Here's why:
* Formation: Coal forms from the accumulation and compaction of plant matter (mainly trees) in swampy environments. Over millions of years, this organic material undergoes a process called coalification.
* Coalification: This process involves burial, pressure, and heat, which transform the plant material into various types of coal (peat, lignite, bituminous, and anthracite). These transformations are diagenesis processes, not metamorphism.
* Metamorphism: Metamorphism involves the transformation of existing rocks (igneous, sedimentary, or even other metamorphic rocks) due to intense heat and pressure, usually deep within the Earth's crust. This leads to changes in the rock's mineral composition, texture, and sometimes even chemical composition.
While coal formation involves heat and pressure, it doesn't meet the criteria for metamorphism:
* Lack of recrystallization: Coalification primarily involves chemical changes and the breaking down of organic matter, not the formation of new minerals through recrystallization.
* Low temperatures and pressures: The temperatures and pressures involved in coalification are relatively low compared to those required for metamorphism.
Therefore, black coal is classified as a sedimentary rock because it forms from the accumulation and transformation of organic matter, a process that doesn't involve the intense heat and pressure characteristic of metamorphism.