Question

Why is earth's surface broken into plates?

Answer 1

Because large convection cells of hot magma in the mantel keep moving upwards toward the crust and thereby break it up.

Explanation:

Mantle plums of hot magma rise as they heat up and as they do they eventually cool and sink back down again producing a convection cell (a little like a pot of bubbling fudge on the stove). This upward motion encounters the crust and tends to pry it open at divergent plate boundaries - such as the one in the middle of the Atlantic Ocean.

See pic.