Plate Boundaries

About this topic

At divergent boundaries, plates move apart and magma rises to form new crust, such as the Mid-Atlantic Ridge and East African Rift. At convergent boundaries, plates collide: oceanic plates subduct under continental plates producing trenches and volcanoes (Andes), and continent–continent collisions build mountains (Himalayas).

At transform boundaries, plates slide past each other, causing earthquakes, as on the San Andreas Fault. The Pacific "Ring of Fire" is a zone of subduction with many volcanoes and quakes.

Imagine two conveyor belts: at a divergent boundary they carry rock away from a central seam, while at a convergent one they meet head-on. Iceland sits on the Mid-Atlantic Ridge and is slowly being pulled apart, with visible rifts at Thingvellir. Where the Nazca Plate dives under South America, the Andes and the deep Peru-Chile trench form, and melting of the sinking slab feeds volcanoes. The Himalayas formed because neither continental plate is dense enough to sink.

The 2004 Indian Ocean and 2011 Japan earthquakes happened at subduction zones, which generate the largest earthquakes (magnitude 9 and above) and tsunamis. The San Andreas Fault runs through California, where Los Angeles on the Pacific Plate is slowly moving toward San Francisco on the North American Plate. Knowing the boundary type helps predict hazards. A common mistake is to think that earthquakes and volcanoes happen randomly; most trace the plate edges. Another is to think that boundaries are always on coasts; many are in mid-ocean or under continents.