Volcanoes go dormant because magma from the Earth's mantle can no longer reach the volcano.  For example, in Hawaii, the line of islands are slowly moving to the northwest. Meanwhile, the supply of magma stays in the same place within the Earth, which currently happens to lie beneath the Big Island. As the islands move away from the supply of magma, volcanoes become dormant, and new volcanoes form over the magma supply . . . such as the newest volcano . . . Loihi.  This can also happen when the angle of subduction changes and magma begins to rise through the mantle in a different location.

General / Misc