Volcanoes erupt because magma (molten or partially molten rock) forms beneath the Earth's surface due to heat from the mantle and crust. This magma rises because it is less dense than the surrounding solid rock. When the pressure inside a magma chamber becomes too great, the magma forces its way through cracks and vents, erupting as lava. The process is driven primarily by heat and pressure:

• Heat from the Earth's interior causes partial melting of mantle and crustal rock, creating magma.
• Gases dissolved in the magma (like water vapor and carbon dioxide) expand as pressure decreases near the surface, increasing pressure inside the magma chamber.
• When the pressure from the magma and expanding gases exceeds the pressure from the overlying rock, it causes fractures, allowing magma to escape explosively or effusively.

Volcanoes most often form and erupt at tectonic plate boundaries or hotspots where magma can rise more easily. Thus, volcanic eruptions occur because magma generated deep inside the Earth rises and escapes to the surface when pressure conditions allow it.