Coal is classified as an organic sedimentary rock because it is primarily composed of the remains of plants that have undergone a series of physical and chemical changes over millions of years. These plant materials accumulate in swamps and wetlands, where conditions are favorable for preservation rather than decay.
As layers of plant debris build up, they are buried by sediments, which creates pressure and temperatures conducive to the formation of coal. This process transforms the organic material through compaction and chemical alteration, eventually resulting in different types of coal such as lignite, bituminous, and anthracite.
The key aspect that qualifies coal as an organic sedimentary rock is its origin from biological material, unlike other sedimentary rocks that may be formed from mineral fragments or chemical precipitation. Therefore, coal’s unique formation process highlights its classification within the geological category of organic sedimentary rocks.