![]() If deformation is permanent, then removing the stress does not result in the rocks snapping back to their original shape. Earthquakes at Divergent and Transform Plate BoundariesĮarthquakes along divergent and transform plate margins are shallow (usually less than 30 km deep) because below those depths, rock is too hot and weak to avoid being permanently deformed by the stresses in those settings. Locations include the Great Rift Valley area of Africa, the Lake Baikal area of Russia, and Tibet. Some are related to the buildup of stress due to continental rifting or the transfer of stress from other regions, and some are not well understood. ![]() Click the image for terms of use.Įarthquakes are also relatively common at a few locations away from plate boundaries. Source: Lisa Christiansen, Caltech Tectonics Observatory (2008) view source. Wide bands of scattered earthquakes mark continent-continent convergent margins (e.g., between the Indian and Eurasian plates), or continental rift zones (e.g., in eastern Africa). Wider bands with earthquakes at a range of depths are subduction zones. Narrow bands with shallow earthquakes (marked in red) indicate transform boundaries or mid-ocean ridge divergent boundaries. ![]() ![]() Bands of earthquakes mark tectonic plates. Figure 12.16 Earthquakes greater than magnitude 5, from 2000 to 2008. Wide swaths of scattered earthquakes may also correspond to continental rift zones, such as in eastern Africa. Wide swaths of scattered earthquakes may correspond to continent-continent collision zones, such as between the Eurasian plate and the African, Arabian, and Indian plates to the south. Bands of earthquakes are wider along subduction zones because they take place throughout the subducting slab that extends beneath the opposing plate. Subduction zones have earthquakes at a range of depths, including some more than 700 km deep. Mid-ocean ridges and transform margins have shallow earthquakes (usually less than 30 km deep), in narrow bands close to plate margins. The depths of earthquakes, and the width of the band, depend on the type of plate boundary. For instance, the india-Asia collision has formed the huge uplifted Tibetan Plateau, a series of mountain ranges to the north including the Tien shan and Karakoram, and deformation of the continents extends far into Asia, as far as Lake Baikal.Bands of earthquakes trace out plate boundaries (coloured dots, Figure 12.16). in continent-continent collisions, deformation may be very diffuse and extend beyond the normal limit of plate boundary deformation that characterizes other types of plate interactions. In some cases, subduction brings two continental plates together and they collide, forming huge mountain belts like the Himalayan mountain chain. Often, slices of the old ocean floor are caught in these collision zones (these are called ophiolites), and the process by which they are emplaced over the continents is called obduction (opposite of subduction). suture zones are complex and include folded and faulted sequences of rocks that form on the two colliding terranes and in any intervening ocean basin. Mountain belts or orogens typically mark the places where lithospheric plates have collided, and the zone that they collided along is referred to as a suture. Since the plates are in constant motion, island arcs, continents, and other terranes often collide with each other. A simple model for the origin of the continental crust is that it represents a bunch of island arcs which formed at different times and which collided during plate collisions. Island arcs are extremely important for understanding the origin of the continental crust because the magmas and sediments produced here have the same composition as the average continental crust. These volcanoes form a volcanic arc, either on a continent or over an oceanic plate, depending on which type of crust the overlying plate is composed of. since subduction zones are long narrow zones where large plates are being subducted into the mantle, the melting produces a long line of volcanoes above the down-going plate. These melts then move upward to intrude the overlying plate, where the magma may become contaminated by melting through and incorporating minerals and elements from the overlying crust. As the oceanic slabs sink downward, they experience higher temperatures that cause the release of water and other volatiles from the subducting slab, generating melts in the mantle wedge overlying the subducting slab. Oceanic lithosphere is being destroyed by sinking back into the mantle at the deep ocean trenches in a process called subduction.
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