Sedimentary rocks form from accumulated material on the Earth’s surface, and their classification depends on the material’s origin. Geologists categorize their formation into mechanical, chemical, and organic routes. Rock salt requires a detailed look at the two primary formation paths—detrital and chemical—to determine its classification.
Defining Detrital Sedimentary Rocks
Detrital sedimentary rocks, also called clastic rocks, form from solid fragments of pre-existing rocks. This formation begins with mechanical weathering, where forces like ice wedging or abrasion physically break down source rock into smaller pieces called clasts.
Following the breakdown, the rock fragments are transported by natural agents such as wind, water, or glacial ice. During transport, the clasts become sorted by size and rounded by continuous abrasion. When the transporting energy decreases, the fragments are deposited in layers, often in low-lying areas like river deltas, lakes, or ocean basins.
The final stage is lithification, the process that turns loose sediment into solid rock. This involves compaction, where the weight of overlying sediment squeezes out water, and cementation, where dissolved minerals precipitate and fill the spaces between the clasts, binding them together. Examples of detrital sedimentary rocks include sandstone, which is cemented sand, and shale, which forms from compacted clay-sized particles.
The Formation Mechanism of Rock Salt
Rock salt (halite) does not form through the mechanical accumulation of fragments. Its formation is a purely chemical process that relies on the supersaturation of water, making it a specific type of chemical sedimentary rock called an evaporite.
The mechanism begins in restricted basins, such as shallow seas or saline lakes, located in arid climates. As water evaporates from the surface, the concentration of dissolved ions, including sodium and chlorine, steadily increases. The water eventually reaches a state of supersaturation, meaning it holds more dissolved mineral than it can naturally maintain in a liquid state.
Once this point is reached, the dissolved ions precipitate out of the solution, forming solid mineral crystals. Halite accumulates in thick, layered deposits as the water continues to evaporate. These beds are later buried by other sediments, where pressure and time complete the process of forming rock salt beds.
Categorizing Rock Salt
Rock salt is classified as a chemical sedimentary rock due to its method of origin. It forms through inorganic chemical precipitation, contrasting with the mechanical accumulation that defines detrital rocks. The crystalline texture of rock salt, where individual halite crystals interlock, is a characteristic feature of chemically formed sedimentary rocks.
Because rock salt forms as a direct result of water evaporation, it is specifically categorized as an evaporite. Other evaporites include rock gypsum and some varieties of limestone.