Convergent Boundary
A convergent boundary is a place where two tectonic plates move toward each other, producing subduction zones, volcanoes, trenches, or mountain ranges.
Definition
A convergent boundary is where two plates push into each other, so that one plate may dive under the other or both may crumple up to make mountains. When oceanic plate meets continental plate, the denser oceanic plate subducts, forming a trench, a volcanic mountain chain, and deep earthquakes; ocean-ocean convergence forms a trench and a volcanic island arc; and continent-continent convergence produces no subduction because both plates are buoyant, so the crust thickens and folds into high mountains. In subduction, the downgoing slab dehydrates at depths near $100\ \text{km}$, releasing water that lowers the melting point of the overlying mantle wedge and generates andesitic arc magma. The Wadati-Benioff zone of earthquakes traces the slab to depths of $700\ \text{km}$, and accretionary wedges of scraped-off sediment, forearc basins, and back-arc extension form a characteristic sequence. Continental collision after ocean closure produces crustal thickening, thrust stacking, high-grade metamorphism, and plateau uplift.
Example
The Himalaya, the tallest mountains on Earth, are still growing because India is pushing into Asia. The Andes and the Cascades are ocean-continent boundaries, Japan and the Aleutian Islands are ocean-ocean island arcs, and the Himalaya and, $300$ million years ago, the Appalachians, are continent-continent collisions. The 2011 Tohoku earthquake (magnitude $9.1$) ruptured the Japan Trench megathrust and displaced the seafloor by tens of meters, generating the tsunami. The Cascadia subduction zone off the Pacific Northwest last ruptured in January 1700, dated by Japanese tsunami records and drowned coastal forests.
Key Insight
Convergent boundaries are where the biggest earthquakes and the most explosive volcanoes happen. Most of the "Ring of Fire" around the Pacific is a chain of convergent boundaries, and its volcanoes and earthquakes trace where the Pacific seafloor is being consumed. Subduction zones are Earth's recycling centers. Oceanic crust, its sediment, and its water are carried back into the mantle, and volatiles released along the way build the atmosphere and oceans over geologic time.