A volcanic eruption is one of the most violent natural events that occur on Earth, as rock, water and gases are heated in the Earth’s interior and break through the surface (Figure 1). The energy for this heating comes partly from radioactive decay within the Earth. Turn to pages 16–17 of this issue for more on radioactivity.
Earth has about 600 active volcanoes, which have claimed over 70000 lives in the past century. One of the most obvious dangers comes from flowing lava (molten rock), which can engulf buildings and crops within a few kilometres of the eruption. Ballistic projectiles (solid rocks hurled at high speed) are a hazard at distances of up to about 10 km. Projectile motion is the subject of Mathskit on pages 11–13.
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