User:BZenith/sandbox

Purpose of this page
I am editing the Accretion (geology) page to be more accurate (as it includes misleading/incorrect terminology) and to have better citations. I am also adding a section with examples, although this is covered more thoroughly on some of the pages of linked topics.

Accretion, in geology, is a process by which material is added to a tectonic plate at a subduction zone, frequently on the edge of existing continental landmasses. This material may be sediment, volcanic arcs, seamounts, oceanic crust or other igneous features.

Description
Accretion involves the addition of material to a tectonic plate via subduction, the process by which one plate is forced under the other when two plates collide. The plate which is being forced down, the subducted plate, is pushed against the upper, over-riding plate. Sediment on the ocean floor of the subjecting plate will often be scraped off as the plate descends. This causes the sediment to accumulate as a mass of material called an accretionary wedge (accretionary prism), which is pushed against and attaches to the upper plate. In addition to accumulated ocean sediments, volcanic island arcs or seamounts present on the subducting plate may be almalgated onto existing continental crust on the on the upper plate, increasing the continental landmass.

Continent Building
Continental crust differs significantly from oceanic crust. Oceanic crust is primarily composed of basaltic rocks which have a higher density than the rocks making up the majority of continental crust. Island arcs and other volcanic rocks are also lower in density than the oceanic crust, and are therefor not easily subducted along with the oceanic crust that surrounds them. Instead, these less-dense bits of crust will collide with existing continental crust on the upper plate once the oceanic crust separating them is completely subducted. Most continents are composed of multiple accreted "terranes," pieces of low-density continental crust with different origins. For example, western North America is made up of multiple accreted terranes, some that came together in the Proterozoic and others that collided later. The Nankai accretionary complex of Japan, the Barbados Ridge in the Caribbean, and the Mediterranean Ridge are specific examples of accreted terranes including accretionary wedges.

Resources

 * Robert, Ballard D. Exploring Our Living Planet. Washington D.C.: The National Geographic Society, 1983.
 * Watson, John. "This Dynamic Planet." US Geological Survey. 6 December 2004