A clast is a fragment of rock or mineral that exists within a larger rock mass or as part of unconsolidated sediment. These fragments, which can range in size from microscopic clay particles to house-sized boulders, are the building blocks of a major category of rocks known as clastic sedimentary rocks. Understanding clasts is fundamental to geology because they provide evidence of the breakdown and recycling of Earth’s crustal materials.
How Clasts Form and Travel
The creation of a clast begins with the breakdown of a pre-existing source rock through the processes of weathering and erosion. Weathering involves both physical mechanisms, like the repeated freezing and thawing of water that cracks rock, and chemical mechanisms, such as the dissolution of minerals by slightly acidic rainwater. This action produces loose fragments, or sediment, ready for transport.
Erosion is the mechanical process that removes and carries these clasts away from their source location. Transport is accomplished by dynamic agents like flowing water in rivers, wind across arid landscapes, moving sheets of glacial ice, or gravity acting on steep slopes. The speed and energy of the transporting medium determines the size and quantity of material carried; for instance, fast-flowing rivers can carry large cobbles, while slow wind can only suspend fine silt and clay.
As clasts are transported, they are continually modified through abrasion, which is the physical scraping and bumping against other particles and the surrounding environment. This process shapes the fragments and often separates them based on their size and density, eventually depositing them when the energy of the transporting agent decreases.
Categorizing Clasts by Size
Geologists rely on a standardized measurement scale to classify clasts purely by their diameter, as size is the primary parameter for naming the resulting rock. The coarsest category is gravel, which encompasses all clasts greater than 2 millimeters (mm) in diameter, and includes pebbles, cobbles, and boulders. Boulders are the largest fragments, exceeding 256 mm, while pebbles fall within the 2 mm to 64 mm range.
The next major size class is sand, defined as particles ranging from 0.0625 mm up to 2 mm in diameter. Below the range of visible particles is silt, with diameters between 0.004 mm and 0.0625 mm. The smallest clasts are classified as clay, which includes any particle smaller than 0.004 mm.
These extremely fine particles are primarily composed of clay minerals, whereas sand-sized particles are often single mineral crystals like quartz, and larger gravel clasts are typically fragments of the original rock.
Reading the History in Clast Properties
Beyond simple size, the shape and uniformity of clasts offer insights into their history of transport and deposition. One property is rounding, which describes the degree of smoothing of a clast’s edges and corners due to abrasion during transport.
Rounding
A clast that is angular retains sharp, jagged edges, indicating minimal transport and deposition close to its source, such as in a landslide or talus slope. Conversely, a well-rounded clast has had its edges completely smoothed, which implies a long duration of transport or a highly energetic environment like a fast-flowing river or a beach with strong wave action. The rate of rounding is also influenced by the clast’s hardness, as softer materials will round more quickly than resistant minerals like quartz.
Sorting
Another revealing property is sorting, which refers to the range of clast sizes present within a sediment deposit. Well-sorted sediment contains clasts that are all roughly the same size, such as fine sand found on a dune or beach, suggesting a selective transport process over a long distance. Poorly-sorted sediment contains a wide mixture of sizes, from clay to boulders, often indicating rapid deposition over a short distance, such as in a glacial till deposit or a debris flow.
Composition (Provenance)
The composition of the clasts, or their provenance, determines the original source rock from which the sediment was derived. By identifying the specific minerals or rock fragments present, geologists can trace the material back to a particular geological region or mountain range.
Rocks Built from Clasts
Clastic sedimentary rocks are the final, lithified products of these fragments, formed when the loose sediment is buried, compacted, and cemented together. Rocks formed from the coarsest clasts, those larger than 2 mm, are called either conglomerate or breccia.
Conglomerate is defined by having well-rounded clasts, which suggests the material was transported a significant distance and deposited in a high-energy environment. In contrast, breccia is characterized by angular clasts, indicating the fragments were deposited very close to their source, often in environments like alluvial fans or fault zones.
Sedimentary rocks composed of sand-sized clasts are known as sandstone, a common rock type that forms in varied settings from deserts to riverbeds. When the primary clasts are silt-sized or clay-sized, the resulting fine-grained rocks are collectively known as mudrocks, which include siltstone and shale or mudstone. These fine-grained rocks form in very low-energy, quiet water settings like deep ocean floors or lake beds, where the smallest particles can settle out of suspension.