Vesuvius has a large cone ( Gran Cono ) which is partly surrounded by the top of a caldera which was made when an earlier and much higher structure called Monte Somma collapsed . Vesuvius is a distinctive `` humpbacked '' mountain , consisting of a large cone ( Gran Cono ) partially encircled by the steep rim of a summit caldera caused by the collapse of an earlier and originally much higher structure called Monte Somma .