Transcript Volcanic Activity - Ysgol Rhyngrwyd IGCSE Geography

Volcanic Activity
Year 10
Geography
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Volcanic Activity
• Volcanic activity is important right across the
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earth’s surface.
There are over 500 active volcanoes which still
erupt from time to time.
Each year there are about 30 or 40 eruptions.
Some are slight, some are serious causing loss of
life and damage to land and property.
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• The majority of the active volcanoes
are found at destructive plate
margins, where one tectonic plate is
sliding down, or subducting, beneath
another one.
• Although not all of them are – what
other kind of plates has volcanoes
along it
• Can you think of a pace that has one?
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• Many volcanoes are found around the edge
of the Pacific Ocean – down the west coast
of North and South America, through the
Philippines, Indonesia and Japan.
• This circle of active volcanoes is called the
Pacific Ring of Fire.
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Ring of Fire
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Parts of a volcano
• A volcano is a crack in the earth’s surface which
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provides an outlet for lava, steam and ash.
The lava passes through a pipe called a vent from
inside the earth’s surface where it erupts.
After the eruption a crater is left on the
surface. Lava which has erupted hardens on the
surface.
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Cross-section
the one in the textbook is better!!
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Acidic Cone Volcanoes
• These are usually found at destructive
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boundaries.
Cone volcanoes are tall and steep-sided.
Cone volcanoes are formed by eruptions of thick,
viscous (sticky) lava.
Viscous lava tends to be acidic.
The thick lava moves relatively slowly and
hardens quickly to form new rock - this explains
the formation of a cone shape.
Eruptions tend to be violent.
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• E.g. Mount Pelee on Martinique (an island in the
Caribbean)
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Shield Volcanoes
• These are usually found at constructive
boundaries.
• They are low, with gently sloping sides.
• Shield volcanoes are formed by eruptions
of thin, runny lava which is not so acidic,
and therefore are often referred to a
basic lava
• Eruptions here tend to be frequent but
relatively gentle.
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Composite Volcanoes
• These volcanoes are composed (made up) of
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alternating layers of lava and ash (other
volcanoes just consist of lava).
The eruptions from these volcanoes may be
a pyroclastic flow rather than a lava flow.
A pyroclastic flow is a mixture of hot
steam, ash, rock and dust.
A pyroclastic flow can roll down the sides
of a volcano at very high speeds and with
temperatures of over 400° C.
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Direct
measurements of
pyroclastic flows
are extremely
dangerous!!!
• A truck carrying
volcanologists and a film
crew attempting to out
run a pyroclastic flow ….
• the pyroclastic flow was
traveling at about 25-30
meters per second…..
• they made it….
• just!
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Comparing sizes
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Tectonic hazards in LEDCs
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Mount Pinatubo
Introduction
Where: Mount Pinatubo, Philippines, Asia
When: June 12th-15th, 1991
Type of volcano: composite volcano
Type of eruption: Explosive - the second
biggest eruption this century
• Deaths: 300 people died, 1000's were
evacuated
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• Mount Pinatuba is 110km NW
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of Manila, the capital of the
Philippines.
In 1991, the volcano had been
dormant for 600 years.
During this time, the ash and
lava had weathered to
become fertile soil where
rice was grown.
But then things began to
happen …..
Clark
Air
base
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What happened?
• Steam on April 2nd 1991
• Quakes from April 6th – June 7th
• Ash Ejections from June 3rd
• Dome Formation was seen from an ariel
survey
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Then it really began to happen!
• Day One, Sunday, 9 June 1991
• Eight hours of ash-laden steam
clouds ejection ushered one of the
world’s most violent and
destructive eruptions of the
century, beginning at around 6 AM.
• This was followed by pyroclastic
flows which flowed down Pinatubo’s
gullies. These pyroclastic flows
reached some 4-5 kilometres from
the centre of activity.
• Philvolcs (Philippine Institute of
Volcanology and Seismology )
issued Alert Level 5 at around 3:15
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What happened
• Day Four, Wednesday, 12 June 1991
• At around 8:51 on Independence Day morning,
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intense seismic activity was followed by three
major explosions, the most powerful of which
ejected a "huge, grey, mushroom-shaped
cloud" that reached 20 kilometres above the
vent. Ash, pumice, and other larger volcanic
fragments were also ejected by Pinatubo.
Major rivers and tributaries radiating from
the volcano overflowed with cascading
pyroclastic flows.
A second series of strong explosions hurled
clouds of ash and pyroclastic materials 25
kilometres into the air.
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Pyroclastic flow 2 weeks later
• It was still too hot to go near
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• Day Five, Thursday, 13 June 1991
• Only 24 hours after the first major
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explosions, another violent eruption
occurred triggering heavy ashfalls
that blanketed most of surrounding
area. Prevailing winds also blew the
ash hundreds of kilometres in all
directions.
Day Six, Friday, 14 June 1991
The fourth major eruption occurred
ejecting another vertical ash column
that reached a maximum height of 25
kilometres above the vent.
Later, a much bigger eruption
produced a cauliflower-shaped column
that ejected volcanic debris to a
maximum height of 30 kilometres.
Pyroclastic flows ran down reaching 15
kilometres from the source.
What
happened
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What happened
• Day Seven, Saturday, 15 June 1991
• Two explosions occurred at dawn accompanied
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by incandescent pyroclastic flows that moved at
speeds between 70 kph to 80 kph.
These were followed by seven more eruptions
lasting up to midmorning, producing ash columns from
15-18 kilometres wide at the base and heights of up
to 25 kilometres above sea level. This time,
pyroclastic flows advanced to areas as far as 16
kilometres away from the centre of activity.
Then a violent eruption ejected a 40 kilometre high
eruption column.
