Magmas erupting in arcs range in composition from MgO-rich basalt to MgO-poor rhyolite. This broad compositional range is due to the sequential crystallization and separation of minerals with different chemical compositions from the cooling magma (a process known as fractional crystallization), to mixing between magmas that have undergone different degrees of fractional crystallization, and to the assimilation of rocks, in which high temperatures allow magmas to partially melt and assimilate the