Question

why formation of coal is a slow change​

Answer 1

million years ago,dead plants and animals get burried under the earth,and due to high pressure and temperature, they turn into coal , petroleum etc.....

Answer 2

Due to high pressure and temperature inside the earth, and in the absence of air, the woods of buried trees was slowly converted into soil . The slow process by which the dead plants buried deep under the earth have become coal is called carbonisation.