That should be interesting, one plume under the sea bed and one under the crust.
The August 2009 issue of National Geographic had a detailed discussion and graphics of this phenomenon. Quite humbleing, makes me not want to live anywhere near there or downwind either as they show the ash dispersion on the last 3 major eruptions of this hotspot. Even more interesting are the 18 million years worth of other hot spots. There is one really big one around the time that early homonids were supposed to have begun differentiating from the ape line around 6 million years ago. Global climate change?
Regarding the Hawaiian chain, if you look at an underwater topography you will see the sea mounts (islands that have been worn down to below sea level) run to the north west. Then about (I think the figure I have read was 43 million years ago) the direction was much more directly to the north. What would have caused such a dramatic shift in the movement of the hot spot or the earth’s crust?