Understanding Evaporite Sequences: The Basics of Cyclic Evaporation
Evaporites are sedimentary rocks formed from the precipitation of minerals as water evaporates in restricted basins, such as ancient seas or lakes. These deposits are key to understanding salt's geological origins, building on the overview of rock-salt deposits.
In a cyclic evaporation process, water bodies like shallow marine basins experience repeated cycles of inflow and evaporation due to arid climates or tectonic restrictions. As water evaporates, its salinity increases progressively, leading to the saturation and precipitation of minerals in a predictable sequence.
- Initial stage: Fresh or brackish water evaporates slowly, concentrating dissolved ions.
- Progressive concentration: As more water is lost, the first minerals to precipitate are calcium sulfate (gypsum, CaSO₄·2H₂O), forming when salinity reaches about 100-150 g/L.
- Cyclic repetition: Refilling with seawater and re-evaporation creates layered sequences, reflecting multiple cycles.
This process can be modeled simply by the evaporation equation for salinity increase: if initial volume is $V_0$ and evaporation removes fraction $f$, final salinity $S_f = S_0 / (1 - f)$, where $S_0$ is initial salinity.