how did soft,spongy peat become hard,black coal?

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as more time passed,the land soak,and the peat was buried under many feet of mud or sand.the wieght of the mud and sand pressed some of the moisture out of the spongy mass.as the layers grew thicker and heavier,they pressed harder.this pressure,along with heat from within the earth,gradually turned the peat into a hard,black rock coal.