Rock salt (a.k.a. Halite) deposits on Earth are extremely common, with many very extensive deposits thousands of metres deep, and probably totaling trillions of tonnes * ^{See note below.}

In geological 'textbook' terms, rock salt deposits are normally classed as an 'evaporite' - in other words they formed by evaporation - due to sunlight - from previously existing salty oceans.

This view, however, has recently been challenged :

Over the past twenty years, many works have been published that have expressed doubts about several aspects of the current paradigm for salt formation via solar evaporation of seawater. The work presented here is a review of peer-reviewed published literature where several issues are documented that are NOT compatible with the current paradigm for the formation of salt giants via solar evaporation alone. This also includes observations of minerals that have been formed together with the salt by the same processes, such as sulphates, carbonates, borates, and silicates. Another important observation is that the large salt deposits do not contain marine fossils but instead traces of terrestrial plants (e.g., pollen). This observation alone significantly weakens any claims that major salt deposits were formed via direct evaporation of seawater."

Source : Minerals, Volume 14, Issue 5 (with references to the relevant papers which question the textbook view)

*Note : Wikenigma has not been able to find any reference for estimates of the total global tonnage of rock salt deposits.