Naturally occurring hydrothermal systems can be broadly categorized as either vapor-dominated or liquid-dominated. Within part of a vapor-dominated system, steam is the pressure-controlling phase. Vapor-dominated conditions within natural hydrothermal systems may be extensive areally (to tens of square kilometers) and vertically (to more than 3 km depth), as at The Geysers, California, or they may be very localized, confined to a few fractures or fracture zones. Although the model of vapor-dominated hydrothermal systems formulated by White et al.  is generally accepted, how vapor-dominated systems evolve and how they behave in the natural state is not well-understood, partly because of their scarcity and partly because of the difficulty of quantitatively describing two-phase systems.
S. E. Ingebritsen, M. L. Sorey: Vapor-Dominated Zones Within Hydrothermal Systems: Evolution and Natural State, Journal of Geophysical Research, vol. 93, no. B11, pages 13,635-13,655, 1988