Volcanic hazards

• Lava flows


  Flows or streams of molten rock that pour from an erupting vent.

  Acidic lava is thick and does not flow easily.

  Basaltic lava is free-flowing and travels far.

  Speed also depends on; whether lava flows as a broad sheet, through a confined channel or down a lava tube; viscosity; steepness of ground.
• pyroclastic flows; nuées and ardente 


  Flows of gas and tephra over 500 degrees and flow down the side of volcanoes at speeds over 100 km/h.
• Gas emissions


  carbon dioxide, carbon monoxide, sulphur dioxide and chlorine escape through fumaroles.

  When sulphur dioxide combines with water, acid rain is produced, leading to weathering and pollution.
• Tephra


  Volcanic bombs and ash ejected into the atmosphere.

  Size ranges from ash to larger bombs of >6 cm diameter.
• Lahars and flooding associated with melting ice


  Lahars are a type of mud flow.

  Rock, ash and soil mix together and destroy and bury anything in the path of the rapid flow of material as they follow valleys.

  Snow and ice on a volcano summit melt during the eruption, leading to flooding (jökulhlaups)
• Tsunamis


  Violent eruption of an island volcano can displace oceanic water and lead to a tsunami, which is a wave travelling at speeds up to 600 km/h.
• Icelandic lava eruptions 


  Persistent fissure eruption.

  Large quantities of basaltic lava build up vast horizontal plains.
• Hawaiian eruption


  More noticeable central activity than Icelandic lava eruptions.

  Runny, basaltic lava travels down the sides of the volcano in lava flows.

  Gas escapes easily.

  Occasional pyroclastic activity occurs but this is less important than the lava eruption.
• Strombolian eruptions


  Characterised by frequent gas explosions which blast fragments of runny lava into the air to form cones.

  Large quantities of pyroclastic rock thrown out.

  Eruptions are commonly marked by a white cloud of steam emitted from the crater.
• Vulcanian eruptions


  Violent gasa explosions blast out plugs of sticky or cooled lava.

  Fragments build up into cones of ash and pumice.

  Very viscous lava solidifies rapidly after an explosion.

  Eruption clears a blocked vent and spews large quantities of volcanic ash into the atmosphere.
• Vesuvian eruptions


  Gas rushes up through sticky lava and blasts ash and fragments into the sky in a huge explosion.

  Immense clouds of gas and volcanic debris several km thick.

  Gas clouds and lava can also rush down the slopes.

  Part of the volcano may be blasted away during the eruption.
• Super-volcanoes are volcanoes that erupt more than 1000 km squared of material in a single event e.g. Yellowstone in Wyoming, which has a caldera measuring 75 km in diameter.
• Volcanic Explosivity Index (VEI) combines magnitude and intensity to measure the power of an eruption. On a scale from 1 (least explosive) to 8 (most explosive), an increase in number represents a ten-fold increase in explosivity.
• Hot spots

  Eruptions not occurring at plate boundaries.

  Places where a plume of magma rises from the mantle and erupts at the surface.

  Associated with intense volcanic activity and eruptions of basaltic lava e.g. Hawaiian chain of volcanic islands.
• Effusive eruptions


  Divergent plate boundaries.

  Basaltic lava.

  Low % of silica, low viscosity, higher temperature at eruption.

  Gas bubbles expand freely; limited explosive force.

  Erupts gas and lava flows.

  Tend to be more frequent; an eruption can continue for many months.

  Shape of volcano; gently sloping sides; shield volcanoes; lava plateaux when eruption from multiple fissures.

  Basic magma flows freely and covers large areas: flood basalts.
• Explosive eruptions

  Convergent plate boundaries.

  Rhyolite lava (more asid); andesite lava (less acid)

  High % of silica, high viscosity, lower temperature at eruption.

  Violent bursting of gas bubbles when magma reachers surface; highly explosive; vent and top of cone often shattered.

  Erupts gas, dust, ash, lava bombs and tephra.

  Tend to have long periods with no activity.

  Shape of volcano is steep-sided strato-volcanoes; caldera.

  Repeated eruption of Rhyolite lava form composite cone volcanoes.

  Internal lava flows form sills and dykes.