Accretion (geology)

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Accretion (geology)

Accretion is a process by which material is added to a tectonic plate

Tectonic Plate

#REDIRECT Plate tectonics...
or a landmass

Landmass

A landmass is a large continuous area of landform. Although it may be most often written as one word to distinguish it from the usage 'land mass' to mean the measure of a land area, it is also used as two words....
. This material may be sediment, volcanic arc

Volcanic arc

A volcanic arc is a chain of volcanos or mountains formed by plate tectonics as an oceanic tectonic plate subduction under another tectonic plate and produces magma....
s, seamount

Seamount

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Oceanic Continental Convergence Fig21oceancont

Accretion is a process by which material is added to a tectonic plate

Tectonic Plate

#REDIRECT Plate tectonics...
or a landmass

Landmass

Volcanic arc

A volcanic arc is a chain of volcanos or mountains formed by plate tectonics as an oceanic tectonic plate subduction under another tectonic plate and produces magma....
s, seamount

Seamount

A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of 1,000?4,000 meters depth....
s 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

Subduction

In geology, subduction is the process that takes place at convergent boundary by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge....
. The plate which is being subducted (the plate going under), is floating on the asthenosphere

Asthenosphere

The asthenosphere is the mechanically weak ductily-deforming region of the upper Mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km below the surface, but perhaps extending as deep as 400 km ....
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

Accretionary wedge

An accretionary wedge or accretionary prism is formed from sediments that are Accretion onto the non-Subduction tectonic plate at a Convergent boundary....
, 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

Alluvium

Alluvium is soil or sediments deposited by a river or other running water. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel....
, often containing precious metals, on riverbanks and in river deltas.

Plate accretion

Evidence

Continental plates

Tectonic Plate

#REDIRECT Plate tectonics...
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

Lithosphere

File:Plates tect2 en.svgFile:Earth-crust-cutaway-english.svgThe lithosphere is the rigid outermost shell of a rocky planet....
. 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

North America

North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and almost totally in the western hemisphere....
, the eastern coast of Australia

Australia

Australia, officially the Commonwealth of Australia, is a country in the southern hemisphere comprising the Australia of the world's smallest continent, the major island of Tasmania, and numerous list of islands of Australia in the Indian Ocean and Pacific Oceans....
, and New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses , and numerous Islands of New Zealand, most notably Stewart Island/Rakiura and the Chatham Islands....
. New Zealand consists of areas of accreted rocks which were added on to the Gondwana continental margin

Continental margin

The continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....
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

Rocky Mountains

The Rocky Mountains, often called the Rockies, are a mountain range in western North America. The Rocky Mountains stretch more than 4,800 kilometre from the northernmost part of British Columbia, in Canada, to New Mexico, in the United States....
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