Why are salt deposits in a salt pan arranged like bathtub rings?

Salt deposits in a salt pan are arranged in zones resembling bathtub rings due to the varying solubility of minerals as brine evaporates. This process mirrors the evaporation of brine in a dish, where minerals precipitate in a specific, orderly sequence based on their chemical properties.12

The zoning typically follows this pattern:

• Outer Margin: The first minerals to precipitate from the evaporating brine are the least soluble, such as calcium carbonate and magnesium carbonate. These form the outermost ring.1
• Intermediate Zone: As evaporation continues, the next ring forms, consisting of sulfates of calcium and sodium, such as gypsum or thenardite.1
• Center: The final, highly soluble minerals—such as sodium chloride—precipitate last as the remaining brine reaches its highest level of salinity, concentrating these deposits in the center of the pan.1

This systematic deposition occurs because as the volume of the brine decreases, the concentration of dissolved solids increases, forcing minerals out of the solution according to their individual solubility limits.2