There are different ways for states to (attempt to) acquire new territory. Accretion is a mode of acquiring territory, whereby new land is created naturally, for example through volcanic eruptions, as in the case of the island of Surtsey, Iceland (1963). Such naturally formed land becomes part of the state in whose territory or territorial waters it arises.

By contrast, man-made additions – sometimes called artificial accretion – do not create territorial rights under international law; for instance, artificial islands constructed in the South China Sea cannot generate new sovereign territory.

Accretion is the opposite of erosion, where land gradually disappears due to natural forces. It must also be distinguished from avulsion, which involves sudden or violent geographical changes (e.g. through earthquakes). Importantly, only gradual changes due to accretion can shift international boundary lines between states.