World Library  
Flag as Inappropriate
Email this Article

Accretion (geology)

Article Id: WHEBN0005183852
Reproduction Date:

Title: Accretion (geology)  
Author: World Heritage Encyclopedia
Language: English
Subject: East African Orogeny, Main Uralian Fault, Sonoma orogeny, East European Platform, East European Craton
Publisher: World Heritage Encyclopedia

Accretion (geology)

Oceanic-continental convergence: The required conditions for plate accretion

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, 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 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


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.


  • 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 [1]
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.