Volcanoes form when magma escapes through weak points in the Earth's crust called volcanic vents. Magma rises from the mantle through cracks and fills magma chambers, building pressure until an eruption expels ash, lava, and rocks. The deadliest eruption was in 1815 at Mount Tambora in Indonesia, killing over 92,000 from starvation after blotting out the sun. Other deadly eruptions like Krakatoa in 1883 caused thousands of deaths from tsunamis rather than lava or ash. Volcanoes can be active and monitored as explosions risks, dormant if inactive for thousands of years but magma chambers could reactivate them, or extinct with zero eruption risk.