Sedimentary rocks form from the accumulation of sediment derived from the physical and chemical breakdown of pre-existing rocks through weathering and erosion. This weathered material is transported by wind, water, or ice and deposited in layers. The loose material transforms into solid rock through lithification, a process involving compaction from the weight of overlying layers and cementation by minerals precipitating from water. Classifying these rocks is necessary to interpret Earth’s history, as their characteristics reflect the ancient environments in which they formed and aid in locating natural resources.
Three Modes of Sedimentary Rock Formation
The highest level of classification for sedimentary rocks is based on the origin of the material, leading to three fundamental categories. Clastic, or detrital, sedimentary rocks are composed of physically transported fragments (clasts) of older rocks and minerals. These clasts result from mechanical weathering processes that break down source material without significantly changing its chemical composition.
Chemical sedimentary rocks form through an inorganic process where minerals precipitate directly out of a water solution, such as a lake or ocean. This precipitation often occurs when water evaporates, leaving behind a concentrated mineral residue that crystallizes.
Biochemical sedimentary rocks involve living organisms that extract dissolved ions from water to create shells or skeletal structures. When these organisms die, their hard parts accumulate and are lithified, forming a rock primarily composed of biological material. Organic sedimentary rocks are a specific type of biochemical rock formed from the accumulation and burial of plant or animal matter, such as decaying vegetation found in ancient swamps.
Classification of Clastic Rocks by Texture
Clastic sedimentary rocks are classified primarily by their texture, specifically the size of the grains. Geologists use a standardized scale to sort these fragments into size groups, which correlates directly to the rock’s name. This focus on texture reflects the energy of the environment that transported and deposited the sediment.
The coarsest category, gravel, consists of particles larger than two millimeters in diameter. If the gravel fragments are angular and sharp, suggesting they were transported only a short distance, the rock is specifically named breccia. If the fragments are rounded, indicating significant abrasion during long-distance transport, the rock is classified as conglomerate.
Particles between \(1/16\) of a millimeter and two millimeters are classified as sand, and a rock composed predominantly of this size material is called sandstone. Finer-grained sediments are divided into silt, which feels gritty when rubbed, and clay, which feels smooth. Lithified silt forms siltstone, while a rock composed mostly of clay or a mixture of silt and clay that breaks into layers is termed shale or mudstone.
Beyond grain size, two secondary textural characteristics refine the classification: sorting and rounding. Sorting describes the uniformity of the grain sizes within the rock, with well-sorted rocks suggesting consistent transport energy. Rounding refers to the shape of the individual grains, where highly rounded grains imply extensive travel and abrasion from their source rock.
Classification of Chemical and Biochemical Rocks by Composition
Unlike clastic rocks, chemical and biochemical sedimentary rocks are classified based on the dominant mineral composition. The main groups are distinguished by the chemical makeup of the precipitated or biologically produced material. Carbonate rocks are the most abundant of this type and are defined by the presence of calcium carbonate, either as the mineral calcite or aragonite.
Limestone is the general name for a carbonate rock, but it can form in two distinct ways. Chemical limestone forms when calcium carbonate precipitates inorganically from warm, shallow seawater, while biochemical limestone forms from the accumulation of skeletal remains and shells of marine organisms. Examples of biochemical limestone include coquina, which is loosely cemented shell fragments, and chalk, which is composed of microscopic shells from plankton.
Evaporites are a distinct group of chemical rocks that form as water bodies dry up, leading to the supersaturation and subsequent precipitation of dissolved salts. Classification within this group is based on the specific mineral that crystallized, such as rock salt, which is composed of the mineral halite, or gypsum rock, composed of the mineral gypsum. These deposits often form in restricted marine basins or desert lakes.
Siliceous rocks are defined by a high content of silica, typically in the form of microcrystalline quartz. Chert is the most common example, which can form either chemically through the precipitation of silica from solution or biochemically from the accumulation of silica shells from organisms like diatoms and radiolarians. Organic sedimentary rocks, like coal, are classified by their carbon content, ranging from low-grade peat to high-grade bituminous coal, reflecting different degrees of heat and pressure exposure.