Subduction zones are the flip side of mid-ocean ridges in plate tectonics. They are destructive plate boundaries - sites where it is inferred that plates are taken back down into the asthenosphere. The evidence for their existence largely comes from seismology. Earthquake focal depths pick out zones of faulting passing from the ocean trenches down over 600 km into the mantle. These are called Wadati-Benioff zones after the two geophysicists who independently established the pattern of earthquakes. These earthquake zones coincide with panels of anomalously high seismic velocities (high velocities are generally thought to represent cold mantle) suggesting plates diving down. You can see these relationships for Japan by clicking here. Computer models can predict the temperature structure of subduction zones and indicate that the downgoing slab can be many hundreds of degrees cooler than the surrounding mantle. The dip of Wadati-Benioff zones are hugely variable. You can see a selection by clicking here.
Evidence for plate convergence at subduction zones comes from the images of scraped up sediment at trenches and from earthquake focal mechanisms.
Destructive plate margins are marked by, commonly explosive, volcanism. Visit the "ring of fire" by clicking here. Theoretically the volcanism could be due to a number of processes. But crucially, the volcanoes are found above subducting slabs which have reached depths of 120-160 km. Visit the Java area for an example. This relationship suggests that it is not the slab itself that melts (its too cool at these depths) but from fluids being driven by metamorphic reactions in the slab reacting with the over-riding mantle wedge. The generation of magma is very complex - click here to see this. The composition of igneous rocks varies - creating volcanic island arcs in oceanic crust above subduction zones and "Andean-style" cordillera in continental crust above subduction zones. The variation in part represents the role of remelting continental crust (for Andean examples). However, the formation of island arcs can be hugely variable - depending on how the mantle melts and how effectively the magma passes upwards to the surface. The rising magma can partly solidify (called fractional crystallisation) and can become contaminated from the material through which it ascends.
You can see some of the rocks that formed in an island arc by visiting an ancient example caught up in the Himalayan collision belt of Pakistan.
It is common at ocean-ocean destructive plate margins for the plate above the subduction zone to stretch out, forming new crust. For example, the Lau basin is forming behind and above the Tonga subduction zone. A stylised version of the architecture of destructive plate margins can be seen by clicking here.
The image at the top of the page is a topographic image of the region around Sumatra and Java, SE Asia. The deep ocean trench to the south of Java and other volcanic islands of the archipelago is the surface manifestation of subduction. The sea bed of the Indian ocean is being consumed, diving down northwards beneath the islands.