The interesting geologic features that plate tectonics can create are amazing. What is plate tectonics, you ask? The Earth's crust is broken up into about 20 different plates (there are 7 major ones) or sections of the crust. At the boundaries of these plates, there are different kinds of movement: if two plates slide past each other, it is called a transform fault (ex: the San Andreas Fault in California); if they move away from each other, it is called a divergent plate boundary (ex: mid-ocean ridges where new ocean crust is formed); and if they move towards each other, it is called a convergent plate boundary (ex: subduction zones and mountain building).
In the Straits of Sicily, between Sicily and Tunisia in the Mediterranean Sea, there is a convergent boundary with the African plate subducting underneath the Eurasia plate. This means that the oceanic crust of Africa is moving underneath the Eurasian plate and making its way down into the Earth to be re-melted and eventually turned back into crust. The African ocean crust is old, cold and heavy; that makes it move quickly into the Earth at a steep angle. This plate movement causes some extension, or pulling apart, on both sides of this subduction zone. The Straits of Sicily between Sicily and Tunisia is one of those areas of extension (see image below). A rift zone has formed in this area, which means that the crust is thinning and being pulled apart which causes faults and allows magma to come up to the surface. And voilà! Volcanoes form much to the delight of geologists.