Volcanic Eruptions and Hazards
Volcanic Eruptions and Hazards
Volcanic Eruptions and Hazards
and Hazards
What is a volcano?
vent
• A volcano is a vent or
'chimney' that connects
cone molten rock (magma)
from within the Earth’s
crust to the Earth's
surface.
• The volcano includes the
conduit surrounding cone of
erupted material.
magma
chamber
How and why do volcanoes erupt?
• Hot, molten rock (magma) is buoyant (has a lower density
than the surrounding rocks) and will rise up through the crust
to erupt on the surface.
– Same principle as hot air rising, e.g. how a hot air balloon works
• When magma reaches the surface it depends on how easily it
flows (viscosity) and the amount of gas (H2O, CO2, S) it has in
it as to how it erupts.
• Large amounts of gas and a high viscosity (sticky) magma will
form an explosive eruption!
– Think about shaking a carbonated drink and then releasing the
cap.
• Small amounts of gas and (or) low viscosity (runny) magma
will form an effusive eruption
– Where the magma just trickles out of the volcano (lava flow).
Explosive Eruptions
• Explosive volcanic
eruptions can be
catastrophic
• Erupt 10’s-1000’s km3 of
magma
• Send ash clouds >25 km
into the stratosphere
• Have severe Mt. Redoubt
environmental and climatic Above: Large eruption column and
effects ash cloud from an explosive
• Hazardous!!! eruption at Mt Redoubt, Alaska
Explosive Eruptions
• Three products
from an explosive
eruption
– Ash fall
– Pyroclastic flow
– Pyroclastic surge
Pyroclastic flows on
Montserrat, buried
the capital city.
Direct
measurements of
pyroclastic flows
are extremely
dangerous!!!
Effusive Eruptions
• Effusive eruptions are
characterised by outpourings
of lava on to the ground.
Hawaii
Courtesy of www.swisseduc.ch
Practical Exercise 1.
Courtesy of www.swisseduc.ch
Pyroclastic Flow
• For example,
eruption of
Vesuvius in 79
AD destroyed
the city of
Pompeii
Pompeii (79AD)
before after
29,000 people died….
Only 2 survived! Why?
How do pyroclastic flows cause
devastation?
Pyroclastic Flow - direct impact
Courtesy of www.swisseduc.ch
Pyroclastic Flow - burial
Pyroclastic Flow - burns
Pyroclastic Flow - lahars
• Hot volcanic activity can melt
snow and ice
• Melt water picks up rock and
debris
• Forms fast flowing, high
energy torrents
• Destroys all in its path
Pyroclastic Fall
• Ash load
– Collapses roofs
– Brings down power
lines
– Kills plants
– Contaminates water
supplies
– Respiratory hazard for
humans and animals
Lava Flow
• It is not just explosive volcanic activity that
can be hazardous. Effusive (lava) activity
is also dangerous.
Lava Flow - Heimaey, Iceland
• Iceland, January
23,1973.
• Large fissure
eruption
threatened the
town of
Vestmannaeyjar.
Lava Flow - Heimaey, Iceland
Assessing Volcanic
Hazards
So….
How do we minimize the risk of
active volcanoes?
Volcano Monitoring
Volcano Observatories
are set up on all active
volcanoes that
threaten the human
population. These are
designed to monitor
and potentially to
predict the eruptive
behaviour of the
volcano in question.
Volcano Monitoring
• Seismicity
• Deformation
• Gas Output
– (on volcano and
remote sensing
techniques)
These three
things are the
most important
precursors to an
eruption.
Seismic Activity
• Earthquake activity commonly precedes an eruption
– Result of magma pushing up towards the surface
– Increase volume of material in the volcano shatters the
rock
– This causes earthquakes
Seismic Activity
• Earthquake activity is measured by Seismographs
– Seismographs are stationed on the flanks of the
volcano
– These record the frequency, duration and intensity
of the earthquakes and report it back to the volcano
observatory.
Deformation Monitoring
• “Tiltmeters” are used to measure the deformation
of the volcano
– The tiltmeters measure changes in slope as small as one part per
million. A slope change of one part per million is equivalent to
raising the end of a board one kilometer long only one millimeter!
Deformation Monitoring
• Tilltmeters can tell you when new material enters the magma chamber.
Note the
B presence of
earthquakes in
relation to the
deformation.
Often it is a
combination of
events that
fore-warns of
an eruption.
Gas Monitoring
• Commonly gas output from a volcano increases or changes
composition before an eruption.
– As magma rises to the surface it releases (exsolves) much
of its gas content.
– This can be measured
Gas Monitoring
• Gas samples are collected from fumaroles
and active vents.
Destruction after a
volcanic induced
earthquake in Japan