Qualitative & Quantitative Information: Plate Boundaries

Convergent Margins

Crust is destroyed as two plates move towards each other. The heavier plate dives (subducts) beneath the more buoyant plate.

Oceanic-oceanic convergence—When two oceanic plates converge, one subducts beneath the other, and in the process a trench is formed. (Example: The Marianas Trench, the Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth’s interior (nearly 11,000 m) than Mount Everest, the world’s tallest mountain, rises above sea level (about 8,854 m).) Inboard of the overlying plate a volcanic arc forms. This is because water being released from the subducting slab facilitates melting of the overlying mantle. The melt rises to form volcanoes. (Ex. the Aleutian Islands.)

Oceanic-Continental convergence—An oceanic plate converges on a continental plate and the denser crust of the oceanic plate sinks beneath the more buoyant continental plate. In this type of convergence, trenches, strong, destructive earthquakes and the rapid uplift of mountain ranges are common. Water released from subducting slab facilitates melting. Rising melt builds volcanic arc. (Examples: Juan de Fuca plate subducting beneath the North American plate off shore in the Pacific Northwest. Off the coast of South America along the Peru-Chile trench, the oceanic Nazca Plate is being subducted beneath the South American Plate. In turn, the overriding South American Plate is being lifted up, creating the towering Andes mountains.)

Continental-continental convergence—When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. (Example: The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world.)