Definition of back-arc
: a geological region that forms at a subduction zone when the overriding plate thins and begins to spread out —often used before another noun The hot springs there are not connected at all to the midocean ridge system: they lie on what is called a back-arc spreading center, a geologic feature that occurs not where two plates separate but where they collide.— Robert Kunzig, The Restless Sea, 1999On the opposite end of a tectonic plate from its spreading zone may be a subduction zone, a span where two plates collide and one plate is forced beneath the other. Weak spots emerge near the edge of the disappearing plate, permitting magma to punch through and form conventional, conical volcanoes that happen to be underwater. These vulnerable areas are known as back-arc basins.— Janet Raloff, Science News, 7 Oct. 2006back-arc vents
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