VOLCANIC ERUPTIONS

AND HAZARDS
• Source for this lecture is Internet….
• Also material from main web site of volcanos….
 THEEM OF LECTURE
• TO Understand Volcanos.
• Types of volcanos
• Effect of Volcanos.
 WHAT IS A VOLCANO?
vent
• A VOLCANO IS A VENT OR
'CHIMNEY' THAT CONNECTS
MOLTEN ROCK (MAGMA)
cone FROM WITHIN THE EARTH’S
CRUST TO THE EARTH'S
SURFACE.
• THE VOLCANO INCLUDES
conduit
THE SURROUNDING CONE
OF ERUPTED MATERIAL.

magma chamber
• A volcano is a place on the Earth’s surface where hot, molten rock (called
magma) breaks through. As we will see there are many different types of
volcanoes and material that is erupted.
• In general a volcano is classed as “active” if it erupts lava, rock, gas or ash,
or if it shows seismic (earthquake) activity.
• A volcano is dormant if it hasn't erupted for a long time (less than 1
million years) but it could erupt one day again.
• Different terms are define as
• Magma: Molten rock beneath the surface of the earth.
• Magma chamber: The subterranean cavity containing the gas-rich liquid
magma which feeds a volcano.

•
• Conduit: A passage followed by magma in a volcano.
• Vent: The opening at the earth's surface through which volcanic
materials issue forth.
• Cone: A volcanic cone built entirely of loose fragmented material
(pyroclastics) and (or) lava flows erupted from the vent. Erupted
material builds up with each eruption forming the cone.
 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).
 • Melting of lower crust and mantle = molten rock = magma
• At depths > 20 km the temperature = 800-1,600 degrees Celsius
• The density of the magma is less than the crustal rock, therefore it rises to the
surface
• Source of this heat?
• Residual from the cooling of the Earth (& solar system)
• Radioactive decay
• Convection in the mantle
• - Brings hot rock up from near the interior of the Earth and returns cooler material
towards the centre of the Earth for reheating.
- Shock/impact melting
- E.g. meteorite impacts produce instantaneous heat and melting from high energy
collisions
- Two styles of volcanic eruption:
- Explosive and Effusive:
- Explosive: where rapidly escaping gas bubbles (= vesicles) rip
apart the magma, fragmenting it.
- Effusive: where the magma leaks out onto the surface passively as
lava flows.
Some effusive eruptions involving highly viscous lava may turn into
explosive eruptions. If the magma is too viscous (sticky) it can block up the
volcanic vent, trapping gas inside the volcano. If this gas builds up enough
to break through the blockage an extremely dangerous explosive eruption
may form.
• - Some of the most explosive eruptions have formed this way, e.g.
Pinatubo 1991
 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 environmental
and climatic effects
Above: Large eruption column and
• Hazardous!!! ash cloud from an explosive
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.
Explosive eruption forms:

• Ash fall: The fallout of rock, debris and ash from an explosive eruption
column.
• An explosive volcanic eruption will propel large volumes of volcanic
rock, ash and gas into the atmosphere. The larger (most dense)
particles will fall out of the air quickly and close to the volcanic vent.
The smaller particles (ash) can be suspended in the atmosphere for
days to weeks before they fall back to Earth. Whilst in the atmosphere
the wind can transport the ash particles large distances.
• Pyroclastic flow:
Pyroclastic flows are hot, turbulent, fast-moving, high particle
concentration clouds of rock, ash and gas.
• They can travel 100s km/h and are commonly >400°C.
• Its one of the hazardous component of explosion bcz of the reason it
contain high concentration of particles and low concentration of Gas.
• Pyroclastic surge:
• Pyroclastic surges are low particle concentration (low density) flows of volcanic
material. The reason they are low density flows is because they don’t have a high
concentration of particles and contain a lot of gas.
• Pyroclastic surges are very turbulent and fast (up to 300 km per hour).
• They overtop high topographic features, and therefore are not confined to valleys.
• Pyroclastic surges usually do not travel as far as pyroclastic flows, but pyroclastic
surges can travel up to at least 10 kilometers from the source.
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
Volcanic Hazards
• Pyroclastic flow
• Lahars/Mud flows
• Pyroclastic fall
• Lava flow
• Noxious Gas
• Earthquakes

Courtesy of www.swisseduc.ch
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 - Heimaey, Iceland
• However, the potential damage was reduced by spraying
seawater onto the advancing lava flows.
• This caused them to slow and/or stop, or diverted them away
from the undamaged part of the town.
 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!
 Gas Monitoring
• Gas samples are collected from fumaroles and
active vents.

• Gas levels may also be monitored by remote

sensing techniques
In Summary..

• Volcanoes are extremely hazardous.

• However, the volcano can be studied, monitored
and understood.
• Each volcano is different, and offers a unique set
of dangers
• Plans may be emplaced to help control
potential damage.
Earthquakes
• Large volumes of magma moving through the
shallow crust can cause large earthquakes.
• This can lead to building collapse, slope failure
and avalanches
Earthquakes

Destruction after a
volcanic induced
earthquake in Japan