Volcanoes

• Volcano concepts

  Volcano is a landform created when lava erupts onto the earth‘s surface

  As more lava erupts & accumulate, a cone-shaped mountain forms

  1. Crater
  2. Active vent
  3. Conduit
  4. Magma chamber
  5. Volcanic cone

  
• Divergent plate boundaries

  1. Plates move apart, crust stretches & fractures develop
  2. The decrease in overlying pressure causes the underlying mantle to melt, forming magma
  3. Magma contains dissolved gases & is less dense than surrounding materials
  4. Magma rises through weak areas in the crust to the earth’ surface to erupts as lava, causing a volcanic eruption
  5. Lava cools, solidifies & accumulates over time to form a volcano
• Convergent plate boundaries

  1. Plate move towards each other & denser plate subducts beneath the other
  2. As the subducting plate sinks into the mantle, high pressure forces water out its oceanic crust.
  3. Water lowers the melting point of the overlying magma, causing it to melt, forming magma
  4. Magma contains dissolved gases & is less dense than the surrounding materials
  5. Magma rises through weak areas of the crust to the earth’s surface to erupt as lava, causing a volcanic eruption
  6. Lava cool, solidifies & accumulates over time, forming a volcano
• Explosivity of volcanic eruptions depends on how easily dissolved gases can escape from the magma
• High silica magma

  • More viscous magma
  • The dissolved gases in the magma cannot escape easily
  • More pressure builds up until gases escape explosively
  • Results in violent & explosive eruptions
  • Generally associated with stratovolcanoes
• Low silica magma

  • Less viscous magma
  • The dissolved gases in the magma can escape easily
  • Less pressure builds up
  • Results in gentle & effusive eruptions
  • Generally associated with shield volcanoes
• There are volcanoes with viscous magma that do not result in explosive eruption as the magma rises in a way that allows gases to escape
• Stratovolcanoes formation

  1. High viscosity magma rises through weak areas in the crust to the earth’s surface & erupts explosively as lava, ash & rocks
  2. Ash & rocks settle on the sides of the volcano, and are later covered by lava
  3. Over successive eruptions, a tall volcano consisting of alternating layers of ash & lava develops
  4. As the highly viscous lava travels a shorter distance before cooling & solidifying, the volcano has steep sides & a narrow summit
• Shield volcanoes formation

  1. Low viscosity magma rises through weak areas in the crust to the earth’s surface & erupts effusively
  2. Over successive eruptions, a volcano consisting of layers of lava develops
  3. As the less viscous lava travels a longer distance before cooling & solidifying, the volcano has gently sloping slides & a board summit
• Volcanic eruption measurement

  Volcanic Explosivity Index (VEI) measures the magnitude of different volcanic eruptions bases on explosivity:

  • Volume of ejected materials
  • Height of the eruption cloud
  • Duration of the eruption

  Measured on a scale of 0 to 8