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EXTRUSIVE VOLCANIC
LANDFORMS
1.
2.
2 forms of lava:
BASALTIC
– low silica, more fluid, so gas bubbles
escape easily and so NOT so explosive
ANDESITIC AND RHYOLITIC – high silica(acidic) less
fluid (high viscosity), gas does not escape
easily,pressure builds up and leads to violent
explosions.
Types of extrusive volcanic
landforms.
• Lava plateaux – fissure eruptions eg
Iceland. Basaltic in nature, lava flows great
distances and creates flat, featureless
landscapes. Laki, Iceland.
Shield / Basic volcanoes
Free flowing lava, volcanoes
have gentle sides and cover a
large area eg Hekla
Acid/dome volcanoes
Steep sided, convex cones.
Viscous lava(rhyolite)eg Puy
region, France
Ash and cinder cones
Sides are steep and symmetrical
eg Paricutin, Mexico
Composite cones, consist of layers
of ash and lava, usually
andesitic.eg Mt Etna.
Calderas
Massive build up of gas causes
explosion and a crater to form eg
Krakatoa, Indonesia.
Location of Krakatoa, Indonesia.
The nature of volcanic eruptions
• Vulcanologists classify volcanoes according to
the nature of their eruptions.
• Classification is based upon the degree of
violence of explosion, which is a consequence of
the pressure and amount of gas in the magma.
• Volcanic Explosivity Index (VEI) Used since
1982 to describe the relative size and magnitude
of explosivr eruptions, 0-8 index of increasing
explosivity. Each increase in number represents
an increase of 10.
Forms of volcanic eruption
Minor volcanic forms
• GEYSERS- water heated by geothermal
activity reaches such a high temperature
that it becomes a gas and explodes out of
a fissure in the ground. Due to increased
pressure this may not occur until
temperatures reach over
120*c(superheated water).
STROKKUR GEYSER,ICELAND.
HOT SPRINGS/BOILING MUD
• If the heated water does not explode onto
the surface, but mixes with surface
deposits, boiling mud is formed. Iceland.
THE BLUE LAGOON, ICELAND
• Geothermal heating of groundwater
created lagoons, rich in silica, in a lava
field.
Positive impact of tectonic activity!
• TOURISM:
AGRICULTURE:
SOLFATARA
• Small volcanic areas without cones,
produced by gas (initially sulphurous)
escaping to the surface. Eg Bay of Naples
in Italy.
FUMAROLES
• Areas where superheated water turns to
steam as it condenses at the surface.
• Very similar to Solfataras, but without the
high sulphur content.
THE IMPACT OF VOLCANIC
ACTIVITY
• Volcanic activity can affect the local,
national and global area.(spatial context).
• Volcanic activity can be classified as
having primary and secondary effects.
(temporal context).
PRIMARY EFFECTS.
• 1.VOLCANIC GASES-carbon dioxide,
carbon monoxide, hydrogen sulphide,
sulphur dioxide and chlorine. CO2 from
Lake Nyos in Cameroon in 1986
suffocated 1,700 people. Laki in Iceland in
1783 poisoned 40% of the population.
Affected harvests throughout Europe,
caused starvation and the French
Revolution!
TEPHRA
• Solid material of varying grain size from volcanic bombs
to ash particles, ejected into the atmosphere.
• DUST-flour size
• ASH-sand size
• LAPPILLI-little stones
• CINDERS-rough clinker
• BOMBS-up to 1m in diametre
• Light materials such as dust and ash become airborne
and travel great distances around the world. Larger
materials accumulate around the crater and increade
cone height.
Layers of tephra
Definition of Tephra
• Tephra, the Greek word for ash, is used to
describe any material that is ejected by a
volcano into the atmosphere. Tephra includes
dense blocks and bombs, and lighter materials
such as scoria, pumice and ash. As one moves
away from a volcano, the tephra deposits
become finer grained (the particles are smaller)
and thinner. This is because small particles stay
aloft longer and stay within the eruption cloud for
a greater distance from the volcano.
PYROCLASTIC FLOWS
• Very hot(800c),gas charged, high velocity flows
made up of a mixture of gases and tephra.
• Montserrat.1995-97.
• Located in the Soufriere Hills on the island of
Montserrat in the Caribbean.
• Began with ash emissions, steam and many
earthquakes.
• Then in 1996 pyroclastic flows began! On 25th
June 1997 4-5 million cubic metres of pyroclastic
material was emitted. See case study sheet.
Montserrat
Montserrat and Pyroclastic flows
• http://dsc.discovery.com/videos/ultimateguide-to-volcanoes-montserrat.html
Montserrat 1995-97
• http://www.google.co.uk/imgres?imgurl=http://w
ww.juicygeography.co.uk/images/PFJuly27.jpg&i
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2Bclip%2Bof%2BMontserrat%2Berupting%26hl
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sch:1
• Monitoring Volcanoes:
• Case study Montserrat.
http://www.mvo.ms/en/science/monitoring/science/monitoring
Definition of Pyroclasts
• Pyroclastic flows can form either from the collapse of an
energetic eruption cloud or by the collapse of unstable
lava domes that have oozed out of stratovolcanoes. The
hot rock tumbles downhill as an avalanche then rapidly
transforms into an expanding cloud of hot rock, "ash"
and gas as it entrains ambient air and heats it, and as
the hot volatiles within the lava violently decompress.
