The rock type formed by the processes of compaction and cementation is called sedimentary rock. This category of rock is unique in its formation from fragmented material, unlike igneous and metamorphic rocks. The dual mechanisms of squeezing and gluing loose particles together are the final steps in a long geological process. The resulting sedimentary rocks cover about three-quarters of the Earth’s continental surfaces and provide scientists with a detailed record of the planet’s history.
The Origin of Sediment
The entire process begins with the formation of sediment, the raw material for sedimentary rock, which is derived from pre-existing rocks. This precursor material is created through weathering, the mechanical and chemical breakdown of igneous, metamorphic, or older sedimentary rocks. Physical weathering fractures the original rock into smaller pieces without changing the rock’s chemical composition.
Chemical weathering alters the rock’s composition by dissolving minerals; for example, water slightly acidified by carbon dioxide can dissolve limestone. After the rock is broken down, erosion takes over, which is the removal and transportation of these fragments by agents like water, wind, ice, or gravity. Rivers and streams are particularly effective, carrying vast amounts of material down to larger basins.
The transportation phase ends with deposition, where the moving agent loses energy and drops its load of sediment. This settling of particles typically occurs in low-lying areas, such as ocean floors, lake beds, river deltas, or desert basins. As successive layers of this loose material accumulate over time, they begin the transition into solid rock.
The Dual Forces of Lithification
The transformation of loose, unconsolidated sediment into solid rock is known as lithification. This process primarily involves the two forces of compaction and cementation, which work together to reduce the pore space within the sediment. Compaction is the physical process driven by the weight of overlying sediment layers, which can be thousands of feet thick.
This immense pressure squeezes the water and air out from the spaces between the sediment grains, forcing the particles into a tighter arrangement. For fine-grained sediments like mud and clay, compaction alone can significantly reduce the volume and cause the material to harden into rock.
Following or concurrent with compaction, cementation acts as the chemical binder, gluing the particles together. Water containing dissolved minerals, such as silica, calcite, or iron oxides, circulates through the remaining pore spaces. These dissolved minerals precipitate, or crystallize, in the gaps, forming a mineral cement that locks the grains in place. The cement acts like mortar between bricks, creating a rigid, durable rock structure from the once-loose sediment.
Major Categories of Sedimentary Rocks
Sedimentary rock is classified into three main categories based on the source material that underwent lithification. Clastic sedimentary rocks are the most common type, formed from fragments of other rocks, known as clasts. These are classified by the size of the fragments, with common examples being:
- Shale (formed from clay and silt)
- Sandstone (formed from sand)
- Conglomerate (formed from rounded pebbles and cobbles)
Chemical sedimentary rocks form when minerals precipitate directly out of water solution without the aid of living organisms. This process often occurs in arid environments or restricted basins where water evaporates, leading to the supersaturation and crystallization of dissolved solids. Rock salt, which forms from the evaporation of seawater, and some varieties of limestone are examples of chemical sedimentary rocks.
The third category is biochemical sedimentary rocks, formed from the remains of living organisms. These rocks are created when marine life extracts ions from water to construct shells or skeletal structures, primarily from calcium carbonate. When these organisms die, their remains accumulate on the seafloor and are later lithified into rock. Examples include:
- Chalk, which is made from microscopic marine shells.
- Coal, which forms from the compaction and alteration of plant material.