By enormous volcano I assume you mean a super-volcano. Think carefully. What causes a volcano? Molten rock. We have Shield Volcanoes, which form from the accumulation of large volumes of Mafic Lavas. Mafic lava has a low viscosity and high temperature allowing the lava to travel great distances before solidifying (this is the main reason for their low slope, making them appear incredibly flat/wide and enormous), giving them their "Shield" like appearance. How enormous? The entirety of the Hawaiian islands mostly shield volcanoes! But as far as truly Enormous volcanoes are concerned. we must talk about Super-Volcanoes, which are an entirely different entity all together. Super Volcanoes are so large we didn't truly start identifying all of them until we could observe them, and their effect on the land around them from space. Super volcanoes leave behind calderas when they erupt. Calderas form when a giant (several cubic kilometers) magma chamber empties and land above it collapses inward. This means super volcanoes form strictly underground, and are apart of the crust. So the magma reservoir which fuels these super volcanoes are very large portions of the crust melted by mantle material that is closer to the surface than usual. In these instances, mantle material doesn't break through the surface, it just melts crustal material that builds up more and more over thousand of years until it erupts. If you want to know how bad super volcanoes can be, research Lake Toba Caldera and Yellowstone Volcano.