Why black coal is considered to be a metamorphic rock?

Answer 1

Coal is generally considered a sedimentary rock as it is made up of deposited material (in this case organic matter, mostly from plants) and is eventually covered up by overburden. I'm not going to go into the details of the physical change but the reason black coal might be considered metamorphic is that it has been subjected to more pressure over time which furthers the coalification process.

The order of coalification coal:

peat -> lignite (brown coal) ->sub bituminous -> bituminous ->anthracite (very rare)

everything after lignite is black in colour.

If you need this for a report I'd probably look it up though, its been a while since I've studied coal in depth.

Answer 2

Black coal is considered a metamorphic rock because it is formed from the compaction and heating of plant material over millions of years. This process transforms the original sedimentary material into a more dense and carbon-rich rock. The increased pressure and temperature causes the chemical and physical properties of the original material to change, leading to the formation of coal.