2 tectonic plates collided, the African and the eurasion plate causing the volcano to erupt. Or for a more simple answer, the magma inside bolied until it erupted...
When Mt. Vesuvius erupted, the towns of Herculaneum and Pompeii were buried under the ash that shot up from the volcano. The ash flew high into the atmosphere where it cooled and hardened, then...
Vesuvius has erupted about three dozen times since 79 A.D., most recently from 1913-1944. The 1913-1944 eruption is thought to be the end of an eruptive cycle that began in 1631.