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

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