accretions

Accretions refer to the growth or enlargement of an object or system through the gradual accumulation of material from its surroundings. In general use, accretion implies growth by external addition, as opposed to internal expansion or division. The term is widely used in geology, astronomy, and atmospheric science, among other fields.

In astronomy and planetary science, accretion describes how small particles collide and stick together to form larger bodies. Dust and ice in a protoplanetary disk can coagulate into planetesimals and, over time, into planets. Gas and dust may also accrete onto preexisting stars or compact objects such as white dwarfs, neutron stars, or black holes, releasing energy that can be observed as infrared or X-ray emission. Accretion disks form around young stars and compact objects when material spirals inward under gravity.

In geology, accretion refers to the addition of material to a continent or crust, often at convergent plate boundaries. Sediments scraped from subducting slabs accumulate to form accretionary wedges, and buoyant terranes can accrete to continental margins, enlarging landmasses over geological timescales.

In atmospheric and planetary science, accretion also describes growth by collection of matter from the surroundings, such as ice accreting onto particles in clouds or droplets growing by collision and coalescence.

Accretion processes are driven by gravity, electromagnetism, and hydrodynamic forces, and occur across a wide range of scales and timescales. The specifics—rate, mechanism, and resulting structure—depend on the environment and the material involved.