumuleu-Ciuc volcanic centres. South of Ciomadul the Murgul shoshonites were erupted 2.3—1.5 million years ago; they represent cryptodomes. Andesite lava flows from Pilisca underlie the Ciomadul deposits in some places.
Composition
The principal rock is dacite, which defines a potassium-rich calc-alkaline suite. The rocks have a porphyric appearance and contain few vesicles. They are also very rich in crystals, with the dominant phenocryst-forming minerals being biotite, hornblende and plagioclase. Less important are allanite, apatite, clinopyroxene, olivine, orthopyroxene, quartz, sphene and zircon. The groundmass contains plagioclase, pyroxene, silicon dioxide and oxides of iron and titanium. Clots formed by various felsic crystals are common. The composition of Ciomadul's rocks has been fairly constant throughout its evolution albeit with two shifts 1 million and 650,000 years before present, and this diversity of its components indicate that the genesis of Ciomadul magmas involved mixing between felsic and mafic magma. The phenocryst compositions at Ciomadul are unlike these at other volcanoes in the Carpathians. The magmas derive from the upper mantle lithosphere, which underwent metasomatic alteration.Compositionally, the tephras of Ciomadul have been subdivided into two groups, one called Tușnad‐type and the other Bixad‐type. A large proportion of crystals in the rocks consists of antecrysts and xenocrysts, making radiometric dating of the rocks difficult. These include amphibole, biotite, feldspar and zircon. The zircons formed almost continuously over hundred thousands of years within Ciomadul's magma chamber, indicating a steady crystallization of the chamber. Differences in magma temperature, crystal content and the participation of pre-existent crystal mushes determine whether an eruption will be effusive or explosive.
The temperature of the magma chamber has been estimated to be about, with heating of over occurring before some eruptions according to thermometry calculation. Volcanic activity was most likely triggered by the injection of basaltic magma into the felsic magma chamber before the actual eruption, as has been observed at other silicic volcanoes around the world, but the magma chamber probably kept being recharged even between eruptions. The amphiboles in the rocks formed at depths of. The magma output of Ciomadul is about while magma chamber recharge may have reached.