Ciomadul

Ciomadul is a volcano in Romania, and is known as in Hungarian. It is in the Carpathians, close to the towns of Băile Tușnad and Bixad. It is part of a volcanic chain known as the Călimani (Kelemen) – Gurghiu (Görgényi) – Harghita (Hargita) chain and lies at its southeastern end. Ciomadul consists of several lava domes with two embedded explosion craters known as Mohos and St. 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 St. Ana and Mohos 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, historical volcanism has not taken place in Continental Europe. The most recent volcanic activity 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 have occurred in the Holocene. The last volcanism occurred at Ciomadul in the last glacial age. Sparse basaltic volcanism has also occurred in the area, forming monogenetic volcanic fields.
A volcanic arc lies in the Carpathians. In its southern segment, also known as the Călimani (Kelemen) – Gurghiu (Görgényi) – Harghita (Hargita) chain, volcanism has migrated between 9 and 0.22 million years ago southward, forming a long 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 Ciomadul 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 Vrancea zone, 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 volcanism at Ciomadul and the Persani volcanic field, south of Ciomadul, 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 Trotus line.
Volcano.
Ciomadul is located in the southeastern Carpathians, at the end of the Călimani (Kelemen) – Gurghiu (Görgényi) – Harghita (Hargita) volcanic chain, and is also known as Ciomadu. 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 Mohos swamp to the St. Ana lake. The basement of the volcano is formed by flysch of Cretaceous age and by older volcanics; in some places volcanic rocks overlie fluvial deposits.
The idea that Ciomadul could be a still active volcano was first proposed in 1780 on the basis of its young appearance and the release of gas. While a publication in 1964 postulated that the tuffs of Ciomadul were reworked Pliocene volcanites, the late Pleistocene age was established soon afterwards.
Ciomadul is formed by a complex of lava domes and other volcanic material that form a south-tilting ridge that rises above the high surrounding Lower Ciuk Basin. Individual lava domes form cone-shaped hills, which reach heights of and widths of . Individual domes include Haramul Ierbos (Fű-Haram in Hungarian), Haramul Mare (Nagy-Haram), Haramul Mic (Kis-Haram), Vf. Cetăţii (Vár-tető), Vf. Comloş (Komlós-tető), Vf. Surduc (Szurdok-tető) 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 (Nagy-Csomád) 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 Mohos and St. 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 St. 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 deep crater lake, which once may have been over deep. This large lake is known as Lake Sfânta Ana () and lies at an altitude of .
The Mohos crater lies at an altitude of . It is larger than St. Ana with a diameter of and not as deep. with its bottom lying above sea level. It is filled with a large and thick "Sphagnum" peat bog and its rim is cut by the St. Ana crater. 
Unlike St. Ana, the Mohos 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 has been suggested, encompassing both St. Ana and Mohos.
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 Tusnad 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 St. Ana Phreatomagmatic Activity". Each comprise a number of individual tephra layers.
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 2.5–1.5 million years old Pilisca centre, the 2.8–2.2 million years old Cucu centre and the 4.3–3.6 million years old Luci-Lazu and Sumuleu-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, rich in potassium. 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. 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.
Compositionally, the tephras of Ciomadul have been subdivided into two groups, one called Tuşnad‐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 the amphibole, biotite, feldspar and zircon.
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. The amphiboles in the rocks formed at depths of . The magma output of Ciomadul is about .
Climate and vegetation.
Ciomadul is located in a temperate climate zone. Rainfall reaches , resulting in strong erosion. The annual mean temperature is at Sfintu Gheorghe, the nearest meteorological station. Around St. Ana, July mean temperatures are and January temperatures .
While some glaciation occurred in the Carpathians during the ice ages, no glacial activity is recorded at Ciomadul. The volcano was unforested at that time, with steppe and tundra vegetation comprising most of the reported flora. Drill cores from the Mohos peat bog have been used to reconstruct the past climate and hydrology of the area.
