Weathering is a fundamental geological process that breaks down existing rock formations, producing the basic components necessary for the formation of sedimentary rocks. Sedimentary rocks are products of accumulated fragments or dissolved minerals, covering a significant portion of the Earth’s surface. They form a complex record of Earth’s history, reflecting the weathering processes that supplied their material.
Creating the Raw Materials
Weathering is the process by which rocks and minerals are broken down at or near the Earth’s surface into smaller pieces or dissolved ions. The resulting material is categorized based on how it was produced: physically broken fragments or chemically altered substances.
Mechanical weathering physically disintegrates the rock without changing its chemical composition. Processes like frost wedging, where water freezes and expands in rock cracks, or abrasion from wind and water, reduce large rock masses into smaller clasts. This physical breakdown increases the total surface area of the rock, which speeds up the rate of chemical attack.
Chemical weathering involves a change in the original rock’s mineral composition, yielding new minerals and dissolved substances. For example, the reaction of water with carbon dioxide forms a weak carbonic acid that dissolves minerals like calcite in a process called dissolution. Other chemical processes include hydrolysis, which alters silicate minerals into clays, and oxidation, where minerals containing iron rust upon exposure to oxygen. The end products of chemical weathering are dissolved ions, newly formed clay minerals, and stable quartz sand grains.
The Process of Rock Formation
The loose material generated by weathering must undergo lithification to become solid rock. Lithification converts unconsolidated sediment into a cohesive sedimentary rock. This transition requires burial, which subjects the sediment to pressure from the weight of overlying layers.
The first step in lithification is compaction, where the accumulated weight squeezes the sediment grains closer together, reducing the space between particles and expelling much of the water. This is particularly effective for very fine-grained sediments like mud and clay. Following compaction, the second process, cementation, occurs as mineral-rich water seeps through the remaining pore spaces.
The dissolved minerals in this water precipitate, or crystallize, in the gaps between the sediment grains, effectively gluing the particles together. Common cementing agents include silica (quartz), calcium carbonate (calcite), and iron oxides. This final binding action completes the lithification process, resulting in a solid, layered sedimentary rock.
Clastic Rock Types
Clastic sedimentary rocks are formed from the physical fragments (clasts) produced by mechanical weathering, transport, and deposition. These rocks are classified according to the average size of the particles they contain, which provides information about the energy of the environment in which they were deposited. High-energy environments, such as swiftly moving rivers or turbulent shorelines, can carry and deposit larger, coarse-grained material.
Coarse-grained rocks are composed of fragments larger than 2 millimeters (gravel-sized clasts). If these large fragments are well-rounded, meaning they have been transported a long distance, the resulting rock is called a conglomerate. If the fragments are angular and sharp, indicating less transport, the rock is a breccia.
Medium-grained clastic rocks are dominated by sand-sized particles, ranging from 1/16th of a millimeter up to 2 millimeters in diameter. The lithified equivalent of sand is sandstone, which often contains high amounts of quartz due to its resistance to chemical weathering. Sandstones form in environments like beaches, deserts, and river channels.
Fine-grained clastic rocks are composed of silt and clay particles, which are too small to be seen without magnification. Siltstone is formed from silt-sized grains, while rocks dominated by the smallest clay-sized particles are termed mudstone or shale. These fine sediments are typically deposited in low-energy, calm water environments, such as deep ocean basins, lakes, or floodplains.
Chemical and Organic Rock Types
Chemical and organic sedimentary rocks form through processes related to dissolved ions and biological material. Chemical sedimentary rocks form when mineral matter precipitates directly out of water solution without the aid of organisms. This often occurs when the water becomes oversaturated with dissolved ions, usually due to high evaporation rates.
Evaporites are chemical rocks that form when water bodies dry up, leaving behind layers of crystallized minerals. Examples include rock salt (halite) and gypsum. Some types of limestone also form chemically when calcium carbonate precipitates directly from water.
Organic sedimentary rocks (biochemical rocks) are formed from the accumulation and lithification of biological remains. Organisms extract dissolved ions from water to build their shells or skeletal structures. Limestone is the most common organic rock, composed of accumulated shells, corals, and skeletal fragments of marine organisms made of calcium carbonate.
Coal is another organic rock, formed from the burial and compression of vast quantities of plant matter. Over long periods, this vegetation is transformed into a carbon-rich rock through heat and pressure. Other examples include chert, which is formed from silica-secreting organisms.