Modern theory believes the continents all started out lumped together in a single, super-continent called Pangaea, where the rest of the world was ocean. Given the analysis of the crust, we find this is incorrect. Indeed, there was at one time, a super-continent of Pangaea, but the Earth was only a fraction of its current size—the whole of the land mass was Pangaea comprising the entire surface of the planet. The oceans had not yet formed.
The expansion due to the cooling of the core cracked Pangaea into a number of large pieces, with magma breaking through those cracks to fill in the gaps. A core flare occurred (the planetary core equivalent of a nova flare of a white dwarf star—same cause and reason). The eruption of the explosive gases pushed the Americas apart from Eurasia, along what is now the mid-Atlantic fault. This formed the first ocean bed—the Atlantic.
Water eventually filled in the basin, and formed the Atlantic Ocean.
The next core flare occurred in the Pacific basin, pushing Russia apart from North America. Leaving two super-continents, North America/South America/Antarctica (then attached to the west coast of the Americas), and Europe/Africa/Asia/Australia. This created a great number of weak fracture areas in the Pacific basin, which continue to exhibit the bulk of the expansion of the Earth.
A later core flare separated Antarctica from the Americas, rolling it off South America to its more southerly position, eventually disconnecting it from the continents, altogether.
If you look at a topography map of the ocean floor, the stretch lines are obvious. Continents are not sliding towards or away from each other vectorially, they are all sliding away from each other, in a scalar fashion—because the Earth is expanding. Oceans will grow wider. Other fractures occur as the surface area of the Earth increases, breaking up the large continents into smaller ones. Eventually, the Earth will be a large, ocean world with many large islands, and no major continents.