Video narration
How can a solid-looking mountain suddenly burst open? Think of a fizzy-drink bottle: trapped bubbles force liquid out when the cap comes off. A volcano isn't a hollow mountain filled with fire. Underground, molten rock called magma rises with dissolved gases. As magma rises, pressure drops, so those gases expand, like bubbles expanding. That builds pressure inside. The pressure pushes through cracks or weak spots in surrounding rock, opening a path toward the surface: a vent. Here's the key: the mountain erupts not because fire bursts out, but because expanding gas pressure drives rising magma through that weak point. At the surface, magma becomes lava. The release can send lava, ash, and rock outward. Thick, gas-rich magma can explode; runnier magma can pour out more gently. So what makes it burst? Gas pressure in rising magma drives lava, ash, and rock through a vent to the surface.