Clastic rocks are a major category of sedimentary rocks, characterized by their formation from fragments of pre-existing materials. The term “clastic” derives from the Greek word klastos, meaning “broken.” These rocks are composed of detritus—pieces of mineral and rock fragments called clasts—that have been physically broken down from older igneous, metamorphic, or sedimentary rocks. This physical origin sets them apart from rocks formed by chemical precipitation or biological activity.
The Step-by-Step Formation Process
Clastic rock formation begins with weathering, the mechanical and chemical breakdown of source rock exposed at the surface. Weathering physically shatters the bedrock, while chemical processes like dissolution alter mineral composition. The resulting fragments, ranging from clay particles to large boulders, become the raw material.
The loose sediment is then subjected to erosion and transport by agents such as running water, wind, or glacial ice. The energy of this transport process rounds the fragments and sorts them by size. Faster currents carry larger clasts, while slower water and wind transport only the finest grains.
Deposition occurs when the transporting medium loses energy, causing fragments to settle out of suspension. This settling happens in low-energy environments, such as lake beds, deep ocean basins, or river deltas, where the sediment accumulates in distinct layers. Burial follows, subjecting the lower layers to increasing pressure and temperature.
The final transformation to solid clastic rock is called lithification, involving two primary mechanisms: compaction and cementation. Compaction occurs as the weight of overlying material squeezes water out, reducing pore space between grains. Cementation follows when dissolved minerals, commonly silica or calcite, precipitate from groundwater, gluing the clasts together into a coherent rock mass.
Classification by Grain Size
The most fundamental method for classifying clastic rocks is based on the size of the individual clasts, which directly reflects the energy of the depositional environment. Geologists divide clastic sediments into three main size categories, corresponding to specific rock grades.
The coarsest material, including particles larger than two millimeters (pebbles, cobbles, boulders), is the rudite grade. Its presence indicates a high-energy environment, such as a fast-flowing river or a turbulent beach.
Medium-grained sediments, ranging from 1/16 millimeter up to two millimeters, fall into the arenite grade, corresponding to sand. This suggests deposition in moderately energetic settings like shallow marine environments, desert dunes, or river channels.
The finest clasts (silt and clay) are smaller than 1/16 millimeter and are classified as the lutite grade. These particles settle out only in very low-energy, quiet water conditions, such as deep-sea floors or lakes. Grain size provides a direct clue to the conditions and transport mechanism that formed the rock.
Common Examples of Clastic Rocks
The three most common clastic rock types correspond directly to the grain size classifications. Rocks of the rudite grade (gravel-sized clasts greater than two millimeters) are known as conglomerate or breccia. Conglomerate contains well-rounded clasts, typically due to significant water transport. Breccia is composed of angular, sharp-edged fragments, indicating deposition close to its source, such as a cliff base or fault zone.
Sandstone is the primary example of an arenite, forming from sand-sized grains, usually composed of durable quartz. Sandstone is a porous rock often associated with ancient beaches, river floodplains, and desert environments. Sand composition varies: quartz arenite is nearly pure quartz, while arkose contains feldspar grains, signifying rapid burial or short transport.
The finest clastic rocks, those of the lutite grade, are mudstone and shale, formed from compacted silt and clay particles. Shale is the most abundant sedimentary rock and is distinguished from mudstone by its fissility—the ability to split into thin, parallel layers. This layered structure results from platy clay mineral grains aligning during compaction. Shale and mudstone typically form in the quietest depositional settings, such as deep marine basins or large lakes.
How Clastic Rocks Differ from Other Sedimentary Types
Clastic rocks are separated from the other two major categories of sedimentary rocks—chemical and biochemical—by their formation. Clastic rocks are exclusively products of physical fragmentation and transport, where solid pieces are cemented together. Chemical sedimentary rocks, conversely, form when dissolved ions precipitate out of a solution to create a solid mineral mass.
A common example of chemical formation is evaporites, such as rock salt (halite) or gypsum, which crystallize as water dries up and concentrates the dissolved minerals. This inorganic process does not involve the physical breaking down of pre-existing rocks. The resulting texture is typically crystalline, not fragmental.
Biochemical sedimentary rocks involve the actions of living organisms to form sediment. Marine organisms, such as corals and shelled creatures, extract dissolved ions to build hard parts, primarily calcium carbonate. When these organisms die, their skeletal remains accumulate and become lithified, forming rocks like limestone. This organic accumulation process differs fundamentally from the mechanical fragmentation defining clastic rocks.