lecture 16: volcanoes - explosion

what happens with a magma and water interaction

rapidly conversion of water to steam

what does steam do

greatly increases eruption explosivity

how can explosive magnitude be expressed

as a combination of plume height and volume

pumice

bubble filled lava, formed by depressurisation and cooling

volcanic ash

powerful explosions spray lava into air, which freezes into tiny fragments of glass

lapilli

pea to plum sized fragments of pumice or ash

blocks and bombs

larger (up to several m), made from pre-existing rock and molten rock respectively

tephra and tuff

fragmental deposits of volcanoes which are unconsolidated or transformed to rock respectively

effects of volcanic eruption

possible catastrophe and devastation on a variety of scales - effects range from immediate localised impacts on nearby landscapes and communities to global influences

pyroclastic flows (nuee ardente)

turbulent avalanches of gas, molten rock, ash and lapilli, are glowing, hot (300-800 degrees) and very fast moving (up to 300 km/h) - can travel 10's of km from a volcano - even across water

where do pyroclastic flows come from

gravitational collapse of an explosive eruption column

Mt. Pelée pyroclastic blast

a pressure wave of searing hot gas and ash - raced at up to 140m/s (500 km/h) down from Mt Pelée and through St. Pierre. This was followed by numerous pyroclastic flows à gravity-driven avalanches of gas, ash and lapilli

blast temperature

~250 to 400 degrees C. This was enough to set buildings and ships in St. Pierre ablaze. However, most of the victims were asphyxiated, not burned to death

how many deaths

~30,000 people

lateral blasts - Mt St Helens

On May 18th 1980 the mountainside finally failed and collapsed as a huge landslide, which moved down slope at 70-150 miles per hour. The landslide depressurised the magma below. This dramatic reduction in pressure caused the volcano to explode almost instantly

what highlights the power of the eruption

The dramatic change in the shape of Mt. St Helens

Mt St Helens crater

several explosive eruptions occurred at the summit, excavating a 350m wide and 150m deep crater

facts of the lateral blast

• speed: ~500 km per hour

• temperature: 350 degrees C

• casualties: 57

• thermal energy: 24 megatons

long term exposure to ash

silicosis of the lungs

ash clouds - December 15th 1989 KLM flight

descending to Anchorage Alaska, all four engines failed and had to be replaced: US$80 million in damage to the aircraft

April 14th 2010 – Iceland's Eyjafjallajökull

• Cloud of ash sent over North Atlantic ocean.

• >300 airports in 21 countries closed in Europe from 15th-21st April 2010

• >100,000 flights cancelled, 7 million passengers affected. Losses to Airline Industry estimated at > 1.3 Billion euro

what is Indonesia known as

land of 129 active volcanoes

Indonesia’s famous volcanoes

Merapi, Krakatoa and Tambora

1815 eruption of Tambora

• 160 km3 of erupted tephra (pyroclastic flows and falls)

• 92,000 local fatalities (eruption, tsunami, famine)

1883 Krakatau

dominated by andesites, but may also have involved some basalt and dacite. The forces accumulating beneath Krakatau were so great that they created an explosion that was heard up to 2,250 miles away - created an atmospheric shockwave that circled the globe 4 times

global atmospheric and climatic effects of Krakatau

global temperatures dropped for few years by up to about 1 degree C and spectacular fiery sunsets persisted globally for three years after the eruption

1816

year without a summer in Northern Hemisphere - severe winter and unusually cold summer -> agricultural disruption, famine, food riots, etc

caldera

big circular depressions up to 100 kms across and '00 metres deep

cause of a caldera

the collapse of volcano centre into drained magma chamber

super volcanoes

volcanoes capable of the largest explosive eruptions (i.e. VEI = 8)

super volcanic eruptions volume

 >= 1000 km3, emitted over the course of several days to a few weeks

Lake Toba

• caldera collapse generated the 100km x 30km lake. 15cm ash over entire South Asia and a site in central India has 6m of ash

• estimates of billions of tons of sulphur dioxide.

global impacts of lake Toba

• decrease in average global temperatures is contentious but estimates of 3-3.5 degrees C - decades long volcanic winter

• deforestation and vegetation destruction, severe drought in equatorial regions

• proposed to be linked with a ‘bottle-neck’ in human population about the same time.

Santorini Greece

Minoan Tuff Eruption (1628BC) devastated Santorini and surrounding islands, with evidence of a tsunami striking north coast of Crete

Yellowstone

Eruptions at 2.1Myr, 1.2Myr, 640,000yrs

Most recent, Lava Creek tuff (300km3 ), involved up to 2m thick of rhyolite pyroclastic falls and flows

caldera produced

75x45km across