Mantle wedge

The triangular piece of upper mantle between a subducting slab and the overriding plate, where slab fluids trigger melting and arc magmas form.

Flux melting: Melting caused by added volatile components, chiefly water, that lower a rock's solidus temperature. The wedge is the textbook setting where slab-derived water fluxes solid mantle into melt.
Volcanic arc: A chain of volcanoes above a subduction zone, fed by magmas generated in the mantle wedge. Arc volcanoes sit where wedge melts rise; the wedge is their magma source.
Subduction zone: A convergent plate boundary where oceanic lithosphere descends into the mantle beneath an overriding plate. The wedge exists only as part of this larger convergent-margin system.
Mid-ocean ridge basalt: Dry, low-potassium basalt produced by decompression melting beneath spreading centers. Dry ridge melting yields MORB; wet wedge melting yields water-rich, oxidized arc magmas.
Cascade Range: A volcanic arc in the Pacific Northwest above the subducting Juan de Fuca plate. A slow, cool subduction zone where a thin, weakly heated wedge still feeds Mount St. Helens.
Slab dehydration: The progressive release of water from hydrous minerals in a subducting plate as pressure and temperature rise. The fluids that hydrate and melt the wedge are supplied by this dehydration of the downgoing slab.
Andesite: An intermediate volcanic rock rich in silica and water, typical of subduction-zone eruptions. Wedge melting plus crustal processing yields the andesitic compositions that dominate arcs.
Solidus: The temperature below which a rock is entirely solid; adding water lowers it. Wedge melting happens where slab fluids push mantle peridotite across its wet solidus.
Asthenosphere: The weak, partially molten-in-places upper mantle layer beneath the lithosphere. The wedge is a slab-bounded segment of the asthenosphere with its own circulation.
Japan trench: The subduction boundary off eastern Japan where the Pacific plate descends beneath Honshu. Its dense seismicity maps wedge hydration and flow in unmatched detail.

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