Subduction is the geological process where one tectonic plate slides beneath another into the Earth’s mantle at a convergent boundary. This downward movement is driven by the density difference between the plates, typically with the denser oceanic plate sinking under a lighter plate. Subduction erosion is a mechanism within this dynamic setting that causes the loss of material from the overriding (upper) plate. This process recycles crustal material deep into the planet’s interior.
Defining the Removal Process
Subduction erosion is the large-scale removal of rock and sediment from the lower margin of the upper tectonic plate where it meets the down-going slab. This occurs through two primary mechanical processes operating at the interface, resulting in a net loss of crustal volume over millions of years.
Basal Erosion
Basal erosion involves the mechanical scraping of material from the underside of the overriding plate’s crust. Frictional forces and immense pressures cause the leading edge of the subducting plate to remove rock from the base of the upper crust. This constant undercutting leads to gradual thinning of the overriding plate’s lower margin.
Frontal Erosion
Frontal erosion occurs at the outermost edge of the overriding plate, near the deep-sea trench. This involves the structural failure and collapse of the leading edge (forearc or accretionary wedge) into the trench. Large blocks of the upper plate are consumed and carried down into the mantle.
The removed material is transported into the mantle and incorporated into the deep subduction channel. High convergence rates and a thin sediment cover on the subducting plate often favor this process.
Erosion Compared to Accretion
Subduction erosion must be contrasted with the opposing process of subduction accretion. Accretion involves the addition of material to the overriding plate, causing the margin to grow outward and thicken. This occurs when sediments and fragments of oceanic crust are scraped off the subducting plate and pile up against the margin.
The scraped material accumulates to form a wedge-shaped body known as an accretionary prism. This prism causes the overriding plate’s margin to widen and thicken. The structural result of accretion is a net gain of material, leading to the progradation, or seaward growth, of the continental margin.
Erosion, in contrast, results in a net loss of material, causing the overriding plate margin to thin and narrow. While accretion builds the margin outward, erosion cuts it back. Most subduction zones exhibit a mix of both processes, but they are dominated by one or the other. Global surveys suggest that more than half of all modern subduction zones are primarily erosive.
Large Scale Tectonic Impacts
The continuous loss of material through subduction erosion has significant consequences for the geology and topography of the overriding plate. One effect is the migration of the subduction zone boundary, known as trench rollback. As the overriding plate loses material from its edge, the trench effectively moves backward toward the ocean.
This removal of rock also results in substantial crustal thinning along the continental margin. The systematic loss of forearc material reduces the overall thickness of the crust. This can lead to regional subsidence, or sinking, of the land surface, evidenced by coastal regions experiencing long-term submergence.
Subduction erosion also influences arc volcanism that occurs inland from the trench. The removal of sediment and hydrated rock from the subduction interface alters the composition of the material carried into the mantle. This change affects the chemistry of the magma generated beneath the volcanic arc, altering the chemical signature and location of the resulting volcanic chain. For example, erosion is thought to be a factor in the long-term migration of magmatic belts away from the trench in regions like the Andes Mountains.