Definition

Noun: 1. (Geology) A constituent fragment of a clastic rock: A "clast" is an individual grain or fragment of rock, mineral, or fossil material that has been broken down, transported, and then cemented or compacted together with other such fragments to form a sedimentary rock. Clasts are the primary components of clastic rocks.

Usage

The word "clast" is a technical term used almost exclusively in geology and earth sciences. It refers to the discrete pieces within a rock, and its properties (like size, shape, and composition) are key to classifying the rock and understanding its history.

Examples

• Noun:
  • The sandstone is composed of well-rounded quartz clasts. (The sandstone is made of individual, smooth quartz fragments.)
  • Geologists study the size distribution of clasts to determine the energy of the ancient river that deposited them. (Scientists analyze the range of fragment sizes to understand how fast the prehistoric river was flowing.)
  • A conglomerate is a rock containing large, rounded clasts. (A conglomerate is a type of rock that includes big, smooth fragments.)

Advanced Usage

• Clast-supported: A descriptive term for a rock where the clasts are in direct contact with each other, and the finer-grained matrix fills the spaces between them. This indicates a high-energy depositional environment.
  • The breccia has a clast-supported fabric.
• Matrix-supported: The opposite of clast-supported, where the finer-grained matrix surrounds and separates the clasts, indicating a lower-energy environment.

Variants and Related Words

• Clastic (adj): Describes a rock or sediment composed primarily of clasts. It is the adjective form.
  • Sandstone and shale are common clastic rocks.
• Intraclast (n): A clast derived from erosion within the same depositional basin, often a fragment of partially lithified sediment.
• Extraclast (n): A clast derived from outside the immediate depositional basin.

Synonyms

• Fragment: A general term for a broken piece.
• Grain: Often used for smaller clasts, especially in sandstones.
• Pebble, Cobble, Boulder: Specific size-based terms for larger clasts.

Related Terms (Not Synonyms)

• Matrix: The finer-grained material that surrounds and cements the clasts together.
• Cement: The mineral material (e.g., calcite, silica) that binds clasts together.
• Sediment: Unconsolidated particles, which become clasts when lithified into rock.

Noun

1. (geology) a constituent fragment of a clastic rock