Why do some volcanoes erupt almost all the time but others lie dormant for centuries, millennia, or even longer?  Steve Sparks has turned our ideas about volcanoes upside down.  Not quite literally, but by applying the physics of fluid motion to the rocks and magma below volcanoes, he discovered that magma forms at much greater depths than previously thought, eventually forming an unstable blob that forces its way up through as much as a hundred kilometres of overlying rocks to erupt from a volcano.  It is how quickly such blobs form that determines how frequently a volcano will erupt. 

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