Ciomadul is covered by beech and spruce forests. Around St. Ana lake, the vegetation is mostly formed by "Fagus sylvatica" (common beech) and "Picea abies" (Norway spruce) woods. Other trees include "Acer platanoides" (Norway maple), "Betula pendula" (silver birch), "Carpinus betulus" (common hornbeam), "Pinus sylvestris" (Scots pine), "Salix caprea" (goat willow) and "Salix cinerea" (grey willow). A fen contains "Carex lasiocarpa" (slender sedge), "Carex rostrata" (bottle sedge), "Lysimachia thyrsiflora" (tufted loosestrife) and "Sphagnum angustifolium" (fine bogmoss). At Mohos, vegetation consists of "Alnus glutinosa" (common alder), "Betula pendula" and "Salix". The peat bog contains trees ("Pinus sylvestris" and "Betula pubescens" (downy birch)) and Ericaceae. The region of the volcano is a Site of Community Importance and some endangered plant species have been identified in the Mohos bog.
Eruptive history.
Ciomadul has been active for over half a million years, with the oldest activity between 1,000,000 and 750,000 years ago forming lava domes. Older estimates indicate that activity did not start before 250,000 years ago, while more recent research indicated a start of volcanism over 600,000/ 850,000 years ago. Volcanism at Ciomadul consisted mostly of the extrusion of lava domes, their collapse forming block-and-ash flows and subplinian and Vulcanian eruptions separated by long periods of rest.
A gap of about 500,000 years separates Ciomadul from the activity of other volcanoes in the area. The two oldest dates of 1,020,000 and 850,000 years ago were obtained on peripheral lava domes. Early activity between 850,000 – 440,000 years ago built the southeastern domes. This effusive phase is also known as "old Ciomadul", and eruptions were separated by long pauses without volcanic activity from each other. The dates obtained by potassium-argon dating are much older; there is substantial disagreement between dates obtained by potassium-argon dating or argon-argon dating on the one hand and uranium-thorium dating on the other hand at Ciomadul. These dates indicate that the formation of the central lava domes took place between 590,000 and 140,000 years ago.
The volcanic history of Ciomadul has been subdivided into an effusive phase that lasted until about 440,000 years ago and an explosive phase that began 200,000 years ago during which magma output increased 30-fold and which is known as "young Ciomadul". Around 200,000 - 130,000 or 150,000 - 100,000 years ago a number of lava domes developed. Explosive eruptions became common only about 57,000 years ago. Between 56,000 and 32,000 years ago, explosive activity occurred at Ciomadul. That timespan coincides with the deposition of tephra from volcanoes in Italy in Europe; it is possible that tephra also came from Ciomadul. Indeed, the age of Ciomadul's last eruption overlaps with the age of the Campanian Ignimbrite. Tephra from eruptions of Ciomadul has also been found in the Ursului Cave of the Perșani Mountains. Some tephra layers found in two drilling cores of the Black Sea may have originated at Ciomadul but reliably distinguishing between Ciomadul tephras and these from Nisyros and Anatolian volcanoes is difficult.
Explosive activity may have occurred in two separate episodes, one 57,000–44,000 years ago and the other 33,000–29,000 years ago. An earlier explosive eruption about 55,900 ± 2,300 years ago may be the origin of the Mohos crater, with another proposed potassium-argon date being 220,000 years ago. Mohos crater is probably older than the St. Ana crater. A phreatomagmatic deposit northeast of Mohos was formed by an eruption of the Mohos crater; this eruption may be the source of the "Turia type" phreatomagmatic deposits, which is dated to have occurred about 51,000 ± 4,800 years ago. In one view, a volcanically quiet period followed an effusive eruption 42,900 years ago named "Piscul Pietros" and lasted until 31,510 years ago, when a Plinian eruption occurred. This latter eruption deposited thick ash as far as from the vent at one site. Alternatively, 38,900 ± 1,700 years ago a subplinian eruption occurred at Ciomadul; it may have formed the St. Ana crater. This date would correspond to that of the so-called "MK-202" tephra. Piscul Pietros has been also dated to be 48,000 ± 6,000 or 60,000 ± 5,000 years old. 
