Ciomadul is a

volcano A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface. On Ear ...

Romania Romania ( ; ro, România ) is a country located at the crossroads of Central, Eastern, and Southeastern Europe. It borders Bulgaria to the south, Ukraine to the north, Hungary to the west, Serbia to the southwest, Moldova to the east, and ...

, and is known as in Hungarian. It is in the Carpathians, close to the towns of

Băile Tușnad Băile Tușnad (; hu, Tusnádfürdő, ) is a town in Harghita County, Romania in eastern Transylvania. With a population of 1,617, it is the smallest town in Romania by population. It is located at an altitude of 650 metres in the southern reac ...

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 Crater Lake ( Klamath: ''Giiwas'') is a volcanic crater lake in south-central Oregon in the western United States. It is the main feature of Crater Lake National Park and is famous for its deep blue color and water clarity. The lake partly fill ...

Lake Sfânta Ana Lake Sfânta Ana (; hu, Szent Anna-tó; ) is the only crater lake in Romania located in the volcanic crater of the volcano named Ciomatu Mare of the Eastern Carpathians, near Tușnad in the Natural Reserve of Mohoș, Harghita County, Romania. ...

. The dominant volcanic rock at Ciomadul is

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...

-rich

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite ...

. 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 Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

as far as the

Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...

. 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 Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from lava, from volcani ...

and evidence of a still existing

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...

indicate that Ciomadul is a potentially active volcano.

Geography and geology

Regional setting

With the exception of

Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders ...

and

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

, historical

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

has not taken place in Continental Europe. The most recent volcanic activity occurred between 40,000 and 6,500 years ago in

Garrotxa Garrotxa () is a comarca (county) in Girona, Catalonia, Spain. Its population in 2016 was 55,999, more than half of them in the capital city of Olot. It is roughly equivalent to the historical County of Besalú. Geography Garrotxa borders ...

, the

Massif Central The (; oc, Massís Central, ; literally ''"Central Massif"'') is a highland region in south-central France, consisting of mountains and plateaus. It covers about 15% of mainland France. Subject to volcanism that has subsided in the last 10,0 ...

and the

Vulkaneifel Vulkaneifel () is a district (''Kreis'') in the northwest of the state Rhineland-Palatinate, Germany. It is the least densely populated district in the state and the fourth most sparsely populated district in Germany. The administrative centre of ...

. 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 Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

. The last volcanism occurred at Ciomadul in the last glacial age. Sparse

basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

volcanism has also occurred in the area, forming

monogenetic volcanic field A monogenetic volcanic field is a type of volcanic field consisting of a group of small monogenetic volcanoes, each of which erupts only once, as opposed to polygenetic volcanoes, which erupt repeatedly over a period of time. The small monogeneti ...

s. A long

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

lies in the

Carpathians The Carpathian Mountains or Carpathians () are a range of mountains forming an arc across Central Europe. Roughly long, it is the third-longest European mountain range after the Urals at and the Scandinavian Mountains at . The range stretches ...

. 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 A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

-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 The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

and the Tisza-Dacia

microplate A microplate, also known as a microtiter plate (''Microtiter'' is a registered trademark in the United States, therefore it should not be used generically without attribution), microwell plate or multiwell, is a flat plate with multiple "wells" ...

took place, preceded by a stage of subduction involving a narrow

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

. This is part of the collision between the

African Plate The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American 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 An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

activity; deep earthquakes suggest that a remnant of a

slab Slab or SLAB may refer to: Physical materials * Concrete slab, a flat concrete plate used in construction * Stone slab, a flat stone used in construction * Slab (casting), a length of metal * Slab (geology), that portion of a tectonic plate tha ...

exists beneath the Vrancea zone. This tectonic setting may also be responsible for ongoing exhumation in the southeastern Carpathians, 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 Delamination is a mode of failure where a material fractures into layers. A variety of materials including laminate composites and concrete can fail by delamination. Processing can create layers in materials such as steel formed by rolling a ...

of the lithosphere or roll-back of the subduction zone. Volcanism in this chain is

calc-alkaline The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic m ...

, yielding both

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

, and rhyolite. Three million years ago, a change in the chemistry of volcanism occurred, with an increased content of

in the rocks. This change in composition geographically coincided with the volcanic activity crossing a

lineament ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-ali ...

known as the Trotus line.

Volcano

Ciomadul is located in the southeastern

, 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

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 Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building epi ...

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

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

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

s of Ciomadul were reworked

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58volcanites, the late

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...

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 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 Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...

