A secondary process of plate tectonics that leads to the birth of small, new, rapidly growing and moving plates.
A subducted oceanic plate does not go quietly. As the plate and sediments deposited on the plate during its stint as ocean floor are dragged into the Earth's mantle beneath an overriding continent, or another oceanic plate, they melt (having a lower melting point than the plastic rocks of the upper mantle). Plumes of molten material (known as "diapir"s) float up into the overriding plate. Some crystallize below the surface, forming plutonic igneous rocks; some eventually penetrate through the surface, forming a line of volcanoes known as a volcanic arc.
Plastic rocks have a bit more grip than your average fluid, and the disintegrating plate drags the bottom of the overriding plate's mantle along with it. And so, curiously, a convection cell forms in the edge of the lower lithosphere in the overriding plate. A rising convection column forms just behind the arc. As the material reaches the bottom of the crust, it spreads out laterally, dragging bits of crust towards and away from the active margin. The crust thins out, and frequently, the edge of the hinterland plate breaks off.
Loosely fold over the edge of a piece of newspaper, and slowly push the loop into a vertical surface. The edge of the paper will move back towards you at twice the rate you are pushing on it. Similarly, the motion of the small, new plate is accelerated by the convection process. Mini- and microplates formed by back-arc rifting are among the fastest-moving and fastest-growing plates on the Earth.
Back-arc rifting has happened throughout geologic time, and is active in many places today. Examples incude:
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