is a process by which material is added to a tectonic plate
or a landmass
. This material may be sediment, volcanic arcs
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 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 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.
are formed of rocks that are very noticeably different from the rocks that form the ocean floor.... Read More