What type of plate boundary occurs when tectonic plates move apart?

A plate boundary where tectonic plates move apart is called a divergent boundary.

As the plates separate, hot mantle material rises beneath the opening and may partially melt. Magma solidifies to form new crust, making divergent boundaries constructive or extensional boundaries. Most active examples occur beneath oceans, where they build long mid-ocean ridges.

The Mid-Atlantic Ridge is a classic example: it separates the North American and Eurasian plates in the north and the South American and African plates farther south. Iceland lies across part of this ridge, allowing visitors to see and study a divergent boundary above sea level.

Divergence can also begin within a continent. The crust stretches, fractures, and forms a rift valley; if extension continues, the continent may split and a new ocean basin can develop. This differs from a convergent boundary, where plates collide, and a transform boundary, where they slide laterally past one another. A subduction zone is a feature commonly associated with convergence, not separation.