being buried under soil and mud changes dead vegetation / dead plant matter into peat through extreme heat and pressure in a swamp over a long period of time & being buried under soil and mud changes peat into coal through extreme heat and pressure in a swamp over a long period of time ->  being buried under soil and mud changes dead vegetation into coal through extreme heat and pressure in a swamp over a long period of time  being buried under soil and mud changes dead vegetation into coal through extreme heat and pressure in a swamp over a long period of time & if something changes into something else then that something else is formed from that something ->  coal is formed from dead vegetation  coal is formed from dead vegetation & plants are a kind of vegetation -> coal is formed from dead plants