-Volcanoes
OUTLINE
I.
Volcanoes and Plate Tectonics
II.
Earth Structure
III.
How Magma Forms
IV.
Hawaiian Volcanoes The Nature of Volcanic Eruptions
CONCEPTS
1.
Specific magma compositions and thus volcanoes are
associated with specific tectonic environments
2.
Chemical versus mechanical layering in the Earth
3.
Partial melting
4.
Subduction zone volcanism
5.
Intraplate volcanism
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.
Eldfell, Heimaey, Iceland
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 man rides a donkey on a road covered with ash from an explosion of
Shield Volcanoes of Hawaii
Mahukona
Loihi
32 kilometer
miles
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
Igneous Rock Names
SiO2 content
< 45%
Volcanic Name
45-53%
53-70%
>70%
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 upper continental
crust
crust
Chemical Layering in the Earth
6371 km
CRUST:
oceanic ~10 km thick BASALT
continental ~40 km thick ANDESITE
MANTLE:
~3000 km thick PERIDOTITE SILICATES
CORE:
~3300 km thick IRON
outer core is liquid
inner core is solid
IRON
Volcanoes
Composite Cone
Mt St Helens – USA
Mount Fuji – Japan
Crater
Composite volcano is
built up of alternate
layers of LAVA and
pyroclastic materials.
They can explode with
great violence.
Pyroclastic Flow
Ash layer
Lava Layer
Conelet
Dyke
Flank
eruption
Lava
Flow
Crust
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)
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.
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.
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).
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
A Shield Volcano
Shield Volcano
K’ilaueau Volcano
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
Low-Viscosity Basaltic Lava
A Pahoehoe lava flow
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.
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
300m
Pipe
Layers of Ash
and Cinder
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.
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'
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.
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.
Geothermal energy "makes economic sense
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
http://volcanoes.usgs.gov/ash/index.html
Igneous Rocks
What are They?
• Fire Rocks
• Formed underground by trapped,
cooled magma
• Formed above ground when volcanoes
erupt and magma cools
The role of water
Pillow Lavas
Assimilation and magmatic
differentiation
Convection- heat transfer which
takes place by hot, lower density
material rising and cool, higher
density material sinking
Decompression melting
At a Mid-Ocean Ridge
Plate Boundaries
1. Divergent:
Mid-ocean ridges where hot material rises to
make new ocean floor-BASALT
2. Convergent:
Subduction zones where cold material dives back
into the mantle- ANDESITE
3. Conservative:
Transform boundary where 2 plates slide by each
other