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