There are two types of geologic accretion—plate accretion and landmass accretion.
The first involves the addition of material to a tectonic plate. When two tectonic plates collide, one of the plates may slide under the other, a process known as subduction. The plate which is being subducted (the plate going under), is floating on the asthenosphere and is pushed up and against the other, subducting plate. Sediment on the ocean floor will often be scraped by the subducting plate. This scraping causes the sediment to come off the subducted plate and form a mass of material called the accretionary wedge, which attaches itself to the upper, subducting plate. Volcanic island arcs or seamounts may collide with the continent, and as they are of relatively light material (i.e. low density) they will often not be subducted, but are thrust into the side of the continent, thereby adding to it.
The second involves the addition of sediment to a coastline or riverbank, increasing land area. The most noteworthy landmass accretion is the deposition of alluvium, often containing precious metals, on riverbanks and in river deltas.
Continental plates are formed of rocks that are very noticeably different from the rocks that form the ocean floor. The ocean floor, is usually composed of basaltic rocks that make the ocean floor denser than continental plates. In places where plate accretion has occurred, land masses may contain the dense, basaltic rocks that are usually indicative of oceanic lithosphere. In addition, a mountain range that is distant from a plate boundary but parallel to it suggests that the rock between the mountain range and the plate boundary is part of an accretionary wedge.
This process occurs in many places, but especially around the Pacific Rim, including the western coast of North America, the eastern coast of Australia, and New Zealand. New Zealand consists of areas of accreted rocks which were added on to the Gondwana continental margin over a period of many millions of years. The western coast of North America is made of accreted island arcs. The accreted area stretches from the Rocky Mountains to the Pacific coast. The island of Barbados is a similar process being actively formed in the Atlantic Ocean. The Gonâve Microplate in the Caribbean is expected to eventually become accreted to the North American Plate.
- Robert, Ballard D. Exploring Our Living Planet. Washington D.C.: The National Geographic Society, 1983.
- Sattler, Helen Roney. Our Patchwork Planet. New York: Lee & Shepard, 1995.
- Watson, John. "This Dynamic Planet." US Geological Survey. 6 December 2004