Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features.

There are two types of geologic accretion. The first kind of accretion, plate accretion, 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 plate. Sediment on the ocean floor will often be scraped by the subducted 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 subducting plate (the top 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 form of accretion is landmass accretion. This 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.

Plate accretion

Evidence

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 suggests that the rock between the mountain range and the plate boundary is part of an accretionary wedge.

Examples

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.

Resources

• 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 [290313]