section_path: Volcanoes and humans/Hazards
section_name: Hazards
paragraph_text: Hazards
  
  There are many different types of volcanic eruptions and associated activity: phreatic eruptions (steam-generated eruptions), explosive eruptions of high- silica lava (e.g., rhyolite), effusive eruptions of low-silica lava (e.g., basalt), sector collapses, pyroclastic flows, lahars (debris flows) and volcanic gas emissions. These can pose a hazard to humans. Earthquakes, hot springs, fumaroles, mud pots and geysers often accompany volcanic activity.
  
  Volcanic gases can reach the stratosphere, where they form sulfuric acid aerosols that can reflect solar radiation and lower surface temperatures significantly. Sulfur dioxide from the eruption of Huaynaputina may have caused the Russian famine of 1601–1603. Chemical reactions of sulfate aerosols in the stratosphere can also damage the ozone layer, and acids such as hydrogen chloride (HCl) and hydrogen fluoride (HF) can fall to the ground as acid rain. Excessive fluoride salts from eruptions have poisoned livestock in Iceland on multiple occasions. Explosive volcanic eruptions release the greenhouse gas carbon dioxide and thus provide a deep source of carbon for biogeochemical cycles.
  
  Ash thrown into the air by eruptions can present a hazard to aircraft, especially jet aircraft where the particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. This can cause major disruptions to air travel.
  
  A volcanic winter is thought to have taken place around 70,000 years ago after the supereruption of Lake Toba on Sumatra island in Indonesia. This may have created a population bottleneck that affected the genetic inheritance of all humans today. Volcanic eruptions may have contributed to major extinction events, such as the End-Ordovician, Permian-Triassic, and Late Devonian mass extinctions.
  
  The 1815 eruption of Mount Tambora created global climate anomalies that became known as the " Year Without a Summer " because of the effect on North American and European weather. The freezing winter of 1740–41, which led to widespread famine in northern Europe, may also owe its origins to a volcanic eruption.