The Gaseous Products are important as agents of the eruptions, in promoting the crystallizing of the lavas, and in altering the rocks with which they come in contact. The most abundant is steam. Carbon dioxide is common, especially when the action is failing, and often continues after all other signs of activity have died out. Sulphur dioxide (S02) is very characteristic and is the source of many other compounds. Sulphuretted hydrogen (H2S) is a common volcanic gas, as is also hydrochloric acid (HC1). Several solids are vapourized, such as the chlorides of ammonium, iron, calcium, etc., but these are of little significance.

It is important to emphasize the vast quantity of material which, in many volcanic eruptions, is brought from the interior of the earth and deposited on the surface. Thus, the eruption of Skaptar Jokul in Iceland, in 1783, produced an amount of lava which is calculated as exceeding six cubic miles in volume. The fragmental materials derived from the great explosion of Kraka-toa, in 1883, are estimated at 4.3 cubic miles, while for that of Tem-boro, in 1815, Verbeek gives the almost incredible figures of 28.6 cubic miles.