Tectonic Plates and Faults - Earthquake Engineering for Structural Design - page 149

Figure 5.5 Continental divergent boundary: African Rift Valley (USGS, nd)
O cean-ocean convergent boundaries (Fig. 5.6) occur when two convergent
plates are oceanic ones and one of them subducts under the other, resulting in
trenches and some chains of islands. The most known case is the Marianne trench
(paralleling the Marianne Islands), where the Pacific plate converges against the
Philippine plate. Other cases are the island arcs produced at the ocean surface, such
as the Japanese, Aleutian or Caribbean islands. The most dangerous result of this
convergence is the possibility of forming the tsunami phenomenon. The 2004
Sumatra tsunami is the best example of this risk (see Fig. 4.18).
Ocean-continent convergent boundaries (Fig. 5.7a) occur when an oceanic
plate with higher density subducts a continental plate, forming trenches thousands
of kilometers long and 8 to 10 km deep. The main ocean-continental boundaries
are presented in Figure 5.7b. Off the coast of South America, along the Peru-Chile
trench, the oceanic Nazca plate is pushing into and being subducted under the
continental plate of South America. In turn, this plate lifts up, creating the Andes
Mountains, the backbone of the continent. This subduction zone is one of the
deepest and largest in the world, in some cases being 650 km from the surface.
Another very important subduction zone is the Western zone of Canada, where the
Juan de Fuca plate subducts the North American plate.