Question

What is the cause of the bulging of the earth at the equator?

Answer 1

I think, think of earth as a giant magnet, you know that.
Now magnets' magnetic field have a property which causes the magnetic field lines to contract from north to south pole and expand side ways, and as the magnetic force is strong enough than gravity, so it causes the earth to bulge at equator. The image attached explains this.
Other reason may be due to collisions of earth with the asteroids as large as moon.

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