Accretion (geology)

Accretion is a process by which material is added to a tectonic plate. This material may be sediment, volcanic arcs, seamounts or other igneous features. When two tectonic plates collide, one of the plates may slide under the other. This process is called 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.

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 is present 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 [http://pubs.usgs.gov/pdf/planet.html]


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Accretion — may refer to:*Accretion (finance), predictable changes in the price of certain securitiesAccretion in scienceIn science, accretion is a process in which the size of something gradually increases by steady addition of smaller parts. This term is… …   Wikipedia

  • Geology of Mars — Mars   Mars as seen by the Hubble Space Telescope Designations …   Wikipedia

  • Geology of solar terrestrial planets — The geology of solar terrestrial planet mainly deals with the geological aspects of four planets of the Solar system namely, Mercury, Venus, Earth and Mars and one terrestrial dwarf planet, Ceres. Objects like Pluto are similar to terrestrial… …   Wikipedia

  • Geology of Australia — Australia is a continent situated on the Indo Australian Plate.The geology of Australia includes virtually all known rock types and from all geological time periods spanning over 3.8 billion years of the Earth s history. ComponentsAustralia s… …   Wikipedia

  • Geology of the Moon — The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to lunar science ) is quite different from that of the Earth. The Moon lacks a significant atmosphere and any bodies of water, which… …   Wikipedia

  • geology — /jee ol euh jee/, n., pl. geologies. 1. the science that deals with the dynamics and physical history of the earth, the rocks of which it is composed, and the physical, chemical, and biological changes that the earth has undergone or is… …   Universalium

  • Geology of the Death Valley area — The exposed geology of the Death Valley area presents a diverse and complex story that includes at least 23 formations of sedimentary units, two major gaps in the geologic record called unconformities, and at least one distinct set of related… …   Wikipedia

  • Geology of the Himalaya — [ Fig 1: The earth in the Early Permian. At that time, India is part of Gondwana and bordered to the north by the Cimmerian Superterrane. Paleogeographic reconstructions. By Dèzes (1999), based on Stampfli and Borel (2002) and Patriat and Achache …   Wikipedia

  • Geology of British Columbia — The geology of British Columbia is a function of its location on the leading edge of the North American continent. The mountainous physiography and the diversity of rock types and ages hint at the complex geology which is still undergoing… …   Wikipedia

  • Geothermal (geology) — In geology, geothermal refers to heat sources within the planet. Strictly speaking, geo thermal necessarily refers to the Earth but the concept may be applied to other planets. Geothermal is technically an adjective (e.g., geothermal energy ) but …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.