Answer
Back-arc basins form behind volcanic arcs in subduction zones. When an oceanic plate is subducted beneath a continental or another oceanic plate, it can create tensional forces behind the volcanic arc. This tension leads to the stretching and thinning of the overlying lithosphere, causing the crust to sag and create a basin-like structure. As the lithosphere stretches, magma from the mantle can rise and fill the basin, creating new oceanic crust. These basins are called back-arc basins, and they are often sites of new seafloor spreading and volcanic activity.
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