Question

why Earth is round in shape

Answer 1

The mass of the Earth is big enough that the gravitational force it creates can pull the hard shape of ice, rock and metal into a sphere. 

Answer 2

We already know that Earth is round. But why is it round? And is it perfectly round like a rubber ball? Let's see what science can tell us about the answers to those questions.

Two features of Earth that determine its shape are mass and gravity. These features are, in fact, interrelated. Mass attracts other mass, resulting in the force we call gravity. Smaller objects, such as a car or a house, have tiny gravitational forces. The amount of mass throughout Earth, however, is huge and really adds up.

Scientists theorize that about 4.6 billion years ago, the solar system was a cloud of dust and gas. As gravity pulled the matter in on itself, it began to spin and clump up, forming the sun, planets, moons, and other space objects. Earth's rocky core was the first part of our planet to form, with dense matter sinking to the center and binding together. Then lighter pieces gathered and formed Earth's crust.