1. Process by which an inorganic body grows in size by the addition of new particles to its exterior. It is the mechanism by which primitive planetary bodies are believed to form as a result of the accumulation of minute, cold, homogeneous particles (homogeneous accretion). An alternative hypothesis is that iron-rich cores accumulated first and were later surrounded by silicate material (heterogeneous accretion). Homogeneous accretion yields a planet that initially has the same composition from centre to surface; heterogeneous accretion yields a planet that has a layered structure from the start.
2. The accumulation of sediments from any cause, representing an excess of deposition over erosion.
3. The addition of continental material to a pre-existing continent, usually at its edge. The use of ‘accretion’ in this sense has evolved from theories of nucleation to newer theories of the horizontal addition of allochthonous terranes of initially coherent bodies of continental rock, usually more than 100 km2 in area, which can collide, rotate, and fragment as they become sutured to a continent.