7 Subduction zones, island arcs and active continental margins ubduction zones are created when two litho- Th e third type of convergent plate boundary spheric plates move against each other and represent the active continental margins where S one of the two plates descends under the other oceanic litho sphere is subducted beneath conti- through the process of subduction. However, only nental litho sphere without a marine basin behind oceanic litho sphere is able to sink deeply into the the volcanic arc; rather, the arc is built directly on Earth’s mantle to become reincorporated there. the adjacent continent. Th e continental margin is Continental crustal material is generally too light connected directly to the hinterland, although a to be subducted to great depth. Th e interaction of shallow marine basin may exist behind the volcanic the subduction zone and the asthenosphere of the arc. Examples for active continental margins are mantle generates the melts that rise to feed the the Andes, SE Alaska, and the western and central volcanism typical of island arcs and active conti- Sunda Arc that includes Sumatra and Java. nental margins. Th e forth type of convergent margin occurs Subduction zones are critical to the dynamics of along zones of continent-continent collision. If the Earth because they represent the essential driv- two continental masses collide during continuous ing force behind the movement of plates. Moreover, subduction, they eventually merge. Telescoping of magmatism initiated by subduction is responsible the two plates and the buoyancy of the subducting for the creation of continental crust through a se- continent eventually leads to a standstill of subduc- ries of complex processes. Th e continental crustal tion within the collision zone. Th e oceanic part material generated in this fashion has a low specifi c of the subducting plate tears off and continues to weight and remains at the outer rind of the Earth drop down, a process referred to as " slab breakoff ". and is not reintegrated into the mantle. Without this Continent-continent collisions ultimately result in light-weight continental crust, which forms high the formation of mountain ranges like the Hima- topographic features on the Earth, our planet would layas or the Alps. Fig. 7.1 Convergent have a completely diff erent face. Th e total length plate margins of the Earth of global subduction zones sums to more than Structure of plate margin systems with characterized by ensimatic 55,000 km, a length only slightly shorter than the subduction zones island arcs (underlain by total length of the mid-ocean ridges (60,000 km). Systems of convergent plate boundaries are char- oceanic crust), ensialic island arcs (underlain by Four types types of convergent plate boundaries acterized by a distinct topographic and geologic continental crust), and ac- are recognized (Figs. 7.1, 7.2): subdivision. Although the plate boundary itself is tive continental margins. Th e fi rst type occurs when ocean lithosphere is subducted below other ocean litho sphere ("intra- oceanic subduction zone") to create a volcanic Kamchatka Alaska island arc system built on oceanic crust ("ensi- Nankai Alëutian Kuril Cascade matic island arc"; sima – artifi cal word fi rst used Ryukyu Japan by Wegener made from silicon and magnesium Izu-Bonin Lesser to characterize ocean fl oor and Earth’s mantle). Mariana Antilles Examples for intra-oceanic, ensimatic island arc Philippine Central America systems include the Mariana Islands in the Pacifi c Solomon and the Lesser Antilles in the Atlantic. Vanuatu Sunda Banda Peru Th e second type occurs where oceanic litho- Tonga Chile sphere is subducted beneath continental litho sphere Kermadec South and an island arc underlain by continental crust Sand- forms (" ensialic island arc"; sial – silicon and alu- wich minum for continental crust). Th e island arc of this system is separated from the continent by a marine basin underlain by oceanic crust. Examples for is- active continental margin ensimatic island arc (deep sea trench <8 km) land arc systems underlain by continental crust are ensialic island arc ensimatic island arc (deep sea trench >8 km) the Japanese Islands and the eastern Sunda Arc. W. Frisch et al., Plate Tectonics, DOI 10.1007/978-3-540-76504-2_7, © Springer-Verlag Berlin Heidelberg 2011 Licensed to jason patton<[email protected]>.