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The strength of a volcanic eruption depends on the type of magma and the amount of gases trapped in it. The magma formed when plates pull apart is very fluid. The gases in it can escape easily and therefore, eruption is mild. However, when the plates collide, the magma formed is much thicker and stickier. Gases become trapped in it and escape explosively in a huge cloud of steam and dust thousands of feet high. Surges of red-hot lava flood out of the volcano's crater as long as there is enough pressure to force it to the surface. After such eruptions, the entire volcano often collapses into its empty magma chamber, forming a steep-sided depression called a caldera. The buildup Some signs show that a volcano is going to erupt. The ground starts to shake and the sides of the cone bulge out as magma collects inside it. There is a smell of sulphur as gases escape through cracks in the rocks. The explosion When a volcano erupts, the gases dissolved in the magma are released. If the vent is blocked by hardened lava, the trapped gases escape with a loud explosion. Aftermath As well as lava and ash, the volcano belches out clouds of steam which condense into water. If there is a lot of steam, it might fall as rain and mix with ash to for thick mud that may pour downhill, burying towns and villages. Back (Active or Extinct?) | Next (Underwater Volcanoes) |