THE iconic Japanese volcano Mount Fuji is something of a mystery: it is simply too big and too active for its location. The volcano sits above a subduction zone in which the Philippine Sea plate is sinking beneath Japan. This process melts the rock, creating lots of small pockets of magma. Volcanoes in regions like this tend to be quiet and small as there simply aren鈥檛 large enough volumes of magma to make them bigger and more active.
But Mount Fuji is unusually tall, and produces material at the rate of 10,000 cubic kilometres per 100,000 years, roughly an order of magnitude more than other volcanoes in similar circumstances. What鈥檚 more, its magma is more akin to the type produced at mid-ocean ridges where two plates are pulling apart, allowing the fresh young mantle to emerge.
Now Koki Aizawa and colleagues at Kyoto University think they know why. They claim to have discovered a tear in the Philippine Sea plate directly beneath Mount Fuji that allows large volumes of fresh mantle to replenish Mount Fuji鈥檚 magma chamber, making the volcano taller and more active than its cousins (Geophysical Research Letters DOI: 10.1029/2004GL019477). Aizawa and his colleagues speculate that the rip was created when two nearby continental plates collided around 2 million years ago.
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