A subduction zone is a convergent boundary where two tectonic plates collide. Plates are large, dense masses in the crust of the Earth, the lithosphere, that float on top of liquefied rock in the asthenosphere. They are constantly shifting and moving, so when they subduct, one pushes beneath the other. These zones create geologic formations such as mountain ranges, ocean trenches, and island arcs, as well as phenomena like earthquakes and volcanoes.
Tectonic plates are categorized as either oceanic plates that have large bodies of water above them or continental plates that support land. Geologists have learned about how subduction works by studying earthquakes in seismology and volcanoes in volcanology. They know that younger plates are less dense than older plates. Generally, ocean crust is thinner and denser than continental crust. Subduction zones don't only make certain landscape changes, but they recycle rock by melting it into magma so that crust might form at other tectonic zones.
When two oceanic plates meet, a deep underwater trench forms where the thicker plate plunges beneath the thinner one. At first, great pressure at the thrusting plates causes earthquakes. These open up spaces to the hot, molten magma so it can push through in submarine volcanoes. Eventually, the material builds up in a crest until it breaks the surface of the ocean, forming island "arcs" and archipelagos. This is how the Philippine Islands originated.
An oceanic and continental plate meet under different circumstances. Beneath the coast, their convergence causes the ocean plate to plunge underneath the continental one, piercing the hot athenosphere. Again, the first stage builds up pressure and releases earthquakes, which is why the Pacific "Ring of Fire" experiences so many tremors. Then, over time, mountains form on land as the continental plate buckles. The Andes are one such mountain range.
Two continental plates can also converge, but this might not technically form a subduction zone. Land crust doesn't have sufficiently different densities or thicknesses for one to be subducted. Still, it bulges and forms interior mountain ranges without volcanoes, such as the Himalayas.
Together with divergent zones, convergent zones ensure that the surface area of the earth never grows or shrinks. As the leading edge of the lithospheric plate is devoured when it melts in the hot athenosphere, the trailing edge forms out of cooling rock. Sometimes, smaller plates may entirely disappear, eaten by the magma.