AS

Volcanoes V: Volcanic Explosivity Index and Hazards

Volcanic Explosivity Index (VEI)

  • logarithmic scale from 0-8
  • describes the size of an explosive volcanic eruption
  • key characteristics:
    • volume of ash produced
    • height of eruption cloud above vent
    • duration of eruption
  • mainly depends on total volume of material erupted explosively
  • increase by 1 unit = 10x more volume erupted

Mt. Pinatubo, Phillipines

  • VEI = 6
  • second largest (terrestrial) eruption in the 20th century
  • 28 mile high ash cloud
  • valleys filled with up to 660 ft of volcanic deposits
  • 20 million tonnes of SO2
  • 0.9 degrees F in global temperatures in 1991-1993

What Would A VEI 8 Eruption Look Like?

  • imagine 100 pinatubos erupting at the same time
  • toba caldera (supervolcano) eruption:
    • 75,000 years ago
    • 2800km^3 (675 miles^3) of material erupted
    • global temperature decreased by 5.4-6.3 degrees F (as much as 27 degrees F at high latitudes)

Volcanic Hazards

  1. lava

  2. pyroclastic material

    1. pyroclastic (ash) fall
    2. pyroclastic fall

  3. gases

  4. lahars

  5. lateral blast

Lava

  • slow

  • easy to predict its path

  • usually not dangerous

  • hazardous only to buildings and other infrastructure

  • lava flows have low viscosity

  • usually mafic lava

  • two general types:

    1. A’a
    2. Pahoehoe

Lava Flow- A’a

  • mafic lava flow
  • blocky and more vicious

Lava Flow- Pahoehoe

  • mafic lava flow
  • forms ropes and less vicious

Pyroclastic Material

  • rock fragments ejected from a volcano
  • different types distinguished based on particle size
    • ash (<2mm)
    • lapilli (2-64mm)
    • block/bomb (>64mm)

Pyroclastic Material- Ash

  • not good to breathe, but not immediately fatal
  • travels very far
  • hazard to air travel
  • in extreme eruptions, can have long-term climate effects and cause crop failure
    • ex: Mount Tambora erupted in Indonesia in April 1815
    • 1816 was the “year without a summer” due to ash and SO2 in the stratosphere
    • on average it was 5.4 degrees F colder that year

Pyroclastic Material- Lapilli

  • larger pieces and fragments
  • don’t “fly” as far
  • minor hazard near volcanoes

Pyroclastic Material- Block

  • >64mm with jagged edges
  • material ripped from existing rock and tossed by eruption

Pyroclastic Material- Bomb

  • >64mm with rounded edges
  • molted when ejected and cools during flight

Pyroclastic Fall

  • eruption column of ash (up to 25 miles high)
  • widespread distribution of ash and other small pyroclastic material (small lapilli) in downwind direction
  • blankets topography
  • heavy ash can cause building collapse

Pyroclastic Flow

  • a hot (400-1300 degrees F) mixture of ash, rock fragments, and gas that travels rapidly (>50 mph) down side of volcano
    • in past also called “nuee ardente” (“fiery cloud” in french)
  • gravity-driven, flow down slope and channelled in valleys
  • extremely hazardous
  • can’t be outrun

Gases

  • magma/lava contains gases
    • water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), hydrogen chloride (HCl)
  • gases can build up or pool and flow
  • deadly in valleys
  • CO2 built up in Lake Nyos due to volcanic activity
  • suffocated 1746 people and 3500 livestock within minutes

Lahars

  • volcanic mudflows
  • flow will be channeled in valleys
  • mixture of rock debris and water
    • eruption melts snow or ice (glacier), or ejects water from a crater lake
    • can occur without an eruption
    • lots of rainfall loosens volcanic sediment forming a slurry
  • can start off small but grow as they pick up things in their path
    • rocks, soil, vegetation, buildings, etc.
  • travel very far

Lateral Blast

  • special type of explosive eruption
  • erupts to the side instead of up
  • carry rock debris at high speeds
    • destroys 10s to 100s of miles
    • kill all living things by abrasion, impact, burial, and heat
  • ex: mount st. helens