Sedimentary rock is formed from the consolidation of loose sediment, making it a product of the rock cycle’s recycling process. This rock type is a lithified collection of particles, which can be fragments of pre-existing rocks or minerals precipitated from water. Sediment is the original unconsolidated material—loose sand, mud, gravel, or dissolved ions—that accumulates at the Earth’s surface. Recycling sedimentary rock back into sediment requires reversing lithification, accomplished primarily through weathering, which breaks the rock down, and erosion, which removes the resulting loose material.
Mechanical Weathering: Physical Disintegration
Mechanical weathering is the physical breakdown of sedimentary rock into smaller fragments without changing its chemical composition. This process increases the rock’s surface area, making it more susceptible to chemical attack.
Frost wedging occurs when water penetrates cracks and joints. When the water freezes, it expands by nine percent, exerting pressure that widens the cracks and forces the rock apart.
Exfoliation, or pressure release, occurs when overlying rock material is eroded away. The removal of this weight causes the rock underneath to expand, leading to fractures parallel to the surface. This causes the outer layers of the rock to peel off in curved sheets.
Thermal stress weathering involves repeated expansion and contraction caused by significant temperature fluctuations, such as those in deserts. This constant heating and cooling weakens the rock structure, especially when mineral grains have different rates of thermal expansion.
Abrasion is the physical grinding of the rock surface by particles carried by wind, water, or ice. Sand grains carried in a stream act like sandpaper, wearing away the rock and producing new sediment.
Chemical Weathering: Altering Mineral Composition
Chemical weathering involves reactions between rock-forming minerals and external agents like water, oxygen, and acids, changing the mineral’s chemical makeup. This process transforms solid rock components into dissolved ions or new, weaker compounds. Dissolution is a direct form of chemical weathering where certain minerals, notably calcite in limestone, dissolve completely when they contact water, especially if the water is slightly acidic.
Natural rainwater becomes weakly acidic because atmospheric carbon dioxide dissolves into it, creating carbonic acid. This acid enhances the dissolution of soluble minerals, turning them into dissolved ions carried away in the water. Hydrolysis involves the reaction of water molecules with minerals, particularly silicates like feldspar. The water chemically alters the mineral to form new, stable secondary products, primarily clay minerals, which weakens the structure of the original rock.
Oxidation is a chemical reaction in which oxygen combines with elements in the rock, often iron, to form new compounds. When iron-bearing minerals oxidize, they form iron oxides and hydroxides, resulting in the reddish-brown coloration associated with rust. These new compounds are softer and less structurally sound than the original minerals, promoting the disintegration of the rock into sediment.
Erosion and Transportation of New Sediment
The final step in the recycling process is erosion, which is the removal and movement of the new sediment created by weathering. Weathering breaks the rock down at its original location, but erosion transports the debris to a new depositional environment. Running water is the most important agent of erosion, capable of moving particles from microscopic clay to large boulders.
Water transports sediment in several ways, including suspension, where fine particles like silt and clay are carried within the flow, and saltation, where medium-sized grains like sand bounce along the stream bed. Larger fragments are moved by rolling and sliding along the bottom, a process known as traction. Gravity also plays a role through mass wasting events, such as landslides and rock falls, which move large volumes of weathered material downslope.
Wind acts as an abrasive and transporting agent, particularly in arid regions, lifting and carrying sand and dust over long distances. Glacial ice is another powerful agent, capable of carrying both fine sediment and massive rock fragments embedded within the ice sheet.