Accretion is the process in which material accumulates or grows gradually over time by the addition of particles, substances, or objects. This term is used to describe various phenomena in different scientific contexts, including geology, astronomy, and planetary science.
- Continental Accretion: This process occurs when tectonic plates collide, gradually adding landmasses or continental fragments to existing continents. For example, the Himalayas formed due to the collision between the Indian and Eurasian plates.
- Accretionary Wedges: In subduction zones, where one tectonic plate is being pushed beneath another, sediments and oceanic material can accumulate in a wedge-shaped formation called an accretionary wedge.
In astronomy, accretion refers to the accumulation of matter onto celestial objects, often involving the formation of protostars, planets, and galaxies.
- Star Formation: In the star formation process, a molecular cloud collapses under its own gravity, accumulating material at the center. As the material accumulates, it forms a protostar, eventually becoming a full-fledged star.
- Planet Formation: In the early stages of planetary formation, small particles and dust in a protoplanetary disk accrete onto each other, gradually building up larger objects known as planetesimals. These planetesimals then collide and merge to form planets.
- Accretion Disks: Accretion disks are flattened gas and dust structures surrounding certain astronomical objects, such as young stars or black holes. Material within these disks spirals inward due to gravitational forces, leading to accretion onto the central object.
- Galactic Accretion: In galactic evolution, smaller galaxies can be gravitationally captured by larger galaxies, leading to the accretion of stars, gas, and dust.