Ciomadul lies at the southeastern end of the Carpathian volcanic chain and it is the youngest volcano of the Carpatho-Pannonian region.

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.

[9] Magma output progressively decreased during time, with early volcanoes being large stratovolcanoes sometimes featuring caldera-forming eruptions, while more recent activity includes monogenetic volcanoes[10][11] although more precise dating and volume estimation efforts at Ciomad have found an increase of eruption rates over time.

[12] This volcanism occurs in a setting where the collision between the Eurasian Plate and the Tisza-Dacia microplate took place,[13][14] preceded by a stage of subduction involving a narrow ocean.

[21] Other theories on Ciomadul's volcanic activity imply delamination of the lithosphere or roll-back of the subduction zone.

[7] 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.

[9][22][8] Ciomadul is located in the southeastern Carpathians,[4] at the end of the Călimani (Kelemen) – Gurghiu (Görgényi) – Harghita (Hargita) volcanic chain,[9] and is also known as Csomád in Hungarian.

[28] Places around Ciomadul were first mentioned in 1349;[29] the Saxon mineralogist Johann Ehrenwert Fichtel was the first to interpret it as a volcano, in 1780.

[30] 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.

[32] While a publication in 1964 postulated that the tuffs of Ciomadul were reworked Pliocene volcanites, the late Pleistocene age was established soon afterwards.

Individual lava domes form cone-shaped hills,[24] which reach heights of 300–400 metres (980–1,310 ft) and widths of 1–2 kilometres (0.62–1.24 mi).

[43] It is larger than Sfânta Ana with a diameter of 1.9 kilometres (1.2 mi) and not as deep[44] with its bottom lying above sea level.

It is filled with a 10-metre-thick (33 ft) and 800,000-square-metre (80 ha) Sphagnum peat bog and its rim is cut by the Sfânta Ana crater.

[35][45] Unlike Sfânta Ana, the Mohoș crater has been breached by erosion, causing the formation of an outlet valley.

[9] The existence of an even larger crater with a diameter of 2–2.5 kilometres (1.2–1.6 mi) has been suggested,[47] encompassing both Sfânta Ana and Mohoș.

[9] One lapilli layer, 20–23 centimetres (7.9–9.1 in) thick, from Ciomadul has been identified 40 kilometres (25 mi) east of the volcano.

[36] Volcanism at Ciomadul consisted mostly of the extrusion of lava domes, their collapse forming block-and-ash flows and subplinian and Vulcanian eruptions[19] separated by long periods of rest.

[69] Ciomad has produced far-flung tephras, which reached as far as Ukraine[87] and have been recovered from the Ursului Cave of the Perșani Mountains.

[97] Alternatively, 38,900 ± 1,700 years ago a subplinian eruption occurred at Ciomadul; it may have formed the Sfânta Ana crater.

[80] Alternatively, the latest eruption may have occurred at a satellite vent seeing as sedimentation of Lake Sfânta Ana has been ongoing since 26,000 years ago.

[101] After the last eruption, the lava domes were subject to glacial weathering, such as frost shattering that produced stone runs.

[104] Presently, Ciomadul displays seismic activity,[24] release of carbon dioxide from bubbling pools and bogs and mofettas[65] and anomalous heat flow[24] reaching 85–120 watts per square metre (0.0106–0.0150 hp/sq ft).

[105] 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"[14] while Puturosu means "stinky" - [99] and local legends of a "gate to hell".

[16] The Tușnad-Băi springs have temperatures of 15–23 °C (59–73 °F) and discharge salty, carbon dioxide-rich water which emerges from pyroclastic deposits.

[62] Zircon crystallization data imply that the magma chambers of Ciomadul were active over time spans of over 300,000 years.

[123] It is considered to be a potentially active volcano[22] although the risk of impending eruptions has been greatly exaggerated by sensationalist media.

[129] Around Lake Sfânta Ana, the vegetation consists mostly of 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).

[129] The region of the volcano is a Site of Community Importance[109] and some endangered plant species have been identified in the Mohoș bog.