Sedimentary rocks cover about 75% of the continents and much of the ocean floor. These rocks are derived from fragments of pre-existing materials or precipitates from solutions, not volcanic heat. Their formation is a multi-step, sequential process that transforms loose sediment into solid rock over vast periods of geologic time.
Creating the Raw Material: Weathering and Erosion
The initial step in the formation sequence requires the breakdown of older source rock, which may be igneous, metamorphic, or even existing sedimentary rock. This process, known as weathering, can be broadly divided into two main categories: physical and chemical. Physical weathering involves forces that fracture the rock without altering its chemical composition. A common example is frost wedging, where water seeps into cracks, freezes, expands, and pries the rock apart.
In contrast, chemical weathering changes the internal composition of the minerals within the rock structure. This often involves water, oxygen, or carbon dioxide reacting with rock materials. Carbonation, for instance, occurs when atmospheric carbon dioxide dissolves in rainwater to form a weak carbonic acid, which then dissolves minerals like calcite in limestone. Silicate minerals often undergo hydrolysis, reacting with water to form new secondary minerals, most commonly clays.
The material created by both physical and chemical breakdown is called sediment, but it must be removed from its place of origin to continue the process. Erosion is the action that removes and carries away this loosened material, relying on natural agents to mobilize the fragments. This step generates the necessary loose material—from fine clay particles to large boulders—that must be transported and accumulated elsewhere to eventually form a new rock.
Movement and Settling: Transportation and Deposition
Following the creation of sediment by weathering and its removal by erosion, the resulting particles are subjected to transportation. The primary agents for moving these materials are water, wind, ice, and gravity, each affecting the sediment differently. Water is an extremely effective transporter, and movement causes the sediment grains to collide, becoming smaller and more rounded as they travel. Finer particles, like silt and clay, can be carried in suspension for long distances, while heavier materials, like sand and gravel, bounce and roll along the bed.
The transportation phase ends when the carrying agent loses the energy required to keep the load moving, leading directly to deposition. This settling occurs in various environments, such as ocean floors, lake beds, river deltas, or deserts. As sediment settles, it naturally forms distinct layers, or strata, with the coarsest and heaviest material typically dropping out first.
This process of deposition also leads to sorting, where particles of similar size accumulate together, creating the stratified appearance characteristic of most sedimentary rocks. The accumulation of these layers of loose sediment is the prerequisite for the final stage of rock formation. Continuous accumulation means that older, lower layers are buried beneath newer deposits, setting the stage for the pressure-driven processes that follow.
Solidifying the Sediment: Compaction and Cementation
The final transformation of loose sediment into a solid sedimentary rock is known as lithification, which involves two sequential steps: compaction and cementation. As sediment layers accumulate, the immense weight of the overlying material exerts pressure on the deposits beneath. This force drives out the water and air trapped within the pore spaces between the sediment grains, a process called compaction.
Compaction reduces the volume of the sediment and brings the individual grains into closer contact. While this pressure decreases the porosity, the sediment does not fully solidify into rock at this stage, as small spaces still remain between the grains. Compaction is a mechanical act that concentrates the sediment particles.
The full conversion to rock requires cementation, which follows compaction. During this phase, mineral-rich groundwater flows through the pore spaces. Dissolved minerals, such as calcite, silica, or iron oxides, precipitate out of the water and crystallize. These precipitated minerals act as a natural glue, binding the compacted sediment grains together to form a cohesive, solid rock. The sequential chain of processes is weathering, erosion, transportation, deposition, compaction, and finally, cementation.