The shapes and sizes of Earth's continents and ocean basins are in a constant state of slow, geological flux, driven by the movement of the planet's lithospheric plates. This process, known as plate tectonics, involves the interaction of these crustal masses at their boundaries, leading to the creation, destruction, and reshaping of the Earth's surface.

1. Divergent Boundaries: At these zones, tectonic plates pull away from each other. As they separate, molten magma from the mantle rises to the surface, cools, and solidifies, forming new oceanic crust. This process, called seafloor spreading, continuously adds new material to the ocean floor, causing the entire ocean basin to widen. The Mid-Atlantic Ridge is a prime example, where the separation of the Eurasian and North American plates gradually increases the size of the Atlantic Ocean, pushing the continents of Europe and North America further apart. This is the same mechanism that originally rifted the supercontinent Pangaea into the continents we see today.
2. Convergent Boundaries: Here, plates collide. The consequences depend on the types of plates involved. When a dense oceanic plate collides with a lighter continental plate, it subducts, or dives beneath it, creating deep ocean trenches and consuming the oceanic crust. This process shrinks the ocean basin. When two continental plates collide, neither can subduct, so the crust is compressed, folded, and thrust upward, forming massive mountain ranges like the Himalayas. This collision dramatically alters the shape and size of the continents involved.
3. Transform Boundaries: At these boundaries, plates slide horizontally past one another. This motion doesn’t create or destroy crust but can cause significant changes in the shape of landmasses by offsetting geological features over millions of years.

Through these continuous interactions, continents drift, oceans expand and shrink, and the world map is perpetually redrawn.