<Li> Flat - slab subduction (<30 °): occurs when subducting lithosphere, called a slab, subducts horizontally or nearly horizontally . The flat slab can extend for hundreds of kilometers and can even extend to over a thousand . That is abnormal, as the dense slab typically sinks at a much steeper angle directly at the subduction zone . Because subduction of slabs to depth is necessary to drive subduction zone volcanism (through the destabilization and dewatering of minerals and the resultant flux melting of the mantle wedge), flat - slab subduction can be invoked to explain volcanic gaps . Flat - slab subduction is ongoing beneath part of the Andes causing segmentation of the Andean Volcanic Belt into four zones . The flat - slab subduction in northern Peru and Norte Chico region of Chile is believed to be the result of the subduction of two buoyant aseismic ridges, the Nazca Ridge and the Juan Fernández Ridge respectively . Around Taitao Peninsula flat - slab subduction is attributed to the subduction of the Chile Rise, a spreading ridge . The Laramide Orogeny in the Rocky Mountains of United States is attributed to flat - slab subduction . Then, a broad volcanic gap appeared at the southwestern margin of North America, and deformation occurred much farther inland; it was during this time that the basement - cored mountain ranges of Colorado, Utah, Wyoming, South Dakota, and New Mexico came into being . The most massive subduction zone earthquakes, so - called "megaquakes", have been found to occur in flat - slab subduction zones . </Li> <Li> Steep - angle subduction (> 70 °): occurs in subduction zones where Earth's oceanic crust and lithosphere are old and thick and have, therefore, lost buoyancy . The steepest dipping subduction zone lies in the Mariana Trench, which is also where the oceanic crust, of Jurassic age, is the oldest on Earth exempting ophiolites . Steep - angle subduction is, in contrast to flat - slab subduction, associated with back - arc extension of crust making volcanic arcs and fragments of continental crust wander away from continents over geological times leaving behind a marginal sea . </Li> <P> Subduction zones are important for several reasons: </P> <Ol> <Li> Subduction Zone Physics: Sinking of the oceanic lithosphere (sediments, crust, mantle), by contrast of density between the cold and old lithosphere and the hot asthenospheric mantle wedge, is the strongest force (but not the only one) needed to drive plate motion and is the dominant mode of mantle convection . </Li> <Li> Subduction Zone Chemistry: The subducted sediments and crust dehydrate and release water - rich (aqueous) fluids into the overlying mantle, causing mantle melting and fractionation of elements between surface and deep mantle reservoirs, producing island arcs and continental crust . </Li> <Li> Subduction zones drag down subducted oceanic sediments, oceanic crust, and mantle lithosphere that interact with the hot asthenospheric mantle from the over-riding plate to produce calc - alkaline series melts, ore deposits, and continental crust . </Li> <Li> Subduction zones pose significant threats to lives, property, economic vitality, cultural and natural resources, as well as quality of life . The tremendous magnitudes of earthquakes or volcanic eruptions can also have knock - on effects with global impact . </Li> </Ol>

Why does oceanic crust sink beneath continental crust at subduction boundaries