Sedimentary rocks represent a record of Earth’s history, forming from materials accumulated on the Earth’s surface. Unlike igneous or metamorphic rocks, they originate from pre-existing rock fragments, minerals precipitated from water, or biological matter. Covering about 75% of the Earth’s land surface, these rocks hold immense scientific value. They are the primary source for understanding ancient climates, locating hydrocarbon reserves, and preserving the majority of the planet’s fossil record. The journey from loose material to solid rock involves a systematic series of mechanical and chemical transformations occurring over vast stretches of time.
Breaking Down the Source Material
The formation of sedimentary rock begins with the breakdown of parent material, which can be igneous, metamorphic, or older sedimentary rock. This occurs through weathering, which includes both physical disintegration and chemical decomposition. Mechanical weathering physically fractures the rock mass without changing its chemical composition. This often happens through frost wedging, where water freezes and expands in cracks, or abrasion, where particles collide and grind against each other.
Chemical weathering involves reactions that alter the mineral structure of the original material. Common processes include dissolution, where water dissolves soluble minerals, and oxidation, where iron-containing minerals react with oxygen to form rust. These processes weaken the rock structure, creating smaller particles that can be easily moved.
Once the material is fragmented, erosion initiates the removal and transport of the sediment. Erosion agents like gravity, running water, wind, or glacial ice detach the weathered material. This stage focuses on the mobilization of the grains away from the source area.
Moving and Settling the Sediment
Sediment is carried away from its origin through various transportation mediums. The primary agents responsible for this movement are running water in streams and rivers, wind currents, and the movement of glaciers. The size and density of a particle influence how far it is carried; fine clay particles can remain suspended in water for long periods, while heavy boulders are only moved short distances by powerful flow or ice.
During transport, sorting occurs as sediments are naturally separated by size, shape, and density. Well-sorted sediment contains grains of roughly the same size, often resulting from prolonged transport by wind or water. Poorly-sorted sediment contains a mix of different sizes, common in glacial or gravity-driven deposits. This sorting directly impacts the texture of the final rock.
Transportation concludes with deposition, which happens when the kinetic energy of the transport medium decreases sufficiently. When a river slows upon entering a lake or ocean, or wind velocity drops, the carried sediment settles out of the fluid. This accumulation occurs in low-lying areas called sedimentary basins, such as continental shelves or lake beds.
Layers of loose sediment—including rock fragments, mineral grains, and organic remains—pile up in these depositional environments. This accumulated material provides the foundation necessary for the final transformation into solid rock.
Turning Sediment into Rock
The transformation of loose sediment into solid sedimentary rock is termed lithification, a multifaceted process involving mechanical and chemical changes. As new layers accumulate, the weight of the overlying material exerts pressure on the sediment below. This pressure drives the first stage of lithification: compaction.
Compaction forces the sediment grains closer together, reducing the material’s volume and decreasing the open space, or pore space, between the grains. This action expels much of the interstitial water. For fine-grained sediments like mud and clay, compaction can be the dominant mechanism, reducing the sediment volume by over 50 percent.
The second stage is cementation, which acts as the geological glue that binds the compacted grains. This process involves the precipitation of dissolved minerals within the remaining pore spaces. Water circulating through the sediment carries dissolved ions, which then crystallize in the open voids.
The most common cementing agents are silica, calcium carbonate (calcite), and iron oxides. When calcium carbonate precipitates, it fills the gaps between sand grains to form solid sandstone. Cementation increases the rock’s overall strength and rigidity.
The Major Categories of Sedimentary Rocks
The specific path of weathering, transport, and lithification results in three major categories of sedimentary rocks, classified based on the nature of the source material.
Clastic Sedimentary Rocks
Clastic rocks, also known as detrital rocks, are formed from the physical fragments of pre-existing rocks that underwent mechanical breakdown and transport. They are classified based on fragment size. Examples include shale, composed of fine clay particles, and sandstone, formed from cemented sand-sized grains.
Chemical Sedimentary Rocks
Chemical rocks form when mineral components dissolve in water and subsequently precipitate or crystallize out of the solution. This process does not rely on the mechanical breakdown of older rock. Examples include rock salt, which forms from the evaporation of saline water, and gypsum, often found in arid environments.
Biochemical Sedimentary Rocks
Biochemical or organic rocks are derived from the remains of living organisms and incorporate biological material directly into their structure. Coquina and some types of limestone are common examples, formed primarily from the calcium carbonate shells and skeletal fragments of marine organisms. Coal is a unique organic rock derived from the burial and compression of terrestrial plant matter.