<P> The inner trench wall marks the edge of the overriding plate and the outermost forearc . The forearc consists of igneous and metamorphic crust, and this crust may act as buttress to a growing accretionary wedge (formed from sediments scraped off the top of the downgoing plate). If the flux of sediments is high, material transfers from the subducting plate to the overriding plate . In this case an accretionary prism grows and the location of the trench migrates progressively away from the volcanic arc over the life of the convergent margin . Convergent margins with growing accretionary prisms are called accretionary margins and make up nearly half of all convergent margins . If the incoming sediment flux is low, material is scraped from the overriding plate by the subducting plate in a process called subduction erosion . This material is then carried down into the subduction zone . In this case, the location of the trench migrates towards the magmatic arc over the life of the convergent margin . Convergent margins experiencing subduction erosion are called non-accretionary or erosive margins and comprise more than half of convergent plate boundaries . This is an oversimplification, because the same section of margin may experience both sediment accretion and subduction erosion throughout its active time span . </P> <P> The asymmetric profile across a trench reflects fundamental differences in materials and tectonic evolution . The outer trench wall and outer swell comprise seafloor that takes several million years to move from where subduction - related deformation begins to sinking beneath the overriding plate . In contrast, the inner trench wall is deformed by plate interactions for the entire life of the convergent margin . The forearc is continuously subjected to subduction - related deformation and earthquakes . This protracted deformation and shaking ensures that the inner trench slope is controlled by the angle of repose of whatever material comprises it . Because the inner slope of non-accretionary trenches are composed of igneous and metamorphic rocks instead of deformed sediments, these trenches have steeper inner walls than accretionary trenches . </P> <P> The composition of the inner trench slope and a first - order control on trench morphology is determined by sediment supply . Active accretionary prisms are common in trenches near continents where rivers or glaciers supply great volumes of sediment to the trench . These filled trenches may lack the bathymetric expression of a trench . The Cascadia margin of the northwest USA is a filled trench, the result of sedimentation by the rivers of the western United States and Canada . </P> <P> The Lesser Antilles convergent margin demonstrates the importance of proximity to sediment sources for trench morphology . In the south, near the mouth of the Orinoco River, there is no morphological trench and the forearc (including the accretionary prism) is almost 500 km (310 mi) wide . The large accretionary prism reaches above sea level to form the islands of Barbados and Trinidad . Northward, the forearc narrows, the accretionary prism disappears, and, north of ~ 17 ° N, the morphology of a trench dominates . Further north, far from major sediment sources, the Puerto Rico Trench is over 8,600 m (28,200 ft) deep and there is no active accretionary prism . </P>

What is a trench and how does it form