6. Volcanic Eruptions and Impacts

Question 1

What is the volcanic explosivity index?

Answer 1

Used to compare and describe the magnitude of volcanic eruptions on a scale of 0-8.
It’s logarithmic with each number increasing by 10.

Question 2

What does the VEI take into account?

Answer 2

• Amount and height of volcanic material ejected (tephra and ash fall)
• Length of the eruption
• Qualitative, descriptive terms such as gentle, explosive

Question 3

What are volcanoes?

Answer 3

• Openings of the Earth’s crust through which ash, lava and gases erupt.
• As tectonic plates move, pressure builds and hot magma and gases push up to the surface.

Question 4

What is the shape of a volcano linked to?

Answer 4

The type of lava produced-> basalt, andesite, rhyolite

Lava type also has an influence on the explosivity of the eruption

Question 5

Characteristics of basaltic lava:

Answer 5

• 1000-1200 degrees celsius
• low 0.5-2% gas content
• formed by melting of mantle minerals
• thin and runny flow
• gentle, effusive eruption
• location: ocean hot spots, mid ocean ridges, shield volcanoes

Question 6

Characteristics of andesitic lava:

Answer 6

• 800-1000 degrees celsius
• 3-4% gas content
• formed by subjected oceanic plate melts and mixes with sea water etc
• slow flow
• violent, moderately explosive
• location: composite cone volcanoes, subduction zones

Question 7

Characteristics of rhyolitic lava:

Answer 7

• 650-800 degrees celsius
  -4-6% gas content
• formed by melting of lithosphere mantle and slabs of previously subducted plate
• thick and stiff flow
• very violent, cataclysmic
• location: super volcanoes or composite cone volcanoes

Question 8

What are the primary effects?

Answer 8

• Tephra: solid material of varying grain size ranging from volcanic bombs to ash, all ejected into the atmosphere e.g. E15, Iceland
• Pyroclastic flows (nuees ardentes): very hot gas charged, high velocity flows made up of mixture of gas and tephra e.g. Pompeii destroyed by flows from Mt Vesuvius
• Lava flows
• Volcanic gases: e.g. co2 emissions, carbon monoxide, hydrogen sulphide, sulphur dioxide and chlorine e.g. in 1986, co2 emissions from lake in crater of Nyos (Cameroon killed 1700 people)

Question 9

What are the secondary effects?

Answer 9

• Lahars: melted snow and ice as a result of the eruption combined with volcanic ash forms mud flows that can move down the course of river valleys at high speeds
• Flooding: when an eruption melts glaciers and ice caps
• Volcanic landslides
• Tsunamis: sea waves generated by violent volcanic eruptions such as those formed by the eruption of Krakatoa
• Acid rain: volcanoes emit gases which include sulfur. When this combines with atmospheric moisture, acid rain results.
• Climatic change: the ejection of a huge amount of volcanic debris into the atmosphere can reduce global temps and is believed to have been an agent in past climate change