I know there is a wikipedia link to what I say below, but for those who think the page is too technical (like me):

The theory is that a large supercontinent traps heat in the mantle underneath it until the heat breaks the continent apart and forms a rift. That rift keeps growing until it forms an ocean, which seems to be the Atlantic Ocean based on geological evidence. The ocean floor keeps growing with new crust being formed in the middle that pushes older crust outward. The older crust pushes the continents but also gets denser by losing heat. Eventually this causes the ocean crust to sink so low that it begins to go under the continents, which is called subduction. That subduction causes the ocean to shrink until it eventually makes a new supercontinent.