Continental arc

[1] This process can create relatively buoyant magma, which subsequently forms a series of volcanoes at the surface along the subduction zone.

[2][3] Because the subduction zone (which is also the plate boundary) is generally an arc-shape, geologists named those volcanoes volcanic arcs.

[4] The dehydration of the downgoing slab and the partial melting of asthenosphere together generate the primary magma of continental arcs.

Primary magma is composed of olivine tholeiitic basalt because of mixture of peridotites from the mantle wedge and large ion lithophile enriched (LIL-enriched) fluids from the dehydrating subducting plate.

[6] A further enriched source may be provided by the tectonic erosion process that causes scraping and dragging of lower continental lithosphere into the melting zone.

Thus, a high concentrations of Rb, Cs, Ba, K, Th, and LREE (light rare-earth elements) and enriched isotopes can be found in the continental arc magmas.

A process called tectonic erosion happens when friction force during convergence scrapes off huge amount of rocks from the base of continental arcs.