Answer:
The crust broke up because of the convection currents, formed tectonic plates, and is slowly pushed on the surface until it collides with another plate and subducts.
Explanation:
This image gives us the basics of how the plate tectonics theory works. The material in the upper mantle is cooler and denser than the material in the lower mantle, so it drops down and it pushes the hotter and less dense material up. This creates a circular motion in the mantle, and this motion creates enormous pressure on the crust above it and breaks it up.
As the crust is broken up, magma is rising constantly for millions of years, so the new magma pushes the old solidified one further away. This also pushes two pieces of crust away from each other. As the crusts move away, they eventually collide with another plate, and either gradually merge with it, or a subduction zone is created. In the latter, the crust moves below another crust and into the mantle, where it gets melted and recycled.
