Lithifying

Lithifying, or lithification, is the set of processes by which unconsolidated sediment is transformed into solid sedimentary rock. It is a key component of diagenesis, the suite of changes that affect sediments after deposition but before metamorphism.

Lithification generally occurs through two main mechanisms: compaction and cementation. During compaction, increasing overburden pressure reduces the porosity of the sediment by squeezing out pore water and expelling air, causing grains to pack more tightly. In cementation, minerals precipitate from pore fluids and crystallize in the spaces between grains, binding them together. Common cements include calcite, silica (quartz), and iron oxides; cement composition depends on the sediment’s mineralogy and groundwater chemistry. In some rocks, diagenetic recrystallization or replacement can also help to solidify the rock.

Different sediment types lithify in different ways. Siliciclastic sediments (sand, silt, clay) typically become sandstone, siltstone, or shale through compaction and cementation by quartz or calcite. Carbonate sediments (calcareous muds and shells) lithify primarily by carbonate cementation, forming limestone or dolostone after chemical alteration such as dolomitization. Some organic-rich sediments may undergo coalification, a specialized lithification affecting plant material, or become hydrocarbons in sedimentary basins.

Lithification generally occurs at shallow burial, with temperatures and pressures typical of diagenesis, well below metamorphic conditions. The timescale ranges from thousands to millions of years, depending on sediment supply, burial rate, and groundwater chemistry.

Understanding lithification helps explain the formation of aquifers and hydrocarbon reservoirs, and it records past environmental and climatic conditions in the sedimentary rock record.