-Volcanoes

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-Volcanoes

• OUTLINE
• Volcanoes and Plate Tectonics
• Earth Structure
• How Magma Forms
• Hawaiian Volcanoes The Nature of Volcanic
  Eruptions
• CONCEPTS
• Specific magma compositions and thus volcanoes
  are associated with specific tectonic
  environments
• Chemical versus mechanical layering in the Earth
• Partial melting
• Subduction zone volcanism
• Intraplate volcanism

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Volcanic gases billow behind those who have come
to collect the offerings made by Hindus at the
crater of Mount Bromo in East Java, Indonesia.
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Eldfell, Heimaey, Iceland
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Smoking craters of Mount Bromo (front) and Mount
Semeru (back) on the Indonesian island of Java.
Scientists have issued a high-alert warning for
Mount Bromo after it showed signs of increased
activity. The volcano, a popular tourist
destination, last erupted in 2004,
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A man rides a donkey on a road covered with ash
from an explosion of Tungurahua Volcano, in
Bilbao, Ecuador.
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Shield Volcanoes of Hawaii
Mahukona
32 kilometer
miles
Loihi
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• Study Guide to Volcanoes
• What causes volcanoes?
• Are there different types of volcanoes?
• What effects do volcanoes have?
• Can we predict when a volcano is going to erupt?
• Which volcano has killed the most people?
• Do volcanoes do anything good?
• Volcano facts

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Igneous Rock Names
SiO2 content lt 45 45-53 53-70 gt70
Volcanic Name Basalt Andesite Rhyolite
Plutonic Name Peridotite Gabbro Diorite Granite
Description ultramafic mafic intermediate felsic
Viscosity very low low intermediate high
General Color very dark dark intermediate light
Minerals olivine, pyroxene olivine, pyroxene, feldspar feldspar, amphibole feldspar, quartz, mica
Occurrence mantle oceanic crust average continental crust upper continental crust
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Chemical Layering in the Earth6371 km

• CRUST
• oceanic 10 km thick BASALT
• continental 40 km thick ANDESITE
• MANTLE
• 3000 km thick PERIDOTITE SILICATES
• CORE IRON
• 3300 km thick IRON
• outer core is liquid
• inner core is solid

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Volcanoes
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Composite Cone
Composite volcano is built up of alternate layers
of LAVA and pyroclastic materials. They can
explode with great violence.
Mt St Helens USA Mount Fuji Japan
Crater
Pyroclastic Flow
Ash layer
Dyke
Lava Layer
Flank eruption
Conelet
Lava Flow
Crust
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Nature of Volcanic Eruptions 1. Fluid basaltic
lavas generally produce quiet eruptions (Hawaiian
lava flows) 2. Highly viscous lavas (rhyolite or
andesite) produce more explosive eruptions
(Yellowstone Mt. St. Helens hot ash explosions)
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Explosive eruptions at Kilauea are thought to be
caused when water comes into contact with hot or
molten rock (magma) and flashes into steam. In
1924, this happened after the level of the lava
lake in Halema'uma'u Crater dropped below the
water table. The walls of the crater then
collapsed and blocked the opening down which the
lava had drained, allowing steam pressure to
build up and cause violent explosions.
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Many of Kilauea's pre-1924 explosive eruptions
that produced significant ash deposits probably
happened when the volcano's summit crater was so
deep that its floor was below the water table,
letting ground water seep in to form a lake.
Whenever magma erupted into the lake water,
violent explosions of steam and volcanic gases
resulted, fragmenting the magma into tiny ash
particles and driving fast-moving, extremely hot
ash-laden steam clouds (pyroclastic surges) out
of the crater.
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Composite volcanoes, sometimes known as strato
volcanoes, are steep sided cones formed from
layers of ash and lava flows. Composite
volcanoes can rise over 8000 feet. Examples of
composite volcanoes include Mount Fuji (Japan),
Mount St Helens (USA) and Mount Pinatubo
(Philippines).
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Shield Volcanoes
Mauna Loa Hawaii USA
Shield Volcanoes are enormous features built up
only from layers of lava. They produce lots of
lava but they tent not to erupt violently.
Layers of Lava
Lava Flow
10,000m
250 kilometer
Composite volcano to scale
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A Shield Volcano
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Shield Volcano Kilaueau Volcano
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Materials extruded from a Hawaiian Volcano

• Basaltic Lava Flows
• Basaltic lavas are much more fluid
• Types of basaltic flows
• Pahoehoe lava (- twisted or ropey texture)
• Aa lava (rough, jagged blocky texture)
• Dissolved Gases
• 1-6 of a magma by weight
• Mainly water H2O vapor, carbon dioxide CO2 and
  sulfur dioxideSO2

