system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the Earth's crust is divided into tectonic plates, all bordering the adjacent plates, how is it that continents manage to move away/towards each other?

Are there gaps between the plates that allow this constant movement?
gpt
At the scales we're talking about - very long timeframes, very large forces - even rock is somewhat "squishy". The plates compress and stretch as the forces below them push them against one another or pull them apart, so the horizontal size of the plates does not have to remain exactly the same over time.

In some cases, the compression is enough to create tensions that overwhelm the mechanical strength of stone even in the short term, before the rock can "flow" under pressure (which it will, given enough time). In those cases, the stone cracks and moves in different directions on either side of the crack - a "fault". The largest faults are the divisons between the plates themselves (e.g. the famous San Andreas fault, which divides the Pacific plate from the North American plate), while smaller faults occur within plates all the way down to the scale of individual stones you could examine in your hand. These faults can force pieces of land upward, allowing a plate to shrink horizontally while rising vertically.

In other cases, one plate is heavier than another, and the heavier plate (typically an oceanic plate) sinks below the lighter one (typically a continental plate). The sinking plate sinks down into the mantle of the Earth and melts, which replenishes the mantle material that erupts through volcanoes at the surface.

In general, the idea of plates as rigid objects is a reasonable way to think for the purposes of everyday understanding, but plates are neither completely well-defined things nor completely rigid nor permanent over geological timescales. They have substructure (the major plates tend to contain smaller microplates, especially around their edges), they can warp (especially when compressed as in mountainbuilding), and they can be created or destroyed (e.g. when an existing plate splits or is pushed down into the mantle).