Vapours of sulphur escaping from a volcano often form a crust on the rocks. The process involved is an example of

The formation of a sulfur crust from escaping volcanic vapours is a classic example of deposition. In volcanic systems, gases such as sulfur dioxide and hydrogen sulfide are emitted through vents known as fumaroles or solfataras [1]. When these hot volcanic gases come into contact with cooler rock surfaces or the atmosphere, they undergo a phase transition directly from a gas to a solid state without passing through a liquid phase. This specific thermodynamic process is scientifically termed deposition (also historically referred to as sublimation in some geological contexts). The resulting solid accumulations are often described as fumarolic minerals or encrustations. While condensation involves a gas-to-liquid transition and precipitation typically refers to atmospheric moisture or chemical solids falling from a solution, deposition accurately describes the direct crystallization of sulfur from its gaseous form onto volcanic rocks [2].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 11: Volcanism > 1) Exhalative (Vapour Or Fumes) > p. 142
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 11: Volcanism > 11. Volcanism > p. 139