For a long time researchers believed that Etna’s position at the crossroads of these faults was the explanation for its volcanism. The presence of faults, however, accounts only for the ability of magma to reach the surface; it does not explain why the magma is produced in the first place. According to most theories, the prevailing forces in the Sicilian crust are similar to those in rift zones—extensional stresses that cause thinning of the crust and upwelling of the underlying mantle. But at Sicily the African and Eurasian plates are colliding, so one would expect the stresses to be compressive rather than extensional. Moreover, only about 20 percent of the magma erupted at Etna has a chemical composition similar to that of a rift-zone volcano