According to their airborne-detected magnetic anomalies, volcanoes forming the islands, banks, and seamounts along the Hawaiian-Emperor seamount chain erupted predominantly during intervals of normal polarity of Earth's magnetic field. For the past 10 m.y. of hotspot activity, when the field was 50% normal and 50% reversed, 19 of 23 volcanic edifices from Hawaii to Necker Island were dominantly normal, one probably was normal, and three were dominantly reversed. Statistical probabilities, consideration of induction in the present normal field, and possibilities of field reversals during rapid edifice-building nevertheless show that eruptions were not random with respect to the magnetic field. Similar conclusions, where data are fewer, are good for the older Hawaiian Ridge (29 of 36 normal) and are fair for the Emperor Seamounts (3 of 3 known normal).
The correspondence suggests that hotspot plumes are generated near the core-mantle boundary. Spreading-center and magmatic-arc volcanism, on the other hand, are mainly upper-mantle processes. The Jurassic and Cretaceous long normal invervals, however, were times of such high heat flux that sea-floor spreading as well as hotspot activity increased. Larger hotspots under continental lithosphere during those normal intervals may have initiated continental rifting.