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Low-Viscosity Basaltic Lava
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A Pahoehoe lava flow
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Shield Shield volcanoes have gently sloping
sides and are formed from layers of lava.
Eruptions are typically non-explosive. Shield
volcanoes produce fast flowing fluid lava that
can flow for many miles. Examples of shield
volcanoes include the Hawaiian volcanoes.
Although these eruptions destroy property, death
or injury to humans rarely occurs. Dome (Acid
Lava Cones) Acid lava is much thicker than
lava which flows from shield volcanoes. Dome
volcanoes have much steeper sides than shield
volcanoes. This is because the lava is thick and
sticky. It cannot flow very far before it cools
and hardens.
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Cinder Cones, Tuff Cones
Diamond Head Koko Head Paracutin - Mexico
The volcano is built up of layers of pyroclastic
materials. When it erupts it is normally with
great explosive force.
Volcanic Bombs
Layers of Ash and Cinder
300m
Pipe
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What are active, dormant and extinct
volcanoes? Volcanoes are found in three states -
extinct, dormant and active. An extinct volcano
will never erupt again. A dormant volcano has not
erupted in 2000 years. An active volcano has
erupted recently and is likely to erupt again.
Where are volcanoes located? Volcanoes are
found along destructive (subducting) (diagram)
plate boundaries, constructive (divergent)
(diagram) plate boundaries and at hot spots in
the earth's surface.
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What is the Ring of Fire? The 'Ring of Fire'
(map) is a volcanic chain surrounding the Pacific
Ocean. It is formed along a destructive
(subducting) plate boundary. The BBC News Web
Site contains an excellent article on the 'Ring
of Fire'
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What are lahars and pyroclastic flows? The most
destructive aspect of volcanoes are lahars and
pyroclasic flows. Lahars are volcanic mudflows
created when water (from rain or meltwater from
glaciers) and ash mix. This deadly combination
can have devestating results on the surrounding
area. When lahars settle they can be metres thick
and as hard as cement. Lahars can occur long
after a volcanic eruption. Pyroclastic flows are
avalanches containing hot volcanic gases, ash and
volcanic bombs. On steep volcanoes pyroclastic
flows can reach speeds of over 100 miles per hour.
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Why do people live close to volcanoes? Volcanoes
have a wide range of effects on humans. These can
be problematic or beneficial. It is usually the
destructive nature of volcanoes which is more
widely documented. However, many people rely on
volcanoes for their everyday survival. Today,
many millions of people live close to volcanoes
for this very reason. People live close to
volcanoes because Geothermal energy can be
harnessed by using the steam from underground
which has been heated by the Earth's magma. This
steam is used to drive turbines in geothermal
power stations to produce electricity for
domestic and industrial use. Countries such as
Iceland and New Zealand use this method of
generating electricity. Volcanoes attract
millions of visitors around the world every year.
Apart from the volcano itself, hot springs and
geysers can also bring in the tourists. This
creates many jobs for people in the tourism
industry. This includes work in hotels,
restaurants and gift shops. Often locals are also
employed as tour guides.Lava from deep within
the earth contains minerals which can be mined
once the lava has cooled. These include gold,
silver, diamonds, copper and zinc, depending on
their mineral composition. Often, mining towns
develop around volcanoes. Volcanic areas often
contain some of the most mineral rich soils in
the world. This is ideal for farming. Lava and
material from pyroclastic flows are weathered
to form nutrient rich soil which can be
cultivated to produce healthy crops and rich
harvests.
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Geothermal energy "makes economic sense
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Can we predict when a volcano is going to erupt?
Scientists who specialize in volcanoes are
called volcanologists. They are getting better at
predicting volcanoes. The most reliable method is
listening to the 'rumbles' inside a grumbling
mountain. Earthquake tremors and ground level
changes are monitored for the active lava dome
buildup. They also look at the changes in the
gases which come out of volcanic mountains, the
angle of the slopes, and bizarrely, the behavior
of animals in the area. Animals often seem to be
able to 'detect' when an eruption is coming, and
they become agitated and worried
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http//volcanoes.usgs.gov/ash/index.html
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Igneous RocksWhat are They?

• Fire Rocks
• Formed underground by trapped, cooled magma
• Formed above ground when volcanoes erupt and
  magma cools

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The role of water
Pillow Lavas
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Assimilation and magmatic differentiation
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Convection- heat transfer which takes place by
hot, lower density material rising and cool,
higher density material sinking
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Decompression melting
At a Mid-Ocean Ridge
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Plate Boundaries

• Divergent
• Mid-ocean ridges where hot material rises to
  make new ocean floor-BASALT
• Convergent
• Subduction zones where cold material dives back
  into the mantle- ANDESITE
• Conservative
• Transform boundary where 2 plates slide by each
  other