Pyroclastic Flows

• Volcanic eruptions produce pyroclastic flows: volatile combinations of ash, volcanic rock and hot gases. As the volcano continues to erupt, these flows travel at significant speeds. The heat and gas can be deadly to any life in their path. Pyroclastic surges are a subset of such flows. Their composite material is less dense, so they project over a wider area.
  
  

Positive Effects

• Eruptions are not purely destructive; they can have positive effects as well. The ash and lava ejected from a volcano is nutrient-rich, sustaining plant life and soil as they break down. The intense heat can also be harnessed for geothermal energy. Moreover, the lava from eruptions eventually cools and forms additional land.
  
  

Volcanic Ash

• Eruptions spew vast quantities of ash. Some of this ash stays up in the atmosphere while other ash falls to earth. Ash in the air can disrupt air travel. It can also partly block out the sun, lowering global temperatures. The 1991 eruption of a Filipino volcano spread the ash plume widely and had a measurable impact on global temperatures for a couple of years.
  
  

Dangers to Humans

• Volcanic eruptions can be a sight to behold, but they can have deadly effects on human populations. They are an unstoppable force. Volcanic ash and lava can wipe out entire towns and cities. Settling ash can make certain places uninhabitable. Consider the ancient example of Pompeii in ancient Rome, when Mount Vesuvius suddenly erupted, destroying an entire city and encasing it in ash.