Lithification

Lithification is the set of processes that transform loose sediment into solid sedimentary rock. It is a key part of diagenesis, occurring after deposition as sediments are buried and subjected to increasing pressure and chemical alteration. The principal mechanisms are compaction and cementation. During compaction, overlying layers press grains together, expelling pore water and reducing porosity, which helps grains lock into a rigid rock. Cementation occurs when mineral-rich groundwater precipitates bonding minerals between grains, producing a cement that binds the sediment. Common cementing agents include silica (quartz), calcite (calcium carbonate), iron oxides, and clay minerals; the specific cement depends on the sediment composition and fluid chemistry.

Authigenic minerals may crystallize within the pore spaces, and diagenetic dissolution and recrystallization can modify grain contacts and porosity. In some cases, dissolution of soluble components followed by precipitation during diagenesis strengthens the rock; in others, cementation reduces porosity further. Lithification can be rapid in some depositional environments and more gradual in slow-burial settings.

Common examples of lithified rocks include clastic rocks such as sandstone and siltstone, which are typically cemented by quartz or calcite, and chemical or biogenic rocks such as limestone and chert, which form through precipitation or recrystallization of calcium carbonate or silica. The term lithification is sometimes used more broadly to include all diagenetic changes that convert sediment into rock, whereas in some definitions it is limited to compaction and cementation.