Clastic rocks are a type of sedimentary rock defined by their composition of fragments or broken pieces of pre-existing minerals and rock. The term “clastic” comes from the Greek word klastos, meaning “broken.” These rocks form from the accumulation of weathered debris, making them a record of Earth’s surface processes. They are categorized as one of the major groups of sedimentary rocks, alongside chemical and biochemical types.
The Fundamental Nature of Clastic Materials
The larger, identifiable pieces within the rock are called clasts, which are rock and mineral fragments derived from older source rocks like igneous, metamorphic, or other sedimentary formations. The composition of these clasts, such as the presence of quartz, feldspar, or rock fragments, provides geologists with clues about the region from which the material originated.
The spaces between these larger clasts are filled by finer-grained material known as the matrix. This matrix is composed of silt- or clay-sized particles that settled around the larger fragments during deposition. Finally, the entire mass is bound into a solid rock by cement, which is a chemical precipitate that crystallizes in the open pore spaces. Common cementing agents include silica (quartz) or calcium carbonate (calcite), which act as the chemical “glue” to lithify the loose sediment.
The Journey: Formation Processes
The creation of a clastic rock begins with weathering, which involves both mechanical forces, like the freezing and thawing of water, and chemical reactions that dissolve or alter minerals. Once broken down, the loose material is then subject to erosion and transport, during which agents like rivers, wind, or glaciers carry the sediment away from its origin.
The energy of the transporting medium determines how far and how quickly the fragments travel, which in turn affects their size and shape. When the energy of the moving agent decreases, the sediments begin to settle out in a process called deposition, often accumulating in layers in environments like floodplains, deltas, or ocean basins. The final stage is lithification, the transformation of loose sediment into a coherent rock mass.
Lithification involves two primary mechanisms: compaction and cementation. Compaction occurs as the weight of overlying sediments increases, squeezing the grains closer together and forcing out excess water. Cementation takes place when dissolved minerals within the remaining pore water precipitate and crystallize, binding the individual sediment grains together.
How Clastic Rocks Are Classified
Geologists classify clastic rocks based on their texture and the size of their constituent grains. The Udden-Wentworth scale provides the standard size ranges used for naming these rocks, which reflects the energy of the environment in which the sediment was deposited. The main size classes range from gravel (fragments larger than 2 millimeters) down to sand (0.0625 to 2 millimeters), and then to silt and clay (less than 0.0625 millimeters).
Beyond size, two other textural features provide context: sorting and rounding. Sorting describes the uniformity of the grain sizes within the rock; well-sorted rocks suggest prolonged transport by a consistent energy source. Rounding refers to the shape of the fragments, where well-rounded grains indicate extensive travel and abrasion, while angular fragments suggest the material was deposited close to its source. Analyzing these textural characteristics allows scientists to reconstruct the ancient geographic and environmental conditions under which the rock was formed.
Common Types and Real-World Examples
The three main families of clastic rocks are grouped by their dominant grain size. Conglomerates and breccias are the coarsest group, composed of gravel-sized fragments. Conglomerates contain rounded clasts, often forming in high-energy river channels where the fragments are tumbled and smoothed. Breccias, in contrast, contain angular fragments and typically form close to the source area, such as at the base of a steep slope or an alluvial fan.
Sandstone is the medium-grained group, composed of sand-sized particles, most commonly quartz. This rock type is common and can form in diverse environments, including coastal beaches, deserts, and riverbeds. For instance, the striking rock formations of the American Southwest are often ancient sandstones deposited in vast desert environments.
The finest-grained clastic rocks are the mudrocks, which include siltstone and shale. Siltstone is composed predominantly of silt, while shale is a very fine-grained rock made mostly of clay minerals that readily splits into thin layers. Mudrocks represent deposition in very low-energy settings, such as deep ocean floors, quiet lake bottoms, or offshore mud flats.