Lithification is the process in which loose sediment becomes solid sedimentary rock.
Lithification mainly involves compaction and cementation. As sediment is buried, the weight of overlying layers squeezes grains closer together and reduces pore space. Groundwater moving through the sediment can deposit minerals such as silica, calcite, or iron oxides between grains, binding them into rock.
The process commonly affects sand, mud, gravel, shell fragments, and volcanic ash. Sand can lithify into sandstone, mud into mudstone or shale, and gravel into conglomerate or breccia, depending on grain shape and arrangement.
Lithification is not the same as metamorphism. Metamorphism changes an existing rock through heat, pressure, or chemically active fluids without completely melting it, whereas lithification turns unconsolidated sediment into rock at comparatively shallow conditions.