1911 Encyclopædia Britannica/Solfatara

SOLFATARA, a volcanic vent emitting vapours chiefly of sulphurous character, whence the name, from the Italian solfo (sulphur). The typical example is the famous Solfatara, near Puzzuoli, in the Phlegraean Fields, west of Naples. This is an old crater which has not been in active eruption since A.D. 1108, but which is continuously exhaling heated vapours, chiefly hydrogen sulphide, sulphur dioxide and steam. These issue from orifices in the crust, on the walls, of which are yellow incrustations of sublimed sulphur, sometimes orange-red by association with arsenic sulphide, whilst the trachytic rocks of the volcano are bleached and corroded by the effluent vapours, with formation of such products as gypsum and alum. Sal ammoniac occurs among the sublimates. The term solfatara has been extended to all dormant volcanoes of this type; and a volcano which has ceased to emit lava or ashes but still evolves heated vapours, is said to have passed into the “solfataric stage.” Examples are to be found in many volcanic districts. By French geologists the term soufrière is used instead of the Italian solfatara. (See Volcanoes.)