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What causes the large and very long lasting magma plumes which rise up from the mantle and create a hot spot such as the ones which built the island chains of Hawaii and Galapagos? The volcanic islands erupt in chains when oceanic plate slowly moves over the hot spot in the course of many millions of years, but what causes these magma plumes to rise in the first place, in other words what initiates them, and is their location purely random?

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