What does clastic mean?
Clastic is an adjective used chiefly in geology to describe rocks and sediments composed of fragments — called clasts — of pre-existing rock, minerals, or organic debris that were broken apart, transported by wind, water, or ice, and then deposited and cemented into new rock. Sandstone, shale, and conglomerate are classic examples: each is essentially an aggregate of worn-down pieces bound together in a finer matrix. The term derives from the Greek "klastos," meaning broken, and entered English through scientific usage. Clastic is contrasted with non-clastic rocks, which form from chemical precipitation or biological accumulation rather than accumulated fragments. Because it belongs mainly to the specialized vocabulary of sedimentology, it appears rarely outside textbooks, field guides, and academic writing, where it serves as a precise descriptor of both composition and texture.
adjectiveComposed of, or relating to, fragments of pre-existing rock or organic material that have been mechanically transported and deposited. In geology, describing sedimentary rocks made of broken pieces of older rock cemented together.
- (Geology) Composed of or consisting of fragments (clasts) of pre-existing rock or minerals, as in sandstone, shale, or conglomerate.
- (More broadly) Relating to fragmentation, or composed of detached pieces or particles of material.
"The canyon walls are built of clastic layers — sandstone and shale pressed from ancient river sediments."
"Sandstone is a classic clastic rock, formed when wind- or water-borne grains of quartz were compacted and cemented over millions of years."
"The geologist noted the coarse clastic texture of the conglomerate, its pebbles still clearly visible in the matrix."
Every sandstone cliff you've ever admired is clastic: a concrete-like glue job holding together the shattered remains of mountains that eroded millions of years ago.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of clastic
Clastic comes from the Greek "klastos" (broken), derived from the verb "klan" (to break), combined with the adjectival suffix "-ic." The word was adopted into English scientific terminology during the nineteenth century as geologists formalized the classification of sedimentary rocks. It shares its Greek root with "iconoclast" (one who breaks images), and with the noun "clast," which names the individual fragment itself.
Related word forms
How clastic is actually used
A technical term used almost exclusively in geology, sedimentology, and related earth sciences; rare in everyday speech. Often contrasted with "non-clastic" or chemical/organic sedimentary rocks such as limestone formed by precipitation.
Easily confused with clastic
"Classic" means of the highest quality or a standard example, while "clastic" refers to rocks or materials composed of fragments.