Dead remains of plants and animals are buried under the soil surface and are decomposed by micro-organisms.
Due to intense heat and pressure under the soil surface, these dead remains take millions of years to change into the soil.
Coal is made up of carbon, hydrogen, nitrogen, oxygen, sulphur, moisture and ash (incombustible mineral matter).
The process by which coal is formed is known as carbonation.
Final Answer-
Through the process of carbonation, dead remains take millions of years to change into coal.