Waters of widely differing chemical compositions have been considered at least in part volcanic in origin, and are commonly associated with each other in the same area. Do any or all of these types contain volcanic components, and if so, how are the different types derived?
To determine the probable characteristics of volcanic waters, the writer has selected hot-spring groups that are particularly high in temperature and associated heat flow, are associated with late Tertiary or Quaternary volcanism, and are therefore most likely to contain some water and chemical components of direct volcanic origin. Of the different types of water that occur in these groups, one of the most common is characterized chemically by a dominance of sodium chloride.
Isotopic evidence indicates that the contribution of water of direct volcanic origin is not large and is probably no more than 5 per cent in typical sodium-chloride springs.
The compositions of volcanic waters are believed to be determined by:  type of magma and stage of crystallization;  temperature and pressure of the emanation at different stages during and after departure from the magma;  chemical composition, relative quantity, and depth of penetration of mixing meteoric water and water of other origin; and  reactions with wall rocks. Although the type of magma and its stage of crystallization are of major interest and have been emphasized in the past, the outstanding characteristics of volcanic emanations at and near the surface of the earth seem to be controlled for the most part by the other factors.
Nonvolatile compounds are slightly to highly soluble in steam at high pressure, and high-density steam has solvent properties similar to those of liquid water. In the volcanic sodium-chloride waters, the high ratio of lithium to sodium and potassium is shown to indicate that alkalies were transported as alkali halides dissolved in a dense vapor. This in turn demands a deep circulation of meteoric water for steam to condense at high pressure and for the halides to remain in solution. The depth of circulation of meteoric water in the sodium-chloride spring systems is believed to be in the order of 2 miles. Where circulation of meteoric water is shallow, the vapors rise and expand at low pressure, which does not permit transport of substances of low volatility; some type of water other than the sodium-chloride type is formed. The common volcanic sodium-chloride waters are therefore concluded to be the diluted product of high-density emanations, modified by reactions with wall rocks and by precipitation of the less soluble components.
Emanations at high temperature and relatively low pressure consist almost entirely of steam and volatile components. Their compositions are therefore relatively simple, and their ability to transport matter of low volatility is very limited.
The sodium-chloride type is probably gradational into acid-sulfate-chloride waters. There is some evidence that, under conditions not well understood, sulfur may be emitted as SO2, SO3, or other sulfur species of intermediate valence, rather than as H2S or S. Other major types of volcanic waters are called sodium bicarbonate, acid sulfate, and calcium bicarbonate; the first two tend to be distinct, but the calcium-bicarbonate type clearly grades into the sodium-chloride type. The writer concludes that, in general, all these are derived from the sodium-chloride waters as a result of physical environment or of reactions with wall rocks.