, 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. Szent Anna tó 3

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 El Chichón, also known as Chichonal, is an active volcano in Francisco León, north-western Chiapas, Mexico. El Chichón is part of a geologic zone known as the Chiapanecan Volcanic Arc. El Chichón is a complex of domes with a tuff ring, made ...

volcano in

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

. This crater lacks a breach and is relatively unaffected by erosion. It contains a deep

, which once may have been over deep. This large lake is known as

() and lies at an altitude of ; its ecosystem and environment has drawn the attention of scientists for two centuries. 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 ''Sphagnum'' is a genus of approximately 380 accepted species of mosses, commonly known as sphagnum moss, peat moss, also bog moss and quacker moss (although that term is also sometimes used for peat). Accumulations of ''Sphagnum'' can store wa ...

peat bog A bog or bogland is a wetland that accumulates peat as a deposit of dead plant materials often mosses, typically sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, mosses, quagmire, and muskeg; a ...

and its rim is cut by the St. Ana crater. Mohos Peat Bog 1

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 A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

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 and took place 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 Dense-rock equivalent (DRE) is a volcanologic calculation used to estimate volcanic eruption volume. One of the widely accepted measures of the size of a historic or prehistoric eruption is the volume of magma ejected as pumice and volcanic ash, k ...

. 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

shoshonite Shoshonite is a type of igneous rock. More specifically, it is a potassium-rich variety of basaltic trachyandesite, composed of olivine, augite and plagioclase phenocrysts in a groundmass with calcic plagioclase and sanidine and some dark-colored v ...

s were erupted 2.3–1.5 million years ago; they represent

cryptodome In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions o ...

Andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

lava flows from Pilisca underlie the Ciomadul deposits in some places.

Composition

The principal rock is

, which defines a

-rich

suite. The rocks have a porphyric appearance and contain few vesicles. They are also very rich in crystals, with the dominant

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

-forming minerals being biotite,

hornblende Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rock ...

and

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

. Less important are

allanite Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rock ...

, apatite,

clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe I ...

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

, orthopyroxene,

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

, sphene and

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

. The

groundmass The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded. The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, ...

contains

, pyroxene,

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

and oxides of

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

and

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

. 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 A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

magma. The phenocryst compositions at Ciomadul are unlike these at other volcanoes in the Carpathians. The magmas derive from the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...

lithosphere, which underwent

metasomatic Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical com ...

alteration. 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 Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

of the rocks difficult. These include amphibole, biotite,

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

and

. The zircons formed almost continuously over hundred thousands of years within Ciomadul's magma chamber, indicating a steady crystallization of the chamber. The temperature of the

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

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 .

Climate and vegetation

Ciomadul is located in a

temperate climate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...

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 A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...

occurred in the Carpathians during the

ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...

s, no glacial activity is recorded at Ciomadul. The volcano was unforested at that time, with steppe and

tundra In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless mou ...

vegetation comprising most of the reported flora.

Drill core A modern core drill is a drill specifically designed to remove a cylinder of material, much like a hole saw. The material left inside the drill bit is referred to as the ''core''. Core drills used in metal are called annular cutters. Core dr ...

s 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 ''Fagus sylvatica'', the European beech or common beech is a deciduous tree belonging to the beech family Fagaceae. Description ''Fagus sylvatica'' is a large tree, capable of reaching heights of up to tall and trunk diameter, though more ...

'' (common beech) and ''

Picea abies ''Picea abies'', the Norway spruce or European spruce, is a species of spruce native to Northern, Central and Eastern Europe. It has branchlets that typically hang downwards, and the largest cones of any spruce, 9–17 cm long. It is very close ...

'' (Norway spruce) woods. Other trees include '' Acer platanoides'' (Norway maple), ''

Betula pendula ''Betula pendula'', commonly known as silver birch, warty birch, European white birch, or East Asian white birch, is a species of tree in the family Betulaceae, native to Europe and parts of Asia, though in southern Europe, it is only found ...

'' (silver birch), '' Carpinus betulus'' (common hornbeam), ''

Pinus sylvestris ''Pinus sylvestris'', the Scots pine (UK), Scotch pine (US) or Baltic pine, is a species of tree in the pine family Pinaceae that is native to Eurasia. It can readily be identified by its combination of fairly short, blue-green leaves and or ...

'' (Scots pine), ''

Salix caprea ''Salix caprea'', known as goat willow, pussy willow or great sallow, is a common species of willow native to Europe and western and central Asia.Meikle, R. D. (1984). ''Willows and Poplars of Great Britain and Ireland''. BSBI Handbook 4. . Des ...

'' (goat willow) and '' Salix cinerea'' (grey willow). A

fen A fen is a type of peat-accumulating wetland fed by mineral-rich Groundwater, ground or surface water. It is one of the main types of wetlands along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as ...

contains '' Carex lasiocarpa'' (slender sedge), ''

Carex rostrata ''Carex rostrata'', the bottle sedge or beaked sedge, is a perennial species of sedge in the family Cyperaceae. Range and habitat The species is native to Holarctic fens and can be found in Canada and the northern part of the United States, and ...

