The Descending Slab
At a subduction zone, dense oceanic lithosphere plunges beneath a lighter plate into the mantle. The oceanic plate, made heavier by cooling over millions of years, is pulled downward by gravity — this 'slab pull' force is the dominant driver of plate tectonics. As the slab descends, it creates a deep ocean trench (up to 11 km at the Mariana Trench) and generates earthquakes along the Benioff zone.
Volcanic Arc Formation
Water trapped in hydrated minerals of the subducting oceanic crust is released at depths of 100–150 km, where temperature and pressure drive dehydration reactions. This water infiltrates the overlying mantle wedge, drastically lowering its melting point and triggering partial melting. The resulting magma is buoyant and rises to build a volcanic arc 100–300 km from the trench. The Cascade Range, Andes, and Japanese archipelago are all volcanic arcs.
Seismicity and the Benioff Zone
Subduction zones produce the full range of earthquake depths. Shallow thrust earthquakes (0–70 km) on the megathrust generate the largest events on Earth. Intermediate-depth quakes (70–300 km) may result from dehydration embrittlement as water is released from the slab. Deep-focus earthquakes (300–660 km) likely involve phase transitions of olivine to spinel. Below 660 km, the slab deflects or penetrates into the lower mantle.
Geochemical Recycling
Subduction is Earth's grand recycling system. Oceanic crust carrying sediments, water, and carbon is transported into the deep mantle. Some material returns to the surface through arc volcanism; the rest is mixed into the mantle over billions of years. This process regulates atmospheric CO₂ on geological timescales, maintains the oceans' chemical composition, and concentrates ore-forming elements like copper, gold, and tin in arc settings.