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© Japan Stock/AlamyWhat's going on below?
Something is brewing under Japan's Mount Fuji. Using rocks ejected by previous eruptions, between 781 AD and 1707, geologists are figuring out what the volcano's internal plumbing looks like.

A team led by Takayuki Kaneko at the University of Tokyo's Volcano Research Center has found that over the centuries the magma's silica levels have gradually increased. High silica tends to indicate large explosions, suggesting eruptions have become more violent. Large amounts of basalt rich in aluminium oxide were also found, which can trigger an eruption when it collides with silica.

Based on the pressures required to form both materials, Kaneko believes the two mineral composites are housed in separate chambers under Fuji: one deep chamber 20 kilometres below the volcano, rich in basaltic magma, and a shallower chamber housing the silica 9 kilometres underground.

He says the deep rumble of low-frequency earthquakes beneath Fuji in 2000 and 2001 suggests movement inside the basaltic magma chamber, and adds he would not be surprised if Fuji erupts in the very near future.

Not everyone agrees. For an eruption, new basaltic magma from deeper underground must intrude into the existing basaltic chamber, pushing it towards the silica, says Phil Shane at the University of Auckland, New Zealand, and we simply don't know if that is happening.