The world’s biggest Phanerozoic magmatic arcs formed above subduction zones and comprise the products of continuous magma emplacement into the crust over periods of up to 500 My. However, the intensity of magmatic activity can vary significantly. Punctuated magmatic events lasting from 5 to 20 My can dwarf the volume of magmas generated through the remainder of an arc’s history: these high-volume events are called “flareups” and can completely rebuild an arc’s crust. In arcs formed on continental lithosphere, flare-ups typically correlate with regional structural events that shorten and/or thicken the crust. Geochemical and isotopic signatures show that these high magmatic addition rate events involve ~50% recycled upperplate crust and mantle lithosphere; the remaining ~50% comes from the mantle wedge.