This was followed by five similarly strong explosions
in such close succession that they appeared to be one
continuous activity.
Philvolcs recorded a total of 19 eruptions that day.
After June 16
After June 16, Pinatubo’s activities declined both in
frequency and magnitude where only minor eruptions
were recorded.
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But it was not half so bad as it could have
been
• Before the cataclysmic eruption, about 1,000,000 people
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lived in the region around Mount Pinatubo, including about
20,000 American military personnel and their dependents
were evacuated
The slopes of the volcano and the adjacent hills and
valleys were home to thousands of villagers.
Despite the great number of people at risk, there were
few casualties in the June 15 eruption.
This was not due to good luck but rather was the result
of intensive monitoring of Mount Pinatubo by scientists
with the Philippine Institute of Volcanology and
Seismology (PHIVOLCS) and the U.S. Geological Survey
(USGS).
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But it was not half so bad as it could have
been
• From early April scientists from PHIVOLCS immediately
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began on-site monitoring and soon declared a 10kilometer radius danger zone around the volcano.
They were joined in a few weeks by American scientists
who brought with them specially designed, portable
instruments, which the joint Philippine-American team
used to quickly set up monitoring networks on and around
Mount Pinatubo.
The team also conducted intensive studies of the
volcano's past eruptive history.
When the data from these efforts indicated that a huge
eruption of the volcano was imminent, the joint team
issued urgent warnings.
These timely forecasts enabled civil and military
authorities to arrange the evacuation of people and
equipment to safe areas before Mount Pinatubo exploded
on June 15.
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But it was not half so bad as it could have been
• They estimate that their forecasts saved at least 5,000 lives and
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perhaps as many as 20,000.
The people living in the lowlands around Mount Pinatubo were alerted
to the impending eruption and many fled to towns at safer distances
from the volcano or took shelter in buildings with strong roofs.
The American Clark Air Base was fully evacuated.
In the eruption, thousands of weaker roofs, including some on Clark,
collapsed under the weight of ash made wet by heavy rains, yet only
about 250 lowland residents were killed.
Of the 20,000 indigenous Aeta highlanders who lived on the slopes
of Mount Pinatubo, all but about 20 were safely evacuated before
the eruption completely devastated their villages.
Property worth hundreds of millions of dollars was protected from
damage or destruction in the eruption. Civil and military aircraft
were also evacuated saving millions of dollars. Stopping flights over
the area, also saved many lives.
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• The results
• Pinatubo unleashed three major destructive agents,
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namely: ashfall, pyroclastic flow and lahar that caused
destruction to Central Luzon's infrastructure and
rendered its vast agricultural lands into virtual
wastelands.
Ash fell over an area of 600km radius
More than 86,000 hectares of agricultural lands and
fishponds were affected by ashfalls and lahars.
Irrigation systems, water service facilities, power
transmission and lateral lines, roads bridges and other
infrastructures were damaged mainly by lahars
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• Houses and public buildings
collapsed from the weight of
accumulated ash deposits.
• Commercial and industrial
operations were suspended
while more than 650,000
workers were forced out of
work because of the
destruction of their farms,
shops, factories and work
places.
• The Americans were forced to
evacuate Clark Air Base after
volcanic dust, ashfall and lahar
rendered its runways useless
and endangered the operations
of its planes.
• The Ninoy Aquino International
Airport was also closed for
four days
• Public buildings were converted
into evacuation centres to
accommodate the increasing
number of refugees from the
devastated areas.
• But more than the destruction
of buildings and infrastructure,
Pinatubo's eruption affected
more than 249,000 families or
about 1.18 million people,
including 847 deaths, 184
injuries and 23 missing.
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Mount Pinatubo
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What is a Lahar?
• Explosive eruptions can deposit huge amounts ash and
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other volcanic debris on a volcano's slopes.
Lahars form when water from intense rainfall, melting
snow and ice, or the sudden failure of a natural dam,
mixes with this loose volcanic material, creating
mudflows that can be particularly dangerous and
destructive.
Although lahars contain a lot of volcanic ash and rock
fragments–making them dense and viscous like wet
concrete–they actually flow faster than clear-water
streams.
These mudflows can rush down the flanks of a volcano at
speeds as great as 65 kilometres per hour and can travel
more than 80 kilometres. Lahars that contain the most
debris move the fastest and are the most destructive.
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Why were Lahars a problem for Pinatubo?
• Annual rainfall on Mount Pinatubo ranges from
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2,000 – 4000 mm.
Nearly all of this rain falls during the time of
the southwest monsoon between June and
October
Ash and other deposits from Mount Pinatubo's
1991 eruption destroyed or buried much of the
lush vegetation that had covered the volcano.
When heavy rain fell on Pinatubo rapid runoff
down the still bare slopes quickly grew into
torrents that became lahars by eroding these
deposits.
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Why were Lahars a problem for Pinatubo?
• But volcanic debris is very fertile and quite quickly the
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vegetation regenerated, so each year this is less of a
problem.
However, in the first few years following the 1991
eruption, lahars have deposited more than 3 km3 equivalent to 300 million dump-truck loads - of debris on
the lowlands surrounding the volcano, burying hundreds
of square miles of land.
They can carry large boulders, cars, or even bridges and
can destroy or bury almost anything in their paths.
When lahars reach low-lying areas, they spread out, slow
down, and deposit their vast loads of debris over many
square miles.
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Ash deposits
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Lahar Warnings
• Radio-telemetric rain gauges provide data
on rainfall in lahar source regions, acoustic
flow monitors on stream banks detect
ground vibration as lahars pass, and
manned watch points further confirm that
lahars are rushing down Pinatubo's slopes.
• This system has enabled warnings to be
sounded for most but not all major lahars
at Pinatubo, saving hundreds of lives.
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