Sediment is loose material, such as sand, mud, or gravel, broken down from pre-existing rock through weathering and erosion. The transformation of this loose material into solid sedimentary rock is a multi-step process known as lithification. Lithification is part of diagenesis, a broader set of physical and chemical changes occurring after deposition and before high-grade metamorphism. This transformation is accomplished through two interconnected mechanisms: the physical reduction of volume and the chemical binding of grains.
Compaction: Reducing Volume Through Pressure
Compaction is the initial physical process where the volume of deposited sediment is significantly reduced. This reduction is driven by the increasing weight of overlying sediment, known as overburden pressure.
The overburden pressure forces the sediment grains into a denser, tightly packed arrangement. This results in a substantial decrease in the sediment’s porosity, the amount of open space between the grains. For fine-grained sediments like mud and clay, porosity can be drastically lowered through this mechanical squeezing.
The pressure also expels water trapped within the pore spaces, effectively dewatering the sediment. While compaction alone can consolidate fine-grained materials like mud into shale, it primarily prepares coarser sediments, such as sand, for the subsequent chemical binding process.
Cementation: The Mineral Binding Process
Following compaction, cementation acts as the chemical process that permanently binds the grains, completing lithification. This mechanism involves the precipitation of dissolved minerals carried by groundwater circulating through the remaining pore spaces. The groundwater becomes saturated with ions leached from the surrounding rock or sediment.
When chemical conditions change, such as variations in temperature or pressure, these dissolved ions precipitate in the spaces between the compacted grains. These newly formed mineral crystals grow around the grain contacts, welding them together into a solid rock structure. Common cementing agents include silica (quartz) and calcium carbonate (calcite).
Iron oxides and various clay minerals can also serve as binding agents, determining the color and hardness of the rock. This precipitation process can take millions of years, depending on the volume of circulating water and the availability of dissolved ions.
Major Types of Sedimentary Rocks
The classification of sedimentary rock is determined by the specific nature of the sediment, which falls into three main categories.
Clastic Sedimentary Rocks
Clastic rocks are formed from the mechanical fragments, or clasts, of pre-existing rocks that have been weathered, transported, and lithified. Examples include sandstone, formed from cemented sand grains, and shale, which originates from compacted clay particles.
Chemical Sedimentary Rocks
Chemical rocks form when minerals precipitate directly from water solutions, often without the direct involvement of living organisms. Rock salt (halite) is a common example that forms when large bodies of water evaporate, leaving behind concentrated mineral deposits. Certain types of limestone also form chemically through the precipitation of calcium carbonate.
Organic Sedimentary Rocks
Organic rocks are derived from the accumulation and lithification of biological material. Coal, for instance, forms from the burial and compaction of vast quantities of plant matter under heat and pressure. Chalk, a type of soft limestone, is another example, formed primarily from the cemented microscopic shells and skeletons of marine organisms.