The inside or structure of a volcano consists of cinder magma pyroclastic materials. There is also a vent that goes straight through the middle. The given volcano also sits on normal dirt, rock, etc. and builds up with each explosions

"All of the molten rock in the asthenosphere finds weaknesses in the crust.The molten rock creates a main passageway called acentral vent. As the magma rises, secondary vents develop. Those secondary ones are just smaller and branch from the main tube."