'' (bottle sedge), '' Lysimachia thyrsiflora'' (tufted loosestrife) and '' Sphagnum angustifolium'' (fine bogmoss). At Mohos, vegetation consists of ''

Alnus glutinosa ''Alnus glutinosa'', the common alder, black alder, European alder, European black alder, or just alder, is a species of tree in the family Betulaceae, native to most of Europe, southwest Asia and northern Africa. It thrives in wet locations wh ...

'' (common alder), ''

'' and ''

Salix Willows, also called sallows and osiers, from the genus ''Salix'', comprise around 400 speciesMabberley, D.J. 1997. The Plant Book, Cambridge University Press #2: Cambridge. of typically deciduous trees and shrubs, found primarily on moist s ...

''. The peat bog contains trees (''

'' and ''

Betula pubescens ''Betula pubescens'' (syn. ''Betula alba''), commonly known as downy birch and also as moor birch, white birch, European white birch or hairy birch, is a species of deciduous tree, native and abundant throughout northern Europe and northern Asia ...

'' (downy birch)) and

Ericaceae The Ericaceae are a family of flowering plants, commonly known as the heath or heather family, found most commonly in acidic and infertile growing conditions. The family is large, with c.4250 known species spread across 124 genera, making it th ...

. The region of the volcano is a

Site of Community Importance A Site of Community Importance (SCI) is defined in the European Commission Habitats Directive (92/43/EEC) as a site which, in the biogeographical region or regions to which it belongs, contributes significantly to the maintenance or restoration at ...

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. 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". An alternative description envisages an "old Ciomadul" between 1,000,000 - 300,000 years ago and a "young Ciomadul eruptive period" between 160,000 - 30,000 years ago, with the latter in turn subdivided into five stages that emplaced about of rock. 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. 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 Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they re ...

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.

Tephras

Ciomadul has produced far-flung

s, which reached as far as

Ukraine Ukraine ( uk, Україна, Ukraïna, ) is a country in Eastern Europe. It is the second-largest European country after Russia, which it borders to the east and northeast. Ukraine covers approximately . Prior to the ongoing Russian inv ...

and have been recovered from the Ursului Cave of the Perșani Mountains. Some tephra layers found in two drilling cores of the

may have originated at Ciomadul but reliably distinguishing between Ciomadul tephras and these from Nisyros and

Anatolia Anatolia, tr, Anadolu Yarımadası), and the Anatolian plateau, also known as Asia Minor, is a large peninsula in Western Asia and the westernmost protrusion of the Asian continent. It constitutes the major part of modern-day Turkey. The ...

n volcanoes is difficult. The Roxolany Tephra has been found as far as Odessa,

, 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 In physics, optically stimulated luminescence (OSL) is a method for measuring doses from ionizing radiation. It is used in at least two applications: * Luminescence dating of ancient materials: mainly geological sediments and sometimes fired pott ...

dates, the Roxolany tephra was deposited around 143,800 years ago.

Recent explosive activity

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 are 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 but probably took place about 30,000 years ago. In 1994,

radiocarbon Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and c ...

dating yielded an age of 10,700 ± 800 years

Before Present Before Present (BP) years, or "years before present", is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Becau ...

from a

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

. 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

s ( versus ) and involving more primitive magma. After the last eruption, the lava domes were subject to glacial

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...

, such as frost shattering that produced stone runs. An 1838 document by an unknown author stated that even old legends do not record eruptive activity at Ciomadul, and there is no evidence in St. Ana lake sediments of

layers that might indicate more recent eruptions.

Current status

Presently, Ciomadul displays seismic activity, release of

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

from bubbling pools and bogs and mofettas and anomalous

heat flow Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

reaching . Outgassing of

, hydrogen sulfide and mostly

abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...

have been found at Ciomadul, forming sulfide deposits in some caverns. The total output of

exceeds about per year, while the output of

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" - and local legends of a "gate to hell". Former alum and sulfur mines east of Ciomadul were abandoned due to the dangers from toxic gases. The carbon dioxide is accompanied by

noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...

es derived from the mantle. At depths of and especially , a

has been identified beneath Ciomadul, based on

magnetotelluric Magnetotellurics (MT) is an electromagnetic geophysical method for inferring the earth's subsurface electrical conductivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface. Investigation depth ra ...

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

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 Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

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,

-rich water which emerges from pyroclastic deposits. They are used in spas in the area. In one cave, autothropic bacterial

biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...

s 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

. However, as demonstrated by the unexpected eruption of Chaiten volcano in

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

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 Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

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 even if there is no positive evidence of ongoing magma generation. 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 although the risk of impending eruptions has been greatly exaggerated by

sensationalist In journalism and mass media, sensationalism is a type of editorial tactic. Events and topics in news stories are selected and worded to excite the greatest number of readers and viewers. This style of news reporting encourages biased or emo ...

media.

References

Bibliography

* * * * * * * * * * * * * * * * * * * * * * * * *

External links

{{Commons category Pleistocene lava domes Volcanoes of Romania Landforms of Romania Natural history of Romania Geography of Harghita County Potentially active volcanoes