The result is a self-supporting dense cloud of hot debris
and gas that flows downhill as a fluid body. A cloud of
"ash" rises above the flow, and it variously scours its
path and/or deposits distinctive linear build ups of rock
and debris, depending on the flow conditions.
LAVA FLOWS
•
•
•
•
•
BASALT
ANDESITIC
RHYOLITIC
Depending upon silica content.
Differing levels of viscosity.
• Lava flow during a rift eruption at Krafla,
Iceland in 1984
More detailed classification of lava.
• Igneous rocks, which form lava flows when
erupted, can be classified into three
chemical types; felsic, intermediate, and
mafic (four if one includes the superheated ultramafic). These classes are
primarily chemical; however, the chemistry
of lava also tends to correlate with the
magma temperature, its viscosity and its
mode of eruption.
MAFIC LAVA IS ALSO CALLED
BASALTIC!
• Mafic or basaltic lavas are typified by their high ferromagnesian
content, and generally erupt at temperatures in excess of 950 °C.
Basaltic magma is high in iron and magnesium, and has relatively
lower aluminium and silica, which taken together reduces the
degree of polymerization within the melt. Owing to the higher
temperatures, viscosities can be relatively low, although still
thousands of times more viscous than water. The low degree of
polymerization and high temperature favours chemical diffusion, so
it is common to see large, well-formed phenocrysts within mafic
lavas. Basalt lavas tend to produce low-profile shield volcanoes or
"flood basalt fields", because the fluid lava flows for long distances
from the vent. The thickness of a basalt lava, particularly on a low
slope, may be much greater than the thickness of the moving lava
flow at any one time, because basalt lavas may "inflate" by supply of
lava beneath a solidified crust. Most basalt lavas are of ʻAʻā or
pāhoehoe types, rather than block lavas. Underwater they can form
"pillow lavas", which are rather similar to entrail-type pahoehoe
lavas on land. This can now be seen on the surface on Iceland.
2 types of basaltic lava
Pāhoehoe and Aa flows
• Aa is rough lava, whilst Pahoehoe is
smooth. Both are Basaltic and are found in
Iceland and Hawaii.
• http://en.wikipedia.org/wiki/Lava
SECONDARY EFFECTS
• LAHARS: volcanic mud flows eg eruption
of the Nevado del Ruiz in Nov 1985
caused the Colombian town of Armero to
be devastated.
Video of lahars
• http://www.bbc.co.uk/learningzone/clips/vo
lcanic-hazards-lahars-inindonesia/3069.html
• Complete ppq on primary and secondary
hazards/lahars.
SECONDARY EFFECTS.cont.
• Flooding- melting of icecaps such as
Eyjafjallajokull causes a lot of flooding.
Bridges had to be rebuilt.
• Tsunamis-From the eruption of Krakatoa in
1883 drowned 36,000 people.
• Volcanic landslides
• Climate change-volcanic debris reduces
global temperatures.
Human responses to natural
hazards.
• 1.This involves the assessment of RISK: the
exposure of people to a hazardous event.
• Why do people expose themselves to risk?
• Unpredictability- when or where and with what
magnitude/explosivity,
• Lack of alternatives- due to economic reasons
or lack of knowledge(LIC),
• Turn a blind eye!!
2. Vulnerability
• This relates to how capable the people are
at predicting,preparing and modifying the
loss!
• Poverty increases vulnerability,
• Wealth and higher levels of technical
ability, as well as education make people
less vulnerable to all kinds of natural
hazard.
• See “Is California worth the risk?”
Human Responses
• 1. Prevention! Impossible with volcanoes and
Earthquakes,
• 2. Modification of vulnerability
a. prediction and warning, how much can be
given? (see later)
b.preparedness, public education,emergency
services provision, land use planning/zoning.
• 3. Modification of loss: a. aid, b. insurance (see
California case study).
Volcano hazard management.
• http://dsc.discovery.com/videos/volcano-video/
• Eruptions can not be prevented, but they can often be
predicted and protection offered to the population.
• PREDICTION:
• Hazard mapping (previous lava flows and pyroclasts by
studying geology),
• Analysing seismic shockwave patterns,
• Sampling gas and lava emissions,
• Remote sensing of changes in topography,
• Heat and gas emissions by satellite.
• Seismic shock waves alerted vulcanologists to the likely
eruption of Popocatepetl, Mexico in 2000 and erupted 24
hours after the local population was evacuated.
Protection
• This involves reducing the risk of damage by
preparing for an eruption. Warnings issued by
the USGS in the US are supported by FEMA
who instruct how to react before, during and
after an eruption.
• http://volcanoes.usgs.gov/publications/2010/icel
and.php
• In the longer term hazard mapping and land use
planning may be used to avoid development in
areas at risk.
http://volcanoes.usgs.gov/activity/
• http://www.fema.gov/hazard/volcano/index.shtm
• Check out the above web-sites to take notes on
how man can predict, prepare and protect
against volcanoes.
• Link this to your case studies from high-income
and low-income countries.
• Take specific notes on the volcano warning
systems.
• PPQ on prep/prediction of volcanoes.