When the plunging plate gets deep enough inside the mantle, some of the rock on the overlying plate melts and forms magma that can move upward and erupt at the Earth's surface.
(the plunging plate; gets; deep enough inside the mantle) (magma; can move; upward) (magma; erupt; L:at the Earth's surface)
the plunging plate magma magma
gets can move erupt
deep enough inside the mantle
upward
L:at the Earth's surface