a. The earth was in a volatile stage.

b. With the increase in density, the temperature also increased.

c. The material inside the earth started separating on the basis of its density.

d. The materials with a heavy density such as iron sunk towards the center of the earth and the materials to work with lower density begin to rise above.

e. As time passed, it cooled, solidified and condensed into a smaller size. This formed the crust.

f. With the process of differentiation, the material separated into different layers on the basis of their density.

g. Starting from the inside, the four layers of the earth aa our inner core, outer core, mantle, and crust.