WOULD a volcano blow its top if it was called petite? Let's hope not, because that's the name given to some unusual volcanoes just discovered in the western Pacific. And they are changing our understanding of how volcanoes form.

Naoto Hirano at the Tokyo Institute of Technology, Japan, and colleagues discovered the volcanoes, which are between 0.005 and 1 cubic kilometre in size, near the underwater Japan Trench. The "petit-spot" volcanoes are 1 to 8 million years old and defy conventional theories of volcanism. They don't sit at tectonic plate boundaries nor at "hotspots" of rapidly rising plumes from the boundary between Earth's core and the mantle (Science, DOI: 10.1126/science.1128235).

The team thinks the mini volcanoes were created when cracks formed in the Earth's crust during the bending of the north-western Pacific plate as it dives under the Kuril and Japan trenches, squeezing partially melted material from the upper mantle out of the cracks. "I was unbelievably excited to discover this volcanism," Hirano says.