Fig. 38. Bee Hive Geyser of Iceland
From Dana's Manual of Geology
Besides water vapor and sulphurous vapors there are other substances that escape from the earth in volcanic districts. Sulphurous acid, together with hydrogen and nitrogen escape from nearly all lava. At Vesuvius chlorine gas is given off. This, however, as soon as it passes into the atmosphere becomes changed into hydrochloric acid. Sulphurous acid is frequently changed into sulphuric acid, which, combining with various substances, forms such materials as gypsum, or sulphate of lime, the chemical name for plaster of Paris; sulphate of soda or Glauber's salt; sodium chloride or common table salt; and sal ammoniac. You will remember in reading the description of Vulcano, in the Grecian Archipelago, that some of these products were collected at the chemical works that had been established on the volcano.
When a volcanic mountain is for the time being passing from an active to an extinct condition, it is sometimes said to be in the fumarole stage, since the presence of the fumaroles are the only indication of its activity. The volcanic heat is still great. When it reaches a still greater decline, the fumaroles disappear, and only solfataras are left. The amount of heat is now only sufficient to produce sulphur vapors and the vapor of water. This is called the solfatara stage.
Of course, as we have already pointed out, fumaroles and solfataras may occur in the neighborhood of a volcano at different distances from its crater.