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Can anybody explain how the St Helena seamount chain originated? - is it something to do with hotspots?

the StHelena seamount chain was formed by a hot spot, your right. a hot spot is a plume of heated magma rising from the lower mantle. it causes volcanos on the surface. the theory goes the a hot spot in the magma is almost staionary but the the tectonic (lithosphereic) plate above is in motion. this means the the hot spot punches through the plate, the plate moves and it punches through again forming a chain of volcanoes which get progressivly older as you move away from the hot spot. the chain "travels" (thats not a good discription) in the opposite direction to the plate's relitive motion.