Accretion is a term used to refer to a situation in which something is growing in size. The term appears in a number of different contexts, from finances to astrophysics. Accretion process are constantly ongoing in a variety of locations around the Earth and the universe. Since most of the settings in which this term is used are in the sciences, this article focuses on accretion in areas like astrophysics, meteorology, and geology.
In the sciences, this process occurs as smaller pieces of material stick to larger pieces of material, gradually allowing the larger piece to grow in size. One example of this type of accretion occurs at some tectonic plate boundaries, as material is transferred from one plate to another. Another occurs with landmasses which grow as a result of the deposition of sediment. Islands, for example, can grow as sediment is deposited on one shore, and beaches often experience accretion during stormy weather as sediment is moved and redeposited.
In the atmosphere, accretion can be involved in some weather processes. For example, hail forms as small seedling ice crystals grow larger as a result of accretion. The crystals attract other crystals, slowly growing until they precipitate out of the clouds and hit the Earth. It is also possible for ice to form around particles of dust and other materials, in another example of an accretion process.
Accretion also occurs in space. Many celestial bodies have attained their size as a result of this process, with gravity in the early stages of their formation attracting particles and other materials from the surrounding area to allow the object to grow. In binary star systems, a phenomenon called an accretion disk is often seen, with a denser star pulling material off another star, causing a halo of material to settle around the denser star. If the conditions are right, the material will be attracted to the surface of the denser star to fuse with it, making it larger.
All of these processes allow objects to grow and shrink in size by moving smaller materials around. The Earth and the universe itself are subject to a number of dynamic processes which ensure that few things remain the same for very long. These processes are involved in everything from the formation of new stars to stormy weather on Earth, constantly repurposing materials which have been used in a variety of ways throughout the history of the universe.