Geology Word of the Week: A is for Accretionary Wedge

Illustration of a convergent plate boundary. I've added a red arrow pointing out the
location of the accretionary wedge. Illustration from TASA graphics and taken from here.
Click to view larger.

def. Accretionary Wedge (aka Accretionary Prism, Subduction Complex):
A wedge- or prism-shaped mass of sediments and rock fragments which has accumulated where a downgoing oceanic plate meets an overriding plate (either oceanic or continental) at a subduction zone. The sediment is generally marine sediment that has been scraped off of the downgoing plate by the overriding plate. However, sediment from the overriding plate can also contribute to the accretionary wedge. Fragments of rock from the colliding tectonic plates can also accumulate in an accretionary wedge.  The sedimentary rocks which form at accretionary wedges are deformed, faulted, poorly-sorted mixtures which are often referred to as "melange" (which means "mixture" in French).