The age of the last eruption is controversial. In 1994, radiocarbon dating yielded an age of 10,700 ± 800 years Before Present from a pyroclastic flow. Later, paleosoils and other samples from the same flow were used to deduce similar ages of over 36,770, 42,650, over 35,670 and over 35,520 years before present, respectively. Thus this youngest age estimate was discarded. In 2010, further research identified two younger eruptions, one occurring 39,000 years Before Present and the other 27,500 years Before Present. Other data obtained by uranium-thorium dating indicate an age of 32,600 ± 1,000 years ago for the youngest eruption. Both of these eruptions took place at St. Ana and imply a repose period between eruptions of over 10,000 years. Much older dates obtained by potassium-argon dating are not considered reliable. Alternatively, the latest eruption may have occurred at a satellite vent seeing as sedimentation of the St. Ana lake has been ongoing since 26,000 years ago. These two recent eruptions were fed by different magmas, with the younger eruption coming from deeper magma chambers ( versus ) and involving more primitive magma. The youngest eruption was probably very violent; the Roxolany Tephra which may be associated with it has been found as far as Odessa, Ukraine, away from Ciomadul. If the Roxolany Tephra was formed by the youngest eruption of Ciomadul, the youngest eruption would have occurred 29,600 calibrated radiocarbon years ago based on independent dates of the tephra. From the other point of view, the clinopyroxene‐bearing Roxolany tephra was unlikely to be derived from Ciomadul, as it differs significantly from Ciomadul typical phenocryst assemblage containing amphibole. Based on new chronostratigraphic model for the Roxolany section, supported by updated magnetostratigraphic results and compiled existing radiocarbon and optically stimulated luminescence dates, the Roxolany tephra was deposited around 143,800 years ago.
Current status.
Presently, Ciomadul displays seismic activity, release of carbon dioxide from mofettas and anomalous heat flow, reaching . Outgassing of carbon dioxide, hydrogen sulfide and mostly abiotic methane have been found at Ciomadul, forming sulfide deposits in some caverns. The total output of carbon dioxide exceeds about per year, while the output of methane amounts to per year. Carbon dioxide concentrations in some places such as caverns can be high enough to become dangerous to people and animals, and is reflected in place names such as Peștera Ucigașă (Gyilkos-barlang) which mean "killer cave" while Puturosu means "stinky". Former alum and sulfur mines east of Ciomadul were abandoned due to the dangers from toxic gases.
At depths of and especially , a magma chamber has been identified beneath Ciomadul, based on magnetotelluric data, and several of magma may still be stored underneath Ciomadul. This magmatic reservoir appears to have about 15% of melt by volume fraction, with a vertical stratification by temperature. A deeper basaltic melt zone may also exist at around depth. Further, a zone of low seismic velocity has been identified with geophysical and seismic modelling in the lower crust and upper mantle beneath Ciomadul, down to depths of or .
Hydrothermal activity has been noted at Ciomadul and Tusnand-Bai, including a high temperature system at depth with temperatures exceeding . The Tusnand-Bai springs have temperatures of and discharge salty, carbon dioxide-rich water which emerges from pyroclastic deposits. In one cave, autothropic bacterial biofilms have been found which subsist on the exhaled gases or the sulfur deposits.
Future activity.
Volcanoes are usually considered to be active if they have had eruptions during the Holocene. However, as demonstrated by the unexpected eruption of Chaiten volcano in Chile in May 2008, even long-inactive volcanoes can become active again. Such volcanoes can constitute a threat to regions with seemingly quiet volcanism. Ciomadul has had repose periods of over 10,000 years and possibly even longer. Zircon crystallization data imply that the magma chambers of Ciomadul were active over time spans of over 300,000 years.
Uniquely, Ciomadul is a still alive volcano in Eastern Europe and its craters have a youthful appearance. There is always the possibility of renewed volcanic activity if the magma chamber has not solidified. Deep earthquake activity at Ciomadul occurs down to a depth of , indicating that the volcanic system between the magma chamber and lithospheric melts is still active. It is considered to be a potentially active volcano.