Just like when two people meet or two football teams meet, almost anything can happen.
Almost. There are a few typical occurrences depending on how the plates meet and what they are made of.
The material of Earth’s crust is recycled over and over in a process that takes millions of years. At the midocean ridges, new crust is formed when magma wells up from beneath the ocean floor.
Crust material gets drawn down into the mantle in ocean trenches, melts, and eventually comes to the surface again in volcanoes. Both ridges and trenches mark the edges of